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Bonding Structures of ZrHx Thin Films by X-ray Spectroscopy
Abstract
The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 Å in the hexagonal closest-packed metal (α-phase) to 3.395 Å in the understoichiometric -ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk δ-ZrH2. For yet lower hydrogen contents, the structures are mixed α and δ-phases, while sufficient hydrogen loading (x>1) yields a pure δ-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.
... The main absorption peak of a-Zr is due to pure Zr 1s → 4p dipole transitions forming a twopeak structure (indicated by arrows). The pre-edge shoulder (pre-peak) in the a-Zr spectrum is [36,37]. The pre-peak is a signature of tetrahedral distortion of the coordination symmetry around the absorbing Zr atoms in the hexagonal α-Zr structure that allows p-d mixing into the Zr 1s → 4p dipole transitions [38,39]. ...
... For a-Zr [34], the main peak is dominated by the Zr-Zr paths at 3.160 Å (half diagonal, same notations as in Fig. 5) and 3.254 Å (cell edge), both with six nearest neighbors in the first coordination shell, which is close to our values obtained from θ/2θ XRD of the a-Zr bulk reference with 3.180 Å and 3.233 Å, respectively [37] [43] and Table III. Our XRD data is in excellent agreement with the bond lengths (3.179 Å and 3.232 Å) calculated from the JCPDS card [34]. ...
The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).
... This pre-edge peak can be attributed to Zr-1s → Zr-4d electron transitions, which are dipole-forbidden but visible as a well-distinguishable peak in the spectrum of bulk-Zr(0) due to the band-mixing of d-and p-like states in the conduction band. 29 For the nanoparticles, this hybridization is less pronounced, whereupon the pre-edge peak appears as only a weak shoulder (Figure 4a). In addition, the absorption edge for the Zr(0) nanoparticles (18,010 eV) is shifted to higher energies in comparison to bulk-Zr(0) (18,008 eV) due to more localized states and therefore located between the absorption edge of bulk-Zr(0) and ZrO 2 (18,013 eV). ...
... In addition, the absorption edge for the Zr(0) nanoparticles (18,010 eV) is shifted to higher energies in comparison to bulk-Zr(0) (18,008 eV) due to more localized states and therefore located between the absorption edge of bulk-Zr(0) and ZrO 2 (18,013 eV). This finding can be 19,22,29 where they are insufficiently coordinated by metal atoms (in comparison to volume atoms) but with THF/DME molecules adhered on the surface ( Figure 4d). Overall, the differences between the spectra of Zr(0) nanoparticles and bulk-Zr(0) are to be expected and confirm the presence of zerovalent zirconium in the nanoparticles. ...
... The main absorption peak of a-Zr is due to pure Zr 1s → 4p dipole transitions forming a twopeak structure (indicated by arrows). The pre-edge shoulder (pre-peak) in the a-Zr spectrum is [36,37]. The pre-peak is a signature of tetrahedral distortion of the coordination symmetry around the absorbing Zr atoms in the hexagonal α-Zr structure that allows p-d mixing into the Zr 1s → 4p dipole transitions [38,39]. ...
... For a-Zr [34], the main peak is dominated by the Zr-Zr paths at 3.160 Å (half diagonal, same notations as in Fig. 5) and 3.254 Å (cell edge), both with six nearest neighbors in the first coordination shell, which is close to our values obtained from θ/2θ XRD of the a-Zr bulk reference with 3.180 Å and 3.233 Å, respectively [37] [43] and Table III. Our XRD data is in excellent agreement with the bond lengths (3.179 Å and 3.232 Å) calculated from the JCPDS card [34]. ...
The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).
... This is also accompanied by a chemical shift with binding energies that are 0.6 eV higher than pure α-Zr indicating significant charge transfer toward H, in particular for x = 1.16. In our x-ray absorption near-edge structure study [34], a chemical shift toward higher energies in comparison to Zr metal was found to be due to changes in the oxidation state that depend on the structure and the formation of Zr-H bonding at sufficient hydrogen loading. Previous valence-band XPS experiments have shown a dominant Zr 4d peak at 1 eV and another peak around 6.4 eV corresponding to a Zr-H bond [11], consistent with DOS calculations [29][30][31]. ...
... Calculated lattice parameters and charge transfer for ZrH x for different x. Experimental values for α-Zr are given in parenthesis[34]. ...
The electronic structure and chemical bonding in reactively magnetron sputtered ZrH x (x = 0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level, and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H 1s-Zr 4d hybridization region at ∼6 eV below the Fermi level. For low hydrogen content (x = 0.15, 0.30), the films consist of a superposition of hexagonal closest-packed metal (α phase) and understoichiometric δ-ZrH x (CaF 2-type structure) phases, while for x = 1.16, the films form single-phase ZrH x that largely resembles that of stoichiometric δ-ZrH 2 phase. We show that the cubic δ-ZrH x phase is metastable as thin film up to x = 1.16, while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated ZrH 1.16 film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr 4p 3/2 and 3d 5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.
... 16. In our x-ray absorption study [34], a chemical shift towards higher energies in comparison to Zr metal was found to be due to changes in the oxidation state that depends on the structure and the formation of Zr-H bonding at sufficient hydrogen loading. Previous valence band XPS experiments have shown a dominant Zr 4d peak at 1 eV and another peak around 6.4 eV corresponding to a Zr-H bond [35], consistent with DOS calculations [31] [29] [30]. ...
The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x=0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at% are investigated by 4d valence band, shallow 4p core-level and 3d core-level X-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity towards the H 1s - Zr 4d hybridization region at about 6 eV below the Fermi level. For low hydrogen content (x=0.15, 0.30), the films consist of a superposition of hexagonal closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type structure) phases, while for x=1.16, the film form single phase ZrHx that largely resembles that of stoichiometric delta-ZrH2 phase. We show that the cubic delta-ZrHx phase is metastable as thin film up to x=1.16 while for higher H-contents, the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge-transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as function of hydrogen content is discussed in relation to the charge-transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.
... The X-ray absorption near-edge structure (XANES, Figure 2a) of unamended Ti showed a characteristic pre-edge peak corresponding to a Ti 1s → 3d transition. 32 The peak was absent in TiH 2 , indicating its cubic (fcc) structure 33 and facilitating TiH 2 assignment in samples of various applied potentials and durations. Ti foil from 4 h of NO 3 RR at −0.4 and −0.6 V RHE showed a pre-edge peak similar to that of the Ti foil but diminished in intensity, while analogous samples from NO 3 RR at −0.8 and −1.0 V RHE showed no pre-edge peak, suggesting a near-surface structure transition from α-Ti (hcp) to TiH 2 (fcc). ...
The electrochemical nitrate reduction reaction (NO3RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between -0.4 and -1.0 VRHE was observed and analyzed with density functional theory (DFT) calculations on TiH2(111). This work underscores the importance of relating NO3RR performance with near-surface electrode structure to advance catalyst design and operation.
... Experimentally, oxidation states and local short-range order atomic coordination symmetry have been probed with the combination of x-ray absorption near-edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy [132] that are complementary tools to long-range order probed by x-ray diffraction (XRD). XANES provides; (i) a quantitative measure of the average oxidation state by the energy shift of the appropriate x-ray absorption edge; (ii) the amount of p-d hybridization in the chemical bonds, and thus the symmetry of the structure; and (iii) information about the coordination symmetry when the XANES line shape is compared to reference materials [133]. EXAFS gives quantitative information on average (i) bond lengths, (ii) coordination number (number of nearest-and nextnearest neighbors in different directions), and (iii) the mean-square disorder (Debye-Waller factor that also depends on the temperature and phonon vibrations). ...
We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... Experimentally, oxidation states and local short-range order atomic coordination symmetry have been probed with the combination of x-ray absorption near-edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy [132] that are complementary tools to long-range order probed by x-ray diffraction (XRD). XANES provides; (i) a quantitative measure of the average oxidation state by the energy shift of the appropriate x-ray absorption edge; (ii) the amount of p-d hybridization in the chemical bonds, and thus the symmetry of the structure; and (iii) information about the coordination symmetry when the XANES line shape is compared to reference materials [133]. EXAFS gives quantitative information on average (i) bond lengths, (ii) coordination number (number of nearest-and next-nearest neighbors in different directions), and (iii) the mean-square disorder (Debye-Waller factor that also depends on the temperature and phonon vibrations). ...
We review the thin film growth, chemistry, and physical properties of Group 4–6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... zirconium) atoms were not considered. These thresholds were chosen considering the HeO distance in the water molecule [45] (0.96 Å) and the ZreH distance in some Zr-H x complexes [46] (2.1 Å). Furthermore, the symmetries of monoclinic zirconia enable to lower the number of calculations. ...
During the corrosion in primary water of zirconium, hydrogen from the water diffuses through the oxide. To better understand this process, we use Density Functional Theory with hybrid functionals to calculate the energetics of interstitial hydrogen ions in defect-free monoclinic zirconia. While there is only one stable site for hydride ions in zirconia, protons have four different sites. We calculate the migration paths and energies between insertion sites to obtain the diffusion coefficients of hydrogen. We find that protons diffuse orders of magnitude faster than hydride ions, proving that protons are responsible for diffusion of hydrogen in monoclinic zirconia.
... XAFS spectroscopy includes X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), which help to characterize element-selective electronic structure and to the short-range atomic order [2][3][4]. XANES spectroscopic studies based on synchrotron radiation source have contributed immensely in understanding the morphology of materials at macroscopic and microscopic levels in diverse scientific fields [3,5]. The XANES covers � 40-50 eV of the spectral region above the absorption edge at which the distance between absorbing atom and its nearest neighbors equals the wavelength of the electron. ...
h i g h l i g h t s Hydrogens tends to stabilize at octahedral sites of B2eZrCu compound. As Hydrogen concentration increases, covalency of lattices improves, which enhances the stability. Crystalline symmetry and packing density are crucial to the hardness/embrittlement. Available online xxx Keywords: ZrCu-based alloys Hydrogen Covalency Electronic properties First-principles calculation a b s t r a c t The influence of hydrogen doping on phase stability, mechanical and electronic properties of B2eZrCu compound was systematically investigated by first-principles calculations. As hydrogen concentration increases, ZreH bonds and CueH bonds makes the metallicity of the system decrease and covalency increase as shown by density of states and electron density difference distribution. Higher covalent character decreases formation enthalpy, which corresponds to better stability. Higher bond covalency also improves hardness and brittleness. However, the lattice structural symmetry and crystallographic packing density play a crucial role in affecting ductility, which make Zr 8 Cu 8 H 2 À<110> display a softer and more ductile character than non-doped Zr 8 Cu 8. Otherwise, the hardness and embrittle-ment will increase monotonically with the increasement of interstitial hydrogen doping concentrations. The combination of covalency, crystalline symmetry and packing density of interstitial hydrogens play a crucial role in mechanical behavior; while the overall bonding character decides the stability.
In diesem Kapitel werden die Elemente der vierten Nebengruppe des Periodensystems der Elemente mit ihren wichtigsten Verbindungen beschrieben. Vor allem ist Titan Bestandteil vieler Gebrauchsgegenstände des täglichen Bedarfs und in technologischer Hinsicht für die Zukunft unverzichtbar, aber auch Zirconium und Hafnium gehen in viele Anwendungen. Es werden ihre chemischen und physikalischen Eigenschaften, ihr Vorkommen, bedeutsame Herstellverfahren, Anwendungen und Patente aufgeführt.
In diesem Kapitel werden die Elemente der vierten Nebengruppe des Periodensystems der Elemente mit ihren wichtigsten Verbindungen beschrieben. Vor allem ist Titan Bestandteil vieler Gebrauchsgegenstände des täglichen Bedarfs und in technologischer Hinsicht für die Zukunft unverzichtbar, aber auch Zirconium und Hafnium gehen in viele Anwendungen. Es werden ihre chemischen und physikalischen Eigenschaften, ihr Vorkommen, bedeutsame Herstellverfahren, Anwendungen und Patente aufgeführt.
Initial structures generated by artificial bee colony algorithm were re-optimized, and structural and electronic properties for (ZrH2)n (n=5-24) were calculated using density functional theory. Zr atoms of (ZrH2)n clusters, tend to occupy the edge positions, whereas the majority of H atoms dissociate to the vertex sites. The formation energies increase from (ZrH2)5 through (ZrH2)21 clusters, showing increased stability. Zr 4d, 5s and H 1s orbitals overlap over a wide range, showing stronger chemical bonding, which increases with increasing cluster size. Cluster (ZrH2)21, is the most stable cluster, with the least formation energy -7.278 eV, and stronger Zr-H chemical bonding. It is observed that ionic bonding exists between Zr-H atoms of (ZrH2)n clusters. By the analysis of Bader charge and charge density difference, we came across the fact that charges transfer from Zr to H atoms. Also, the bond strengths for the maximal and minimal of structures, (ZrH2)5 and (ZrH2)11 clusters, were analyzed.
The electronic structure and chemical bonding in reactively magnetron sputtered ZrH x (x = 0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level, and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H 1s-Zr 4d hybridization region at ∼6 eV below the Fermi level. For low hydrogen content (x = 0.15, 0.30), the films consist of a superposition of hexagonal closest-packed metal (α phase) and understoichiometric δ-ZrH x (CaF 2-type structure) phases, while for x = 1.16, the films form single-phase ZrH x that largely resembles that of stoichiometric δ-ZrH 2 phase. We show that the cubic δ-ZrH x phase is metastable as thin film up to x = 1.16, while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated ZrH 1.16 film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr 4p 3/2 and 3d 5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.
For the first time, various hydride phases in a zirconium–hydrogen system have been prepared in a high-energy synchrotron X-ray radiation beamline and their transformation behaviour has been studied in situ. First, the formation and dissolution of hydrides in commercially pure zirconium powder were monitored in real time during hydrogenation and dehydrogenation, then whole pattern crystal structure analysis such as Rietveld and Pawley refinements were performed. All commonly reported low-pressure phases presented in the Zr–H phase diagram are obtained from a single experimental arrangement.
Reactive sputtering by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS) of a Zr target in Ar/H2 plasmas was employed to deposit Zr-H films on Si(100) substrates, and with H content up to 61 at. % and O contents typically below 0.2 at. % as determined by elastic recoil detection analysis. X-ray photoelectron spectroscopy reveals a chemical shift of ∼0.7 eV to higher binding energies for the Zr-H films compared to pure Zr films, consistent with a charge transfer from Zr to H in a zirconium hydride. X-ray diffraction shows that the films are single-phase δ-ZrH2 (CaF2 type structure) at H content >∼55 at. % and pole figure measurements give a 111 preferred orientation for these films. Scanning electron microscopy cross-section images show a glasslike microstructure for the HiPIMS films, while the DCMS films are columnar. Nanoindentation yield hardness values of 5.5–7 GPa for the δ-ZrH2 films that is slightly harder than the ∼5 GPa determined for Zr films and with coefficients of friction in the range of 0.12–0.18 to compare with the range of 0.4–0.6 obtained for Zr films. Wear resistance testing show that phase-pure δ-ZrH2 films deposited by HiPIMS exhibit up to 50 times lower wear rate compared to those containing a secondary Zr phase. Four-point probe measurements give resistivity values in the range of ∼100–120 μΩ cm for the δ-ZrH2 films, which is slightly higher compared to Zr films with values in the range 70–80 μΩ cm.
Over the last three decades low-dimensional systems have attracted increasing interest both from the fundamental and technological points of view due to their unique physical and chemical properties. X-ray absorption spectroscopy (XAS) is a powerful tool for the characterization of such kinds of systems, owing to its chemical selectivity and high sensitivity in interatomic distance determination. Moreover, XAS does not require long-range ordering, that is usually absent in low-dimensional systems. Finally, this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (diffraction anomalous fine structure, extended energy-loss fine structure, pair distribution function, x-ray emission spectroscopy, high-energy resolution fluorescence detected XAS and x-ray Raman scattering). Subsequently, a selection of significant applications of XAS to two-, one-and zero-dimensional systems will be presented. The selected low-dimensional systems include IV and III-V semiconductor films, quantum wells, quantum wires and quantum dots; carbon-based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.
Dramatic advances in the understanding of x-ray absorption fine structure (XAFS) have been made over the past few decades, which have led ultimately to a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint. The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge, and, to a lesser extent, on x-ray absorption near-edge structure (XANES) closer to an edge. The discussion includes both formal considerations, derived from a many-electron formulation, and practical computational methods based on independent-electron models, with many-body effects lumped into various inelastic losses and energy shifts. The main conceptual issues in XAFS theory are identified and their relative importance is assessed; these include the convergence of the multiple-scattering expansion, curved-wave effects, the scattering potential, inelastic losses, self-energy shifts, and vibrations and structural disorder. The advantages and limitations of current computational approaches are addressed, with particular regard to quantitative experimental comparisons.
We propose two methods for obtaining the atomic-like background
for the x-ray absorption fine structure (XAFS): the methods of smoothing
spline and of Bayesian smoothing. Both are capable of using the prior
information, calculated or experimental, about the background. The XAFS
signals obtained by these techniques are shown to be significantly
corrected in comparison with standard methods. The method of Bayesian
smoothing is the only method that gives the errors of approximation of the
atomic-like background by an artificial smooth function. These errors are
shown to be the main source of the uncertainty of the XAFS function.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The aim of the current work is the synthesis and characterization of metallic Zr nanoparticles. The preparation is carried out by using the RAPET method (Reaction under Autogenic Pressure at Elevated Temperatures) developed in our lab. The RAPET reaction of commercial ZrO(2) with Mg powder was carried out in a closed stainless steel cell, at 750 °C. On completion of the reaction, the additionally formed MgO is removed by treatment with acid. The characterization of the product was performed by XRD, X-ray absorption spectroscopy, SEM, TEM and elemental analysis. The XRD pattern reveals that the product is composed of pure metallic zirconium, without any traces of the MgO by-product.
We propose an efficient, accurate method to integrate the basins of attraction of a smooth function defined on a general discrete grid and apply it to the Bader charge partitioning for the electron charge density. Starting with the evolution of trajectories in space following the gradient of charge density, we derive an expression for the fraction of space neighboring each grid point that flows to its neighbors. This serves as the basis to compute the fraction of each grid volume that belongs to a basin (Bader volume) and as a weight for the discrete integration of functions over the Bader volume. Compared with other grid-based algorithms, our approach is robust, more computationally efficient with linear computational effort, accurate, and has quadratic convergence. Moreover, it is straightforward to extend to nonuniform grids, such as from a mesh-refinement approach, and can be used to both identify basins of attraction of fixed points and integrate functions over the basins.
The characteristics of pre-edge peaks in K-edge x-ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d-electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre-edge peak intensity for Td symmetry is larger than those for Oh symmetry for all 3d elements. The intense pre-edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre-edge peak consists of 1s–3d electric quadrupole transition. The pre-edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d0 and gradually decreases to zero at d10. The features of pre-edge peaks in K-edge XANES spectra for 4d elements and the L1-edge for 5d elements are analogous with those for 3d elements, but the pre-edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.
A description of a new facility for X-ray absorption spectroscopy at the materials science beamline, I811, at MAX-lab synchrotron source, Lund, Sweden, is given. The beamline is based on a superconducting multipole wiggler source inserted in a straight section of the 1.5 GeV MAX-II ring. X-rays in the energy range 2.4–12 keV are extracted by a standard optical scheme consisting of a vertical collimating first mirror, double-crystal monochromator, and a second vertically focusing mirror. The second monochromator crystal provides sagittal focusing. The total flux impinging on the sample at 9 keV is 5 × 10¹¹ photons s⁻¹, for a minimum beam spot of 0.5 mm × 0.5 mm. The beamline has facilities for experiments in transmission, fluorescence and total-electron-yield mode and experiments have been performed by international research groups on a wide range of materials, such as dilute systems with metal concentrations below 10 p.p.m.
With contributions by Paul F. Fewster and Christoph Genzel. While X-ray diffraction investigation of powders and polycrystalline matter was at the forefront of materials science in the 1960s and 70s, high-tech applications at the beginning of the 21st century are driven by the materials science of thin films. Very much an interdisciplinary field, chemists, biochemists, materials scientists, physicists and engineers all have a common interest in thin films and their manifold uses and applications. Grain size, porosity, density, preferred orientation and other properties are important to know: whether thin films fulfill their intended function depends crucially on their structure and morphology once a chemical composition has been chosen. Although their backgrounds differ greatly, all the involved specialists a profound understanding of how structural properties may be determined in order to perform their respective tasks in search of new and modern materials, coatings and functions. The author undertakes this in-depth introduction to the field of thin film X-ray characterization in a clear and precise manner.
The formation of -type dislocation loops in α-Zr is believed to be responsible for the breakaway irradiation growth experimentally observed under high irradiation fluences. However, while -loop growth is well described by existing models, the atomic mechanisms responsible for the nucleation of -type dislocation loops are still not clear. In the present work, both interstitial and vacancy -type dislocation loops are initially equilibrated at different temperatures. Cascades simulations in the vicinity of the -type loops are then performed by adding primary knock-on atoms (PKAs) with different kinetic energies, using molecular dynamics simulations. No -type dislocation loop was formed in cascades simulations with a 10 keV PKA, but -type interstitial loops after the interaction between discontinuous 50 keV PKAs and preexisting -type interstitial loops. The comparisons of cascades simulations in volumes having pre-existing -type interstitial and vacancy loops suggest that the reaction between the PKAs and -type interstitial loops is responsible for the formation of -type interstitial loops.
The local structure and chemical bonding in two-phase amorphous Cr1−xCx nanocomposite thin films are investigated by Cr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies in comparison to theory. By utilizing the computationally efficient stochastic quenching (SQ) technique, we reveal the complexity of different Cr-sites in the transition metal carbides, highlighting the need for large scale averaging to obtain theoretical XANES and EXAFS spectra for comparison with measurements. As shown in this work, it is advantageous to use ab initio theory as an assessment to correctly model and fit experimental spectra and investigate the trends of bond lengths and coordination numbers in complex amorphous materials. With sufficient total carbon content (≥ 30 at%), we find that the short-range coordination in the amorphous carbide phase exhibit similarities to that of a Cr7C3±y structure, while excessive carbons assemble in the amorphous carbon phase.
Skeletal (nanoporous) gold catalysts were prepared from Au-Zr amorphous alloys by immersion in HF solution to extract the Zr moieties. The catalytic performance of the skeletal Au catalyst for the oxidation of benzyl alcohol to benzaldehyde was examined using O2 as an oxidant. The amount of benzaldehyde formed over skeletal Au treated with HF solution for 1 h was the highest among the prepared samples, despite the low surface area. Detailed characterization indicated that residual ZrO2 species on the skeletal Au promoted the oxidation reaction. The oxygen storage capacity (OSC) of skeletal Au was measured for the quantitative estimation of active sites and showed a positive correlation with the catalytic activity of skeletal Au, varifying the presence of Au-ZrO2 interfaces. Skeletal Au was also prepared from a Au-Zr crystalline alloy and the catalytic activity was compared with that prepared from the amorphous alloy. The atomic arrangement of the Au-Zr amorphous alloy had a strong effect on the catalytic activity due to the surface structure (coordination number and morphology) of the skeletal Au. The unsaturated Au atoms created in the skeletal Au catalysts contributed to the reducing activation energy for oxidation of benzyl alcohol.
Zirconium diboride (ZrB2) is an important ceramic due to its extremely high melting temperature of 3245 °C and metallic electrical conductivity, properties that make it an ideal candidate thin film electrode material for high temperature electronics. In this report, thin films of varying B:Zr ratio ranging from 3–0.67 have been grown by e-beam evaporation from elemental sources. X-ray absorption spectra at the Zr K-edge were measured before and after annealing in ultra-high vacuum for 9 h at 1000 °C. Films with compositions near ZrB2 stoichiometry show X-ray absorption fine structure that can be well modeled by crystalline ZrB2 with a small portion of a coexisting tetragonal zirconia (t-ZrO2) phase. Films far from stoichiometry show substantial disorder beyond the nearest-neighbor distances, and after vacuum annealing exhibit high levels of oxidation. Contributions to the X-ray absorption fine structure from a pure Zr phase are very small compared to ZrB2 and t-ZrO2 phases. The fact that nearly stoichiometric (3 < B:Zr < 1.6) as-deposited amorphous films form the same crystalline ZrB2 nanostructure after annealing is particularly encouraging for high temperature thin film electronics applications, because it would allow the production of highly stable electrodes with e-beam evaporation without the need of any high temperature heating during film growth.
This review provides comprehensive coverage of the application of X-ray absorption spectroscopy (XAS, XAFS, EXAFS, XANES) to matrix isolated species. As X-ray absorption spectroscopy provides structural data without the need for long range order it has been applied to a large number of systems to yield unique structural data about both the matrix isolated species, and their interactions with the matrix host. To put the work into perspective there is a tutorial introduction to the theoretical background of X-ray absorption spectroscopy, data content, processing and analysis. In addition there is brief coverage of the use of other synchrotron radiation techniques for the study of matrix isolated species, and a consideration of future perspectives.
The electronic structures of TiHx, ZrHx, and
HfHx have been studied using photoelectron spectroscopy and
synchrotron radiation. Structures in the metal d-derived band within ~3
eV of the Fermi level EF and in the bonding band (~3-10 eV
below EF) are compared with theory. In each dihydride, the
bonding band center falls at -5.5 eV, at approximately the same energy
as previously observed for the dihydrides of Sc and Y. Changes in the
emission features near EF and at -7 eV have been observed in
samples bridging the fcc-->fct distortion in ZrHx,
1.63<=x<=1.94. The changes at EF demonstrate the
Jahn-Teller effect for the electronic states of ZrHx. The
binding energies of the Ti 3p, Zr 4p, Hf 5p, and Hf 4f cores are
observed to be greater than in the elemental metals, consistent with
charge transfer to the hydrogen site.
While titanium has long been of interest for use in creating low resistivity metal contacts on silicon, the commonly resulting epitaxial silicide (TiSi2) is often of C49 phase and is, unfortunately, metastable - transforming to the stable C54 phase at higher temperatures. Zirconium, however, only exhibits a C49 phase disilicide and, being in the same periodic group, is chemically similar to Ti, affording the possibility of alloying small quantities of Zr with Ti in order to stabilize the epitaxial C49 structure. Both Ti and Zr have been reported to show a strongly disordered interface region at low temperatures, but little quantitative structural work has been performed on the Zr:Si system. To this end, an initial structural study of the Zr on Si (111) system has been undertaken. Thin films (100Å) of Zr were deposited in UHV conditions onto atomically clean Si(111) wafers and annealed in situ at fine temperature intervals between 300 and 425°C, over which range Auger spectroscopy indicated Si diffusion to the surface. A comparison will be made with the Ti:Si system for samples of 100Å Ti prepared under the same conditions.
First-principles calculations have been carried out to investigate the structural, mechanic and electronic of transition metal hydrides MH2 (M = Ti, Zr, Hf, Sc, Y, La, V and Cr). It is found that TiH2 is mechanically unstable because of a negative C44 = −21.31 GPa and C11–C12 < 0, the same behavior can be found in MH2 (M = Zr, Hf, and Y) compounds. Also there is a strong interaction between M (Ti, Zr, Hf, Sc, Y, La, V and Cr) and H. On the other hand, the H–H bond orders are always negative or nil reason of brittleness.
K-edge photoabsorption measurements of the 4 d transition metals (Y, Zr, Nb, Mo, Ru, Rh, Pd, and Ag) using synchrotron radiation from the 12-GeV electron synchrotron at the Wilson Synchrotron Laboratory at Cornell reveal considerable fine structure on or above (~ 150 eV) the edge. Systematics in the features closest to the absorption edge are attributed to structure in the "local-band-structure" density of states. This work points to the need for angular-momentum-resolved density-of-states calculations for energies far above the Fermi surface and for L-edge photoabsorption studies.
Structural, mechanical, electronic, and thermodynamic properties of fluorite and tetragonal phases of ZrH2 are systematically studied by employing the density functional theory within generalized gradient approximation. The existence of the bistable structure for ZrH2 is mainly due to the tetragonal distortions. And our calculated lattice constants for the stable face-centered tetragonal (fct) phase with c/a=0.885 are consistent well with experiments. Through calculating elastic constants, the mechanically unstable characters of face-centered cubic (fcc) phase and fct structure with c/a=1.111 are predicted. As for fct0.885 structure, our calculated elastic constants explicitly indicate that it is mechanically stable. Elastic moduli, Poisson’s ratio, and Debye temperature are derived from elastic constants. After analyzing total and partial densities of states and valence electron charge distribution, we conclude that the Zr–H bonds in ZrH2 exhibit weak covalent feature. But the ionic property is evident with about 1.5 electrons transferring from each Zr atom to H. Phonon spectrum results indicate that fct0.885 structure is dynamically stable, while the fcc and fct1.111 structures are unstable in accord with the mechanical stability analysis.
The thermophysical properties of zirconium hydride and deuteride such as thermal, electrical and electronic properties have been studied. The thermal conductivities of zirconium hydride and deuteride are slightly lower than that of zirconium metal, and are not strongly affected by the hydrogen content or temperature. An isotope effect in the thermal conductivity was observed, which was discussed based on the experimental values of the electrical resistivity. The electronic structure of zirconium hydride was also studied by molecular orbital calculations and by X-ray photoelectron spectroscopy (XPS) measurements.
There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multi-electron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye–Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes. To cite this article: J.J. Rehr et al., C. R. Physique 10 (2009).
The present work is devoted to investigate the local atomic environment (of Zr, Y and O) as well as surface modifications associated with excess helium in the cubic phase of (100)-oriented Zr0.8Y0.2O1.9 single crystal substrates. Commercially available oxide crystals have been implanted at various fluences in the range 0.15–2.0×1016 He-atoms/cm2 using a 2.74MeV He+ ion beam passing through a 8.0μm Al foil. The microstructure and surface morphology of the irradiated surface are examined using atomic force microscopy (AFM). The local atomic environments of Zr, Y and O in the implanted layer are studied using synchrotron radiation and by extended X-ray absorption fine structure (EXAFS) measured at glancing angles to probe the implanted layer. From AFM studies it was observed that the surface roughness increases as fluence increases and above a critical fluence stage, small blister-like structures originating from helium bubbles are scattered on the irradiated surface. The radial distribution functions (RDFs), derived from EXAFS data at the Zr K-edge, have been found to evolve continuously as a function of ion fluence describing the atomic scale structural modifications in YSZ by helium implantation. From the pristine data, long range order (beyond the first- and second-shell) is apparent in the RDF spectrum. It shows several nearest neighbour peaks at about 2.1, 3.6, 4.3 and 5.4Å. In the implanted specimens, all these peaks are greatly reduced in magnitude and their average positions are changed, typical of damaged material. A simple model taking into account only the existence of lattice vacancies has been used for the interpretation of measured EXAFS spectra.
Recent research has shown the possible presence of α-based hydride superstructures in the two-phase α-δ equilibrium region of H-Zr. This evidences the relevance of investigating the coherent phase diagram at the atomic scale, a task performed in this paper using a combination of cluster expansion (CE) and electronic structure methods. Our work points out the existence of various metastable ordered structures, some of them CE predicted and confirmed by the ab initio calculations, and shows the H-Zr system as conveniently described by CEs not exceeding fourth-neighbor interactions. Apart from second-order long-range interactions parallel to the c axis, these interactions stabilize these hydrides as stackings of “H biplanes” parallel to the basal plane.
Electrical properties of hydrides and deuterides of zirconium have been investigated between 1.1 and 410°K. The metallic nature of these materials is evident in the fact that for compositions approaching ZrH2, the hydride is a better conductor than is high-purity zirconium. Above ∼150°K the electrical resistivity exhibits an interesting upturn, which arises from scattering from the optical-model lattice vibrations. Excellent fits to the ideal-resistivity data are obtained with a simple additive combination of Grüneisen and Howarth-Sondheimer functions for the respective acoustical- and optical-mode scattering contributions. The corresponding acoustical- and optical-mode characteristic temperatures are in good accord with expectations based on earlier inelastic neutron scattering data. Moreover, the optical-mode characteristic temperature exhibits the expected hydride-deuteride isotope shift of √2. The observed Hall coefficients are large in magnitude (much greater than for pure Zr), and indicate majority hole conduction for the fcc δ phase and majority electron conduction for the face-centered tetragonal ε phase. The thermoelectric power also changes from positive to negative with increasing hydrogen concentration in the range ZrH1.5-ZrH2.
X-ray absorption fine structure (XAFS) spectroscopy is a powerful technique for the investigation of the local environment around selected atoms in condensed matter. XAFS under pressure is an important method for the synchrotron source. We design a cell for a high pressure XAFS experiment. Sintered boron carbide is used as the anvils of this high pressure cell in order to obtain a full XAFS spectrum free from diffraction peaks. In addition, a hydraulic pump was adopted to make in-suit pressure modulation. High quality XAFS spectra of ZrH2 under high pressure (up to 13 GPa) were obtained by this cell.
Self-consistent full-potential linearized augmented-plane-wave (LAPW) band-structure calculations are performed for the fluorite-type dihydrides ScH2, TiH2, VH2, YH2, ZrH2 and NbH2 as well as for the tetragonal low-temperature phases of TiH2 and ZrH2. For all compounds studied which show metallic behaviour, band structures, densities of states, electron densities and the total-energy minima with respect to the lattice parameters are computed. Additionally, the bulk moduli for the cubic phases are given. Good agreement with experimental data is found where they are available. The results obtained confirm previous interpretations of the tetragonal distortion of the cubic unit cell as a Jahn-Teller-type effect and show how the electron densities of the tetragonally distorted phases depend on the splitting of states near the Fermi level. The total-energy curves for TiH2 and ZrH2 as functions of the c/a ratios show two minima each, of which only the ones for c/a<1 have been observed experimentally.
Titanium dioxide (TiO2) films have been deposited on Si substrates using reactive magnetron sputtering. The resulting films, having a polycrystalline anatase phase with a dense columnar structure, were analysed by time-of-flight elastic recoil detection analysis (ToF-ERDA) using 40 MeV I9+ ions. A clear decrease in the areal atomic density (atoms/cm2) of Ti and O was observed during measurement, but the stoichiometry remained essentially constant up to a fluence of 4 × 1013 ions/cm2.To investigate this effect in more detail, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were applied in order to characterize the films prior to and after ion irradiation with fluences in the range of 1010–1013 ions/cm2. Distinct morphological and structural changes of the polycrystalline film were observed. XRD revealed that the crystallinity of the film was gradually destroyed, and the film became amorphous at a fluence above 5 × 1012 ions/cm2. SEM and AFM measurements revealed topographical changes in the form of surface recession and smoothing compared to the pristine polycrystalline surface. The observed change in areal atomic density during ERD measurement is believed to be due to the combined effects of electronic sputtering, amorphization and ion hammering.
We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.
The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated bysoft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of thenanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of theelectronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend onthe TiC carbide grain size. An increased charge transfer between the Ti 3d-eg states and the C 2p states hasbeen identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It issuggested that the charge transfer occurs at the interface between the nanocrystalline-TiC and the amorphous-Cmatrix and represents an interface bonding which may be essential for the understanding of the properties ofnc-TiC/amorphous C and similar nanocomposites.
Zirconium alloys are currently used in nuclear power plants where they are susceptible to hydrogen pick-up. Hydride precipitation may occur when the hydrogen solubility limit is reached. Various Zr hydride phases, gamma, delta and epsilon have been identified since the 1950s. Combining electron precession microdiffraction, electron energy loss spectroscopy and ab initio electronic calculations, a new Zr hydride named zeta has been identified and characterized. It belongs to the trigonal crystal system with space group P3 m1 and it is fully coherent with the alphaZr matrix.
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- W M Mueller
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- Eds
- C Dai
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