Journal of Physics and Chemistry of Solids

Published by Elsevier
Online ISSN: 0022-3697
Publications
Article
Microwave assisted synthesis of iron oxide/oxyhydroxide nanophases was conducted using iron(III) chloride titrated with sodium hydroxide at seven different temperatures from 100 degrees C to 250 degrees C with pulsed microwaves. From the XRD results, it was determined that there were two different phases synthesized during the reactions which were temperature dependent. At the lower temperatures, 100 degrees C and 125 degrees C, it was determined that an iron oxyhydroxide chloride was synthesized. Whereas, at higher temperatures, at 150 degrees C and above, iron(III) oxide was synthesized. From the XRD, we also determined the FWHM and the average size of the nanoparticles using the Scherrer equation. The average size of the nanoparticles synthesized using the experimental conditions were 17, 21, 12, 22, 26, 33, 28 nm, respectively for the reactions from 100 degrees C to 250 degrees C. The particles also had low anisotropy indicating spherical nanoparticles, which was later confirmed using TEM. Finally, XAS studies show that the iron present in the nanophase was present as iron(III) coordinated to six oxygen atoms in the first coordination shell. The higher coordination shells also conform very closely to the ideal or bulk crystal structures.
 
Article
High-resolution X-ray measurements near a nuclear resonance reveal the complete vibrational spectrum of the probe nucleus. Because of this, nuclear resonance vibrational spectroscopy (NRVS) is a uniquely quantitative probe of the vibrational dynamics of reactive iron sites in proteins and other complex molecules. Our measurements of vibrational fundamentals have revealed both frequencies and amplitudes of (57)Fe vibrations in proteins and model compounds. Information on the direction of Fe motion has also been obtained from measurements on oriented single crystals, and provides an essential test of normal mode predictions. Here, we report the observation of weaker two-quantum vibrational excitations (overtones and combinations) for compounds that mimic the active site of heme proteins. The predicted intensities depend strongly on the direction of Fe motion. We compare the observed features with predictions based on the observed fundamentals, using information on the direction of Fe motion obtained either from DFT predictions or from single crystal measurements. Two-quantum excitations may become a useful tool to identify the directions of the Fe oscillations when single crystals are not available.
 
Article
The reaction of N-[3-(trimethoxysilyl)propyl]ethylenediamine with fluorinated carbon nanotubes (F-CNT) produced the corresponding aminoalkylalkoxysilane functionalized carbon nanotubes. Cobalt salt is then complexed to this functionalized carbon nanotubes by the addition of cobalt chloride to form cobalt complexed nanocomposite in high yield. This composite and precursors were characterized by using Fourier transform infra-red spectroscopy (FTIR), transmission electron microscopy (TEM), energy dispersive spectral (EDS) and thermal gravimetric analysis (TGA). The nanoparticulate metal complexes of functionalized carbon nanotubes generate new nanostructure with several practical applications. The reaction of N-[3-(trimethoxysilyl)propyl]ethylenediamine with cobalt (II) chloride salt afford the cobalt complex nanoparticles, with particle sizes less than 10 nm.
 
Article
Drug delivery to the skin is limited by the strong barrier properties of skin's outer layer of stratum corneum. Micron-scale needles have been developed to deliver drugs across this barrier layer and into the skin in a minimally invasive manner. One method of delivery involves coating these microneedles with a drug that rapidly dissolves off within the skin. As a variation on this approach, this study examines microneedles with holes cut through their shafts to form "pockets" that can be filled with drug formulations using a dip-coating method. Our results (i) demonstrated the filling of microneedle pockets having a variety of different sizes and shapes, (ii) quantified the amount of drug that can be filled into pockets and coated onto microneedle surfaces, (iii) developed composite microneedle structures that sequester one model drug within the microneedle pocket and coat another model drug on the microneedle surface and (iv) showed that pocketed microneedles can deliver a model drug to a targeted depth within the skin. We conclude that pocketed microneedles offer unique capabilities for controlled drug delivery to the skin.
 
Article
The skin's outer layer of stratum corneum, which is a thin tissue containing multilamellar lipid bilayers, is the main barrier to drug delivery to the skin. To increase skin permeability, our previous work has shown large enhancement of transdermal permeation using a pore-forming peptide, magainin, which was formulated with N-lauroyl sarcosine (NLS) in 50% ethanol-in-PBS. Mechanistic analysis suggested that magainin and NLS can increase skin permeability by disrupting stratum corneum lipid structure. In this study, our goal was to improve conditions that increase skin permeability by magainin by further optimizing the pretreatment time and concentration of magainin exposure. We found that skin permeability increased with increasing pretreatment time. Skin permeability also increased with increasing magainin concentration up to 1 mM, but was reduced at a magainin concentration of 2 mM. Enhancement of skin permeability to fluorescein (323 Da) up to 35-fold was observed. In contrast, this formulation did not enhance skin permeability to larger molecules, such as calcein (623 Da) and dextran (3,000 Da).
 
Article
Although topical drug delivery is a convenient route of administration to treat various eye diseases, it has serious limitations due to rapid clearance of the formulation from the surface of the eye. In this study, we engineered microparticles for both sustained drug delivery and prolonged residence time on the extraocular surface. Microparticles were fabricated by emulsification using poly(lactic-co-glycolic acid) (PLG) and poly(ethylene glycol) (PEG) as the core material and mucoadhesion promoter, respectively. The particle size was controlled to be less than 10 microm to avoid eye irritation and for eventual clearance through the lacrimal canals. In vitro mucoadhesion tests showed that PLG microparticles with PEG adhered better to the mucous membrane under the conditions employed in this study compared to the microparticles without PEG. When an aqueous suspension of microparticles with PEG was administered topically to the rabbit eye in vivo, microparticles were seen for up to 30 min on the ocular surface in the cul-de-sac, which was a dramatic increase in residence time as compared to conventional eye drop formulations. We conclude that mucoadhesive microparticles are promising vehicles for ophthalmic drug delivery.
 
Article
This letter is concerned with the analysis of high pressure experimental data. It is demonstrated that ln H plots based on the Vinet et al. (1988) universal equation of state are a simple sensitive means for identifying anomalous P-V data in high pressure experiments and for detecting structural and phase transitions in solids subjected to high pressure.
 
Article
As most previous work on non-stoichiometric magnetite Fe3−xO4 has been carried out with fine powders prepared in the metastable domain, with the inherent uncertainties, we have prepared small crystals by quenching from the high temperature stability range.With increasing x, the cell parameter decreases, the Verwey temperature falls below 4.2 K for x = 0.09, the thermal stability decreases (disproportionation occurs at ~550K for x = 0.09), the saturation magnetization at 125 K varies as (4.1 – 2x) suggesting that the vacancies are mainly octahedral; this is confirmed by the Mössbauer effect at 150 K and 5 Teslas. At 4.2 K with 8 T, the Mössbauer spectrum is very complex but displays a mixed-valence signal involving a fraction (~ 10%) of the octahedral sites. This signal is evidence for electron tunnelling. The conductivity, at 295 K, decreases more rapidly than (1 − 3x); the conductivity of stoichiometric magnetite is described by the dynamic correlation model applied to short-range ordering; for x = 0.09 an activation energy of 0.077 eV is deduced between 39 and 120K.These results are interpreted as evidence that the tunnel effect at low temperature is limited to domains separated by vacancies, associated with 5 times more electron holes, and by regions where short-range order prevails.
 
Article
Charge distributions of lithium cobalt nitrides Li2.6Co0.4N and Li1.0Co0.4N were investigated using core-level electron energy loss spectroscopy. The N-K spectra of Li2.6Co0.4N and reference sample Li3N showed that both lithium and cobalt are monovalent in Li2.6Co0.4N. The N-K spectrum of electrochemically lithium-extracted Li1.0Co0.4N indicated that a large amount of holes are introduced into nitrogen 2p orbitals by the lithium extraction and that Li1.0Co0.4N reveals a strongly covalent character. The branching ratio of the Co-L spectrum of Li1.0Co0.4N was almost intermediate between those of typical oxides having high- and low-spin ground states. This is attributed to the mixing of several electron configurations due to the covalency between cobalt and nitrogen.
 
Article
A series of nanocrystalline rare earth doped alkaline earth sulphates, MSO4:xEu3+ (M=Ca,Sr,Ba; x=0.001–0.005), have been prepared by co-precipitation method. Their crystal structures and luminescent properties with and without γ-ray irradiation are compared. It was found that their crystal structures have not changed, but their lattice parameters with irradiation become smaller than those without irradiation. On the other hand, the characteristic emissions of both Eu3+ and Eu2+ are observed in the same nanocrystals with γ-irradiation. The differences in their crystal structures and luminescent properties are also discussed.
 
Article
The redox reaction of Ce4+–Ce3+ promoted by the catalytic function of nickel ions in a (1−x)CeO2–xNiO solid solution was investigated for solar H2 production by the two-step water-splitting reaction. By irradiation using an infrared imaging lamp as a solar simulator, the O2-releasing reaction with (1−x)CeO2–xNiO solid solution proceeded at 1673–1873 K, and its reduced form was produced. The amounts of H2 gas evolved by the reduced form were 1.2–2.5 cm3/g and the evolved gases amounts ratio of H2/O2 was nearly 2, which is equal to the stoichiometric value of the water-splitting reaction (H2O=H2+1/2O2). The maximum amounts of evolved H2 and O2 gases were obtained at the Ce:Ni mole ratio of 0.95:0.05 (x=0.05) in the (1−x)CeO2–xNiO system. The X-ray absorption fine structure (XAFS) measurement showed that the O2-releasing and H2-generation reactions with (1−x)CeO2–xNiO solid solution were repeatable with the redox system of Ce4+–Ce3+, which was enhanced by the catalytic function of Ni2+–Ni0.
 
Article
We review recent studies of the static and dynamic magnetic fluctuations in La2 − xSrxCuO4. In La2CuO4(TN = 325K) the instantaneous two-dimensional (2D) spin correlations have been studied over the temperature range 340K < T ≤ 820K. We find quantitative agreement for the correlation length over the complete temperature range with no adjustable parameters with predictions for the 2D quantum non-linear sigma model in the renormalized classical regime. We have carried out bulk magnetization measurements in La1.96Sr0.04CuO4 which has neither antiferromagnetic long range order nor superconductivity. All of the features that characterize a canonical spin glass transition have been seen: irreversibility, remnant magnetization and scaling behavior. Finally, neutron inelastic scattering experiments have been performed on homogeneous single crystals of La1.85Sr0.15CuO4 with Tc = 37.3K (at onset), higher than any previously studied single crystals. The temperature dependence of the low energy incommensurate peak intensity at (π(1 ± δ), π) and (π, π(1 ± δ)) exhibits a pronounced maximum near 3Tc. In contrast to the results reported on lower Tc crystals, the intensity for energies below 3.5 meV dramatically decreases as the temperature decreases below Tc, vanishing into the background below ~ 15K. The behavior is consistent with predictions based on a dx2 − y2 superconducting order parameter.
 
Article
The elastic, piezoelectric and dielectric constants of [011] poled Y cut length extension bars made of 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 (PZN–4.5%PT) and 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN–8%PT) single crystals were fabricated and measured. The absolute value of piezoelectric coefficient of PZN–8%PT crystal poled along [011] reaches as high as 1887 pC/N, which is almost twice as its value. The electromechanical coupling coefficient of PZN–4.5%PT and PZN–8%PT crystals poled along [011] are 0.73 and 0.86, respectively, showing great application potential for transverse mode sensors, actuators and other electromechanical devices.
 
Article
Bi2(Te1−xSex)3 single crystal solid solutions with x = 0.025 and x = 0.05 have been grown using the travelling heater method. The first step of this work was the study of the BiTeSe ternary phase diagram on the Bi2Te3-rich side and with excess tellurium. The isoconcentration lines for these two compositions have been determined for temperatures ranging from 550 °C to their melting points. The knowledge of this phase diagram allows us to grow homogeneous ingots of high crystalline quality. The solidus line on the Te-rich side for these compositions has been determined by Hall effect measurements. This study shows that it is possible to grow thermodynamically well-defined single crystals in a reproducible way.
 
Article
Polycrystalline iron-based superconducting NdFeAsO0.88F0.12 was prepared via solid-state reaction in ambient pressure. Small amounts of ferromagnetic impurities were detected by Scanning Electron Microscopy (SEM) analysis. To study the influence of magnetism on superconducting properties of the sample, magnetization M(T, H) measurements were performed in fields up to 1.5 T and in the temperature range of 5–60 K. The abnormal behavior was observed in the χ(T) curves, and the magnetization hysteresis loops (MHLs) can be described by a sum of the contributions of superconductive hysteresis, ferromagnetic hysteresis of the impurities and the paramagnetic background of the isolated Nd3+ ions. The influence of the ferromagnetic impurities and the paramagnetic background were investigated and discussed. After the corrections of the magnetism contributions, the intrinsic superconductive MHLs as well as the critical current density were obtained.
 
Article
In this contribution we present the analysis of the third harmonic susceptibility data of the new superconductor NdFeAs1−0.14F0.14. ‘Cole–Cole’ polar plots respect to the magnetic frequency of the exciting field are presented and discussed. Data show that NdFeAs1−0.14F0.14 exhibit a ‘bulk pinning’ with a 3D flux dynamic character. A comparison of the responses of high Tc materials and an evaluation of theoretical critical states is also presented.
 
Schematic structural model for Na 2-X K X Ti 3 O 7 with the emphasis that potassium atoms are accommodated with sodium atoms in the widely opened interlayer space.  
Loss tangent (tand) versus frequency plots at different temperatures for (a) SPT-1, (b) SPT-2, (c) SPT-3.
Relative permittivity (3 r ) versus temperature plots at different frequencies for (a) SPT-1, (b) SPT-2, (c) SPT-3.
Article
The dependence of loss tangent (tanδ) and relative permittivity (εr) on temperature and frequency has been reported for Na2-XKXTi3O7 (with X=0.2, 0.3, 0.4) ceramics. The losses are characteristic of dipole mechanism and electrical conduction. The peaks of εr at high temperature indicate a possible ferroelectric phase transition for all three compositions. The results of a.c. conductivity studies on the same samples have also been reported. The corresponding ln(σT) versus 1000/T plots have been divided into five regions namely I, II, III, IV and V. The various conduction mechanisms in the different regions have been stressed. Furthermore, the log(σ) versus frequency plots for all the above samples reveal that the electronic hopping (polaron) conduction, which diminishes with the rise in temperature, is dominant in the lower temperature region. The interlayer ionic conduction seems to play a major role in conduction towards higher temperature.
 
Article
Electrical conductivity (σ) measurements have been made on powders and hexagonal single crystals of AgI at pressures to and at temperatures to 410 K. Single crystal σ values indicate intrinsic ionic conduction at temperatures greater than 400 K. The enthalpy of activation (ΔH∗) in this region is about 0.9 eV, independent of crystallographic orientation. At lower temperatures, log(σT) is a nonlinear function of T−1.At temperatures between 300 and 400 K, σ increases with pressure in all orientations. This agrees with macroscopic continuum theory that predicts a positive pressure coefficient of diffusivity (negative activation volume ΔV∗) for a material in which point defect formation and migration are related to a negative thermal expansion coefficient. A negative Δ V∗ (−3.1 to − 5.8 cm3/mol at atmospheric pressure) in the supposed extrinsic region means that conduction in this region is caused by either more than one type of charge carrier, or involves nonequilibrium or complexed defects.An irreversible increase in σ in the a-direction and a decrease in the c-direction is observed at pressures in excess of 0.1 GPa at temperatures below 390 K. This is suggestive of a fundamental change in the crystal lattice. However, the crystals are visibly unchanged after pressure cycling and σ gradually changes to equilibrium values with increasing temperature and time. These facts are not consistent with a reconstructive phase transformation and their directional dependence suggests one-dimensional stacking disorder.Between 300 and 400 K we obtained a Grüneisen γ of −0.9 to −1.1 calculated from continuum theory for activated processes. In general these values are in agreement with values of −0.4 to −0.5 using independent physical property measurements and Grüneisen's equation of state. This consistency suggests that at higher temperatures (t > θd) Frenkel defect formation plays a more important role than vibration frequencies in determining the negative thermal expansion coefficient.
 
Article
Brillouin spectroscopy was used to study the phase transitions of LiK0.80(NH4)0.20SO4 mixed crystals in the temperature range 10–300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ16 and the quasi-transverse γ17 mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO4 and LiK0.96(NH4)0.04SO4 crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.
 
Article
The heat capacity of polycrystalline Fe[S2CN(C2H5)2]2Cl has been measured in the temperature range from 0.411 to 19.55 K. The transition between ferromagnetic and paramagnetic states is characterized by a sharp λ-type anomaly centered at the Curie temperature, Tc = 2.412 ± 0.008 K. The enthalpy and the entropy of transition are 40.475 J mol−1 and 11.199 J K−1mol−1 (= 1.347 R), respectively. The transition entropy is only 2.81% smaller than R In 4. This fact ascertains that the spin manifold is really a quartet. The ground spin states of the present compound are characterized by a zero-field splitting of the state into two Kramers' doublets. The overlapping non-cooperative Schottky-type anomaly due to the energy separation between the two Kramers' doublets is separated tentatively from the cooperative heat capacity due to the exchange interaction. Based on a careful analysis of the transition entropy it may be concluded that the magnetic structure of the present complex would be a two-dimensional triangular Ising lattice. The exchange and the Curie-Weiss constants are also determined to be 0.155 and 4.19 K, respectively.
 
Article
Aqueous electrochemical insertion of M+ (Na+ and H+) species into WO3(4,4′-bipyridyl)0.5 has been carried out. The chemical states and structure of the resulting product were analysed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS showed the presence of W6+ as well as the usual reduced W species (W5+) which is responsible for a change in colour. Moreover, the presence of these intercalates correlates with the evolution of the reduced W species. The bulk structure of the layered hybrid, as determined by powder X-ray diffraction, showed no alteration after electrochemistry, in contrast to the same measurements on tungsten trioxide (WO3). This however concurs with single-crystal X-ray studies, which show little change in lattice parameters with Na+ insertion. Four-probe resistance measurements of the layered hybrid coated film display a drop in resistance after electrochemistry, which can be attributed to the injection of charge-carriers into the conduction band.
 
Article
The electrical properties and defect structure of selected compositions in the SrTi1−xFexO3−y system (x = 0–0.8) have been studied using van der Pauw 4-point conductivity measurements and electron energy loss spectroscopy (EELS). Using X-ray powder diffraction and selected area electron diffraction (SAD), the basic crystal structure was determined to be cubic perovskite for all the investigated compositions. A superstructure cell 2·2·1 times the ordinary perovskite cell was found in materials with x = 0.6 and x = 0.8. The conductivity was measured on sintered tablets as a function of the partial pressure of oxygen (pO2 = 10−25 to 1 atm) at 600–1100°C. The materials investigated are predominantly p-type electronic conductors at high, n-type conductors at low, and ionic conductors at intermediate oxygen partial pressures. All conductivity contributions increase with increasing iron content. This can be attributed to the acceptor role of the iron, decreased band gap and decreased activation energy for oxygen vacancy migration. The EELS spectra show a shoulder below the oxygen K-edge, increasing in magnitude with increasing iron content and oxygen partial pressure. This shoulder is assigned to empty electron energy states at some of the oxygen atoms, indicating that electron holes are associated with lattice oxygen in the structure. Spectra from the Fe L-edge showed small changes, suggesting that there are few or no empty states at the iron atoms. © 1997 Elsevier Science Ltd
 
Article
(Na0.5Bix)0.93Ba0.07TiO3 (x=0.500–0.492) ceramics were prepared by a citrate method, and the structure and electrical properties of the ceramics were investigated with respect to the amount of Bi deficiency. It was detected that the Bi deficiency had a considerable impact on the crystal structure and microstructure. The inspection of both the temperature dependence of the dielectric properties (free permittivity ε33T/ε0 and dielectric loss tan δ) and the evolution of the polarization–electrical field (P–E) hysteresis loops with measuring temperature suggests that the Bi deficiency served to increase the depolarization temperature (Td). The Bi deficiency led to an increase in the coercive field (Ec) and mechanical quality factor (Qm) together with a decrease in the remanent polarization (Pr) and piezoelectric constants (d33). The variation of the structure and electrical properties with Bi deficiency amount was qualitatively interpreted in terms of the formation of Bi and oxygen vacancies in the Bi-deficient specimens. This research indicates the importance of adequately controlling Bi stoichiometry of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics in obtaining the desired ferroelectric and piezoelectric properties.
 
Article
In this article, we summarize our studies on the spatial element distribution in (Ga,In)(N,As) quantum wells and epilayers grown on GaAs(0 0 1) substrates by molecular beam epitaxy. Nanometer-sized composition fluctuations are detected in (Ga,In)(N,As) layers with In and N concentration above 20% and 2%, respectively, by dark-field transmission electron microscopy and spatially resolved electron energy-loss spectroscopy. The fluctuations and clustering are inherently present in these quaternary alloys due to the phase separation tendency. Morphological instabilities, such as the surface roughening due to the elastic strain relief (i.e., a 2D- to 3D-growth mode transition), are succeeding processes. The origin of the fluctuations is discussed with respect to the selected growth conditions and the post-growth annealing procedure.
 
Article
We discuss first-principles angle-resolved photoemission (ARPES) intensity calculations for the (001)-surface of YBa2Cu3O7, with focus on spectra for k∥-values along the ΓS symmetry direction. The theoretical spectra show the sensitivity of the spectral shape to the specific surface termination used in the computations. The experimental spectra however appear to be well-described by assuming the first layer to be BaO followed by the CuO2 plane layer. Effects of the direction of polarization vector of the incident light on the spectra are examined; in particular, we show that the predicted behavior of the spectral feature arising from the CuO2 plane bands is in remarkable accord with experiments. This result gives confidence in the reasonableness of the LDA wave functions implicit in the computations.
 
Article
The molar heat capacity of [CrI2(depe)2], where depe = trans-bis[1,2-bis(diethylphosphino) ethane], has been measured with an adiabatic calorimeter in the 14–300 K range, and infrared spectra have been recorded at 300 and 91 K in the 4000−ndash;30 cm−1 range. A sharp heat capacity anomaly arising from the spin-state transition from the low-spin 3T1 to the high-spin 5E electronic ground state was found at 171.45 K. The enthalpy and entropy of the phase transition are ΔtrsH = 6686 ± 33 Jmol−1 and ΔtrsS = 39.45 ± 0.22 JK−1mol−1, respectively. The entropy gain can be well accounted for in terms of the contributions from the change in the spin manifold (10%), the change in the metal-ligand skeletal vibrations (~ 75%) and the change in the barrier heights hindering the internal rotation of the eight methyl moieties of the ethyl groups attached to the phosphine ligands (~ 15%). The extremely sharp phase transition observed for this complex has been explained in terms of the large polarizability and thus large compressibility of the iodide ligands. The “cooperativity” often used for spin-crossover phenomena is discussed.
 
Article
Three nickel complexes with a new multi-sulfur 1,2-dithiolene ligand, (n-Bu4N)[Ni(cddt)2] 1, (Ph4P)[Ni(cddt)2] 2 and [Ni(cddt)2] 3 (cddt=4a, 6, 7, 7a-5H-cyclopenta[b]-1,4-dithiin-2,3-dithiolate), have been synthesized and characterized by electrochemical measurements, IR, EPR and UV–Vis-NIR spectroscopies. The crystal structure of complex 2 is determined. Their optical nonlinearities are measured by the Z-scan technique with an 8 ns pulsed laser at 532 nm and all exhibit NLO absorptive abilities. Complexes 1 and 2 both exhibit effective self-defocusing performance (n2=−5.81×10−10 esu for 1 and −4.51×10−10 esu for 2). The optical limiting (OL) effects were observed with nanosecond and picosecond laser pulses. The OL capability of complex 3 is superior to C60 at the same experimental condition in ns measurements.
 
Article
Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell (a=0.5448 nm, b=1.2758 nm, c=1.9720 nm, density 1.513 g cm−3) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P21/c with cell parameters a=1.990 nm, b=0.500 nm, c=1.240 nm, β=91.7°, density 1.681 g cm−3 (phase II, determined at 1.1 GPa) and a=1.890 nm, b=0.510 nm, c=1.242 nm, β=89.0°, density 1.733 g cm−3 (phase III, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a=0.465 nm, b=1.920 nm, c=1.230 nm and density 1.857 g cm−3. For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa−1 has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II–IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing.
 
Article
We present the spectroscopic identification of the binary hydrogen and 1,4-dioxane clathrate hydrates in which H2 molecules are encapsulated in small cages of the structure II hydrate framework. X-ray diffraction, solid-state NMR and Raman spectroscopy were used to confirm the mixed hydrate structure and more importantly the occupancy pattern of hydrogen in clathrate cages. The corresponding pressure–temperature behavior and direct gas release measurements were also determined to support the spectroscopic results. The direct volumetric measurements revealed that the favorable confinement of hydrogen molecules over 0.4 wt% could be obtained by the compositional tuning of the binary products.
 
Article
The molar heat capacity of the thermochromic compound [Cu(daco)2](NO3)2, where daco = 1,5-diazacyclooctane, has been measured with an adiabatic calorimeter in the range 14–400 K, and i.r. spectra have been recorded in the range 4000-30cm−1 at 108, 299 and 373 K. A large heat capacity anomaly associated with a thermochromic phenomenon was found at TC = 359 K. This anomaly exhibits both superheating and undercooling effects, indicating a first-order phase transition. The color of the crystal is brilliant orange in the low temperature phase, and turns violet above TC. The enthalpy and entropy of the phase transition were ΔH = 8400 ± 220 J mol−1 and ΔS = 23.37 ±0.83 J K−1 mol−1, respectively. The entropy gain is well accounted for in terms of conformational disordering of the daco ligand, Rln 16 (equal to 23.05 J K−1 mol−1), where R is the gas constant. The number of the microscopic states (16) is counted as follows. Each eight-membered daco ring statically coordinates to the central copper ion with a fixed chair-boat conformation in the low temperature phase. However, it may be assumed that dynamic interconversion between the chair and the boat form might take place in the high temperature phase and thus each daco ring will gain four conformations (i.e. chair-chair, chair-boat, boat-chair and boat-boat). Since two daco ligands are involved in the complex, the total configurations per [Cu(daco)2]2+ cation amount to 16 (4 × 4). The change in the ligand-field strength leading to thermochromism is most probably caused by a change in the thermal motion of the dago ligand, from being static to dynamically interconverting between different conformations. An improved method of sample preparation has been achieved, by which the yield is increased to ~90%.
 
Article
We report on the blue-green-red up-conversion spectroscopic properties of Pr3+/Yb3+-codoped oxyhalide tellurite glasses upon excitation of a conventional 980 nm laser diode (LD). Significant enhancement of the blue-green-red up-conversion emission intensity has been observed with increasing PbCl2 doping. The up-conversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. The population of the Pr3+ upper 3P0 emitting level was accomplished through a combination of a ground state absorption, energy transfer and excitated state absorption. 1.3-μm emission in the second telecom window originated from Pr3+:1G4→3H5 transition has also been investigated upon excitation at 980 nm LD. The measured peak wavelength and full width at half-maximum of the fluorescent are 1335 nm and ∼100 nm, respectively. An enhanced 1.3μm emission with increasing PbCl2 doping has also been observed. Codoping of Yb3+ significantly enhance both the blue-green-red up-conversion emission and 1.3-μm emission intensity by way of a nonradiative Yb3+:2F5→Pr3+:1G4 energy transfer.
 
Article
In this work, we have performed synthesis of nitrogen-doped carbon nanotubes using chemical vapor deposition method. Morphology, structure and composition of the carbon nanotubes (CNTs), as well as concentration and distribution of nitrogen inside CNTs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray dispersive spectroscopy and X-ray photoelectron spectroscopy techniques. A bamboo-like structure of the nitrogen-doped CNTs has been observed. Temperature dependency on the synthesis of nitrogen-doped carbon nanotubes has been investigated and discussed. Diameter and growth rate of these hybrid materials are obviously temperature dependent. Nitrogen concentration inside the CNTs increases with declining synthesis temperature. Nitrogen-doped CNTs with nitrogen content up to 10.4 at% can be achieved at a low temperature of 800 oC. Synthesis of the high nitrogen CNTs proposes a feasible way to develop novel nanoenergetic materials. Besides the experimental study, we have carried out Density Functional Theory calculations on five energetic molecules named n-oxides of 3,3′-azo-bis(6-amino-1,2,4,5-tetrazine) (DAATO), where n=1–5 refer to oxygen atoms, encapsulated in CNTs (10,10), in order to investigate the chemical stabilization of filled DAATO_n inside CNTs (10,10). In fact, the predicted adsorption energy values confirmed the chemical stability of the hybrid systems DAATO_n@CNTs (10,10) under normal conditions.
 
Article
The diffuse intensity in the h1, h2, 0 plane of reciprocal space was measured for Ni-rich alloys containing 20.0 and 10.7 at. % Mo after varied thermal history. A very unusual distribution of diffuse intensity indicating considerable short-range order was observed for samples of both alloys which had been quenched from 1000°C.After correcting for atomic size effects, two-dimensional short-range order parameters defined in terms of the usual three-dimensional parameters by were determined. Comparison of these observed Alm with values Alm calculated from models of the short-range structures of the alloys led to the conclusion that small regions existed within which the atomic arrangement was similar to that in long-range ordered Ni4Mo.Reheating the Ni-10.7 at. % Mo alloy near 479°C after quenching from 1000°C resulted in a sharpening of the diffuse intensity distribution corresponding to an increase in the degree of order. Short heat treatments of the Ni-20.0 at. % Mo alloy near 650°C (below the critical temperature, 868°C) caused the short-range order peaks observed in samples quenched from above the critical temperature to split into peaks located near the positions of the superlattice reflections for long-range ordered Ni4Mo. This is interpreted to mean that reasonably well defined domains of order exist at this stage of heat treatment. The critical size of the domains necessary to cause the observed splitting was estimated to be of the order of 10 a0.
 
Article
The heat capacity from 5 to 350 K of In2O3 has been measured by adiabatic calorimetry. For the thermophysical properties at room temperature C(p)-degrees(298.15 K) = (99.08 +/- 0.09) J mol-1 K-1 and S-degrees(298.15 K) = (101.80 +/- 0.06)J mol-1 K-1 have been derived. Enthalpy increments relative to 298.15 K have been measured by drop calorimetry from 502 to 959.2 K: [H-degrees(T) - H-degrees(298.15 K)}/J mol-1 = 109.1383(T/K) + 13.73786 x 10(-3)(T/K)2 + 16.22947 x 10(5)(T /K)-1 - 39,204.2. The thermodynamic functions, including the formation properties DELTA(f)H-degrees(T) and DELTA(f)G-degrees(T), have been derived for temperatures up to 1000 K.
 
Article
The total cross-section of beryllium oxide for cold neutrons of mean wavelength 5.55 Å has been measured in the temperature range from 100 to 1000°K. The results are satisfactorily interpreted in terms of the incoherent approximation using the Placzek expansion. The values of the Debye temperature obtained from these measurements in the low and high temperature ranges are compared with those got by specific heat measurements. A method of choosing the effective mass and effective neutron cross-section of a molecule in a diatomic lattice is suggested.
 
Article
The SnO2(1 0 1) surface can be prepared with a SnO2 or SnO composition and consequently the surface Sn-atoms are either in a Sn(II) or Sn(IV) charge state. For a Sn(II) surface, Sn-5s derived surface states are identified by resonant, angle resolved photoemission spectroscopy (ARUPS). The differences in the interface properties of the Sn(II) and Sn(IV) surfaces of SnO2(1 0 1) are reviewed on the example of benzene and water adsorption. It is found that the difference in work function of these two surfaces causes a shift of the molecular orbitals of benzene by ∼1 eV with respect to the Fermi-level of the substrate. Density functional theory calculations predict dissociation of water on the stoichiometric (Sn(IV)) surface but only weak molecular adsorption on the reduced Sn(II) surface. These predictions are in agreement with ARUPS measurements that show that at 160 K no water adsorbs on the reduced surface but adsorbs dissociatively on the stoichiometric surface. A strong adsorbate induced band bending is also observed for water adsorption on the stoichiometric surface that is likely associated with the formation of surface hydroxyls.
 
Article
Decomposing liquid lithium-ammonia and sodium-ammonia solutions are high-conducting in some critically decomposed states, in which they form a gel with capillaries of the non-decomposed solution extending one-dimensionally through a sponge-like network of the decomposition product (alkali amide). The solutions may be stabilized at room temperature in their critically decomposed state if the ammonia is swept out of the capillaries of the gel in order to inhibit any further decomposition reaction. The xerogel, which retains the reduced structural dimensionality of the gel, can carry current densities of 105 A cm−2.
 
Article
The heat capacity of synthetic α-Fe2O3 has been measured in the range 300–1050K by adiabatic shield calorimetry with intermittent energy inputs and temperature equilibration in between. A λ-type transition, related to the change from antiferro- to paramagnetism in the compound, is delineated and a maximum heat capacity of about 195 JK−1 mole−1 is observed over a 3 K interval around 955 K. Values of thermodynamic functions have been derived and CP (1000K), [H0(1000K)-H0(0)], and [S0(1000K)-S0(0)] are 149.0JK−1 mole−1, 115.72 kJ mole−1, and 252.27 JK−1 mole−1, respectively, after inclusion of earlier low-temperature results [X0 (298.15K)-X0(0)]. The non-magnetic heat capacity is estimated and the thermodynamic properties of the magnetic transition evaluated. The results are compared with spin-wave calculations in the random phase approximation below the Néel temperature and the Oguchi pair model above. An upper estimate of the total magnetic entropy gives 32.4JK−1 mole−1, which compares favorably with that calculated for randomization of five unpaired electron spins on each iron, ΔS = 2R ln 6 = 29.79 JK−1 mole−1 for α-Fe2O3. The critical exponent α in the equation is −(0.50±0.10) below the maximum and 0.15±0.10 above, for Tn = 955.0K. The high temperature tail is discussed in terms of short range order.
 
Article
We investigated the high-pressure structural transformations in gadolinium at room temperature. Up to a pressure of 106 GPa, gadolinium metal undergoes four structural transformations: h.c.p.→ Sm-type → d.h.c.p. → f.c.c. → t.h.c.p. The corresponding transformation pressures are: 1.5 ± 0.2 GPa, 6.5 ± 0.5 GPa; onset at 24.0 GPa and completion at 29.0 GPa; and between 44.0 and 55.0 GPa, respectively. We found a much larger stability range for f.c.c.-gadolinium phase than that reported by others.
 
Article
In Ni-Cr alloys annealing after quenching from high temperatures leads to an increase of electrical resistivity which is attributed to a short range order state often denoted as K-state. Here the kinetics of the formation of the K-state in a Ni 10% Cr alloy is investigated by resistivity measurements for different quenching and annealing temperatures. The measurements are analyzed by an extended Schulze-Lücke method [22] which assumes that the rate of resistivity change is proportional to the vacancy concentration and that this rate as well as the rate of vacancy annihilation is described by a chemical rate equation.This analysis allowed a quantitative determination of the parameters determining the kinetics of short range order formation under different vacancy concentrations as well as the kinetics of annealing out of the quenched-in surplus vacancies. Among other quantities this treatment yielded the activation energies for vacancy formation HF = 1.16 eV, for vacancy migration HM = 1.56 eV and, as an independent cross check, for self diffusion HD = 2.73 eV in good agreement with HF + HM = 2.72 eV.
 
Article
We use an analytic method based on the expansion of the dynamical matrix to determine the thermal dependence of the mean-square displacements 〈u2〉 of volume, and (100) and (110) surface atoms of Cr. For the volume and (110) surface the results are in satisfactory agreement with previous X-ray and low-energy electron diffraction data. In particular, the zero-point thermal motion of Cr-volume atoms is approximately reached at temperatures as high as 170°K in accordance with experiments.A discussion about the temperature at which the atomic vibrations are strongly frozen is carried out for b.c.c. crystals.
 
Article
It is suggested that dislocations in ionic crystals of the NaCl type may be dissociated in (110) planes according to the reaction [110] → [110] + [110]. A rough calculation gives an estimation of the energy for the stacking fault between the two partials. One thus obtains dissociations of the order of some b. A model for the formation of sessile locks is proposed.
 
Article
The energies of the ideal, missing row (MR) and missing column (MC) (110) surfaces have been calculated by using modified embedded atom method (MEAM) for seven face centered cubic (FCC) transition metals Au, Pt, Ag, Pd, Rh, Cu and Ni. The results, that the MC reconstruction can not be formed for all metals, while the MR reconstruction can be formed naturally for Au and Pt, inductively for Ag, Pd, Rh and Cu and difficultly for Ni, are better than EAM calculated results in comparing with experimental results. In addition to the surface energy explanation, the results are also related to the surface topography and valence electron structure.
 
Article
Systematic annealings, at increasing temperatures of up to 1100 °C in ambient air, of X, Y and Z-cuts of synthetic, natural and air-swept synthetic quartz crystals were performed. They show, after a first phase of rearrangements, an outdiffusion of hydrogen from the various bulk OH defects as controlled by their ir band absorptions. Outdiffusion is found to be virtually isotropic for (AlOH) defects in natural and swept quartz; it becomes evident at ~900-700 °C, respectively. Strong anisotropy appears for as grown OH defects in synthetic quartz where the Z-cuts outdiffuse before 550 °C. The relative S4/total OH concentrations reaches a maximum at ~800 °C. Results are quantified using a simple one dimensional classical theory of diffusion which allows diffusion coefficients (Dcm2/s) to be attributed to these outdiffusions.
 
Article
The high sensitivity of the magneto-optical Kerr effect to details of the electronic structure is used to identify a structural transition from fcc to hcp in ultrathin Co(111) wedges on Pt(111). The variation of the Kerr rotation with Co thickness yields two distinct regions: (i) fcc(111) Co in the range 0.4 < dCo < 1 nm and (ii) hcp(0001) Co in the range dCo > 1.2 nm. These regions show characteristic volume and interface magnetic anisotropies.
 
Article
Ferroelectric Pb(Zr0.52Ti0.48)O3 thin films were grown on p-InSb (111) substrates by using a radio-frequency magnetron sputtering method at low temperature (∼350°C). Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements showed that the Pb(Zr0.52Ti0.48)O3 film layers grown on InSb substrates were polycrystalline thin films with smooth surfaces, and Auger electron spectroscopy (AES) measurements indicated that the compositions of the as-grown films consisted of lead, zirconium, titanium and oxygen. Transmission electron microscopy (TEM) and selected-area electron diffraction measurements showed that the grown Pb(Zr0.52Ti0.48)O3 was a polycrystalline layer with small domains and that the Pb(Zr0.52Ti0.48)O3/InSb (111) heterointerface had no significant interface problems. Room-temperature current–voltage and capacitance–voltage (C–V) measurements clearly revealed a metal-insulator–semiconductor behavior for the Pb(Zr0.52Ti0.48)O3 insulator gates, and the interface state densities at the Pb(Zr0.52Ti0.48)O3/p-InSb interfaces, as determined from the C–V measurements, were approximately high 1011 eV−1 cm−2 at the middle of the InSb energy gap. The dielectric constant of the Pb(Zr0.52Ti0.48)O3 thin film, as determined from the C–V measurements, was as large as 803.2. These results indicate that the Pb(Zr0.52Ti0.48)O3 layers grown on p-InSb (111) substrates at low temperatures can be used both for high-density dynamic memories and high-speed varistors based on InSb substrates.
 
Article
On Si(111) surfaces, we observed a special type of Si magic clusters with a variable-temperature scanning tunneling microscope (STM). At temperatures above 400°C, these clusters migrate on Si(111)-(7×7) surfaces as a whole. They can hop within a half-cell of Si(111)-(7×7), but sometimes they hop away from their original halves, leaving the 7×7 structure intact. When this happens, the magic cluster usually reappears at a site a few hundred Å away. We characterize its structure and derive path-specific hopping parameters using Arrhenius analysis. In the long hops, interestingly, magic clusters exhibit a strong bias for moving in the direction of the heating current. Effects of the directed motion in electromigration and those in thermal migration are determined separately and quantitatively. We also observed fluctuations of step edges through detachment and attachment of magic clusters. The filling of two-dimensional (2-D) craters and the decay of 2-D islands are also found to occur preferentially at the cathode side. These observations provide important clues for understanding the atomic processes in epitaxial growth and in electromigration on Si(111) surfaces. Based on our observations, the phase transition of 7×7↔1×1 is also discussed.
 
Article
CuGaS2 films epitaxially grown on Si(111) substrates by molecular beam epitaxy from elemental sources were investigated by various optical methods. Photoluminescence studies at low temperatures reveal a strong dependence of the near-bandgap emission on the composition. Ga-rich films show predominantly donor–acceptor pair recombination, while additionally free-to-bound and excitonic transitions are found for stoichiometric and Cu-rich films. Low lineshape broadening of the free excitonic transitions FXA and FXB/C in the photoreflectance spectra up to room temperature demonstrates the high crystal quality. The assignment of these transitions is confirmed by polarisation dependent photocurrent measurements making use of the optical selection rules and the three different growth directions for the c-axis of CuGaS2 relative to the Si-substrate. A fit to the photocurrent measurements yields the relative volume fraction of each c-axis orientation.
 
Article
In oxidizing etching media, the {111} surfaces of InSb are much less reactive than the } surfaces as determined by direct dissolution rates and by etching of spheres. Related to this difference in reactivity is the fact that dislocation etch pits appear on the {111}, but not on the {} surfaces. Inhibitors such as stearic acid and primary amines decrease appreciably the dissolution rate of only the {} surfaces. Furthermore, etching inhibitors lead to the formation of new dislocation etch pits on the {111} and also on the {} surfaces. Similar effects were observed in GaAs. These phenomena are discussed in terms of the atomic configuration of the surfaces involved and the relative reactivity of the group III and group V atoms.
 
Article
The nuclear quadrupole interaction of the I=5/2 state of the nuclear probes 111Cd and 181Ta in the anatase and rutile polymorphs of bulk TiO2 was studied using the time differential perturbed angular correlation (TDPAC). The fast-slow coincidence setup is based on the CAMAC electronics. For anatase, the asymmetry of the electric field gradient was eta=0.22(1) and a quadrupole interaction frequency: 44.01(3) Mrad/s was obtained for 181Ta. For rutile, the respective values are eta=0.56(1) and quadrupole frequency=130.07(9) Mrad/s. The values for rutile match closely with the literature values. In case of the 111Cd probe produced from the beta decay of 111Ag, the quadrupole interaction frequency and the asymmetry parameter for anatase was negligible. This indicates an unperturbed angular correlation in anatase. On the other hand for rutile, the quadrupole frequency is 61.74(2) Mrad/s and the asymmetry is 0.23(1) for 111Cd probe. The results have been interpreted in terms of the surrounding atom positions in the lattice and the charge state of the probe nucleus. Comment: 14 pages, 2 figures
 
Top-cited authors
Werner P. Wolf
  • Yale University
Hiroshi Suga
  • Osaka University, Japan Toyonaka
Harold Hjalmarson
  • Sandia National Laboratories
Richard Richard A Catlow
  • University College London
Wolfgang Jeitschko
  • University of Münster