[Show abstract][Hide abstract] ABSTRACT: The co-crystal of phenazine (Phz) and chloroanilic acid (H2ca) is ferroelectric below the temperature T[I](c) = 253 K (FE-I phase). Upon cooling, two more phase transitions involve a further reduction of symmetry, until Phz-H2ca is triclinic in the second ferroelectric phase (FE-II phase) stable below T[II](c) = 137 K. Ferroelectricity in all low-temperature phases is believed to be related to partial proton transfer within the hydrogen bonds between the molecules Phz and H2ca. Here we present the crystal structure of the FE-II phase at T = 100 K. Experimental positions of hydrogen atoms indicate that ferroelectricity is mainly governed by half of the hydrogen-bonded chains, whereby proton transfer is observed within one of the two hydrogen bonds in which each molecule participates. A simple point charge model quantitatively reproduces the polarisation of this material. However, a possible contribution to the polarisation is proposed of the O-H...N hydrogen bonds of the second half of the mixed chains, which show elongated O-H bonds similar to those in the FE-I phase. The twofold superstructure with P1 symmetry was successfully solved as commensurately modulated structure employing the monoclinic superspace group P 2(1) (1/2 , s2 , 1/2) 0. The latter shows that the distortions at low temperatures follow a single normal mode of the space group P 2(1) of the FE-I phase, and it thus explains that the direction of the polarisation remains close to the monoclinic axis, despite the lowering towards triclinic symmetry.
Journal of Chemical Crystallography 08/2014; 44(8):387-393. · 0.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: At low temperatures the organic salt adamantan-1-ammonium 4-fluorobenzoate, C10H18N(+)·C7H4FO2(-), possesses an incommensurately modulated crystal structure. The effect of the modulation on the atomic arrangement and intermolecular interactions is studied by analysing single-crystal X-ray diffraction data within the (3 + 1)-dimensional superspace approach and superspace group P21/n(α0γ)00. The modulation strongly affects the position of the atoms as well as their atomic displacement parameters. Nevertheless, the molecular cations and anions are built by rigid moieties, which vary their orientation with respect to each other as a function of the phase of the modulation t. Cations and anions are connected into slabs by dense N-H...O and C-H...F hydrogen-bonded networks, which are characterized by being rather rigid and which show only a little variation as a function of the phase of the modulation t.
[Show abstract][Hide abstract] ABSTRACT: Single crystals of ScxTi1−xOCl (x = 0.005) have been grown by the vapor phase transport technique. Speciﬁc heat measurements prove the absence of phase transitions for 4–200 K. Instead, an excess entropy is observed
over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ScxTi1−xOCl gives broadened
diffraction maxima at incommensurate positions between Tc1 = 61.5 (3) and ∼90 K, and at commensurate
positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase
without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without
long-range order at low temperatures. The commensurate phase is attributed to a ﬂuctuating spin-Peierls state
on an orthorhombic lattice. The monoclinic symmetry and local structure of the ﬂuctuations are equal to the
symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ScxTi1−xOCl (x = 0.005) is
a transformation from one ﬂuctuating phase (the incommensurate phase at intermediate temperatures) to another
ﬂuctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical
temperature, but it proceeds gradually over a temperature range of ∼10 K wide. The destruction of long-range
order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the
canonical spin-Peierls compound CuGeO3. An explanation for the higher sensitivity to doping has not been
found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic
system. The observed ﬂuctuating states with long correlation lengths are reminiscent of Kosterlitz–Thouless-type
phases in two-dimensional systems.
Physical Review B 07/2014; 90:014415. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Single crystals of Cu3TaSe4 are obtained from a precursor that is produced by chemical vapor reaction of Cu, Ta, and Se in a molar ratio of 0.21:1.15:2 (quartz ampoule in two-zone furnace, 1.
[Show abstract][Hide abstract] ABSTRACT: Chemical bonding at the active site of hen egg-white lysozyme (HEWL) is analyzed on the basis of Bader's quantum theory of atoms in molecules [QTAIM; Bader (1994), Atoms in Molecules: A Quantum Theory. Oxford University Press] applied to electron-density maps derived from a multipole model. The observation is made that the atomic displacement parameters (ADPs) of HEWL at a temperature of 100 K are larger than ADPs in crystals of small biological molecules at 298 K. This feature shows that the ADPs in the cold crystals of HEWL reflect frozen-in disorder rather than thermal vibrations of the atoms. Directly generalizing the results of multipole studies on small-molecule crystals, the important consequence for electron-density analysis of protein crystals is that multipole parameters cannot be independently varied in a meaningful way in structure refinements. Instead, a multipole model for HEWL has been developed by refinement of atomic coordinates and ADPs against the X-ray diffraction data of Wang and coworkers [Wang et al. (2007), Acta Cryst. D63, 1254-1268], while multipole parameters were fixed to the values for transferable multipole parameters from the ELMAM2 database [Domagala et al. (2012), Acta Cryst. A68, 337-351] . Static and dynamic electron densities based on this multipole model are presented. Analysis of their topological properties according to the QTAIM shows that the covalent bonds possess similar properties to the covalent bonds of small molecules. Hydrogen bonds of intermediate strength are identified for the Glu35 and Asp52 residues, which are considered to be essential parts of the active site of HEWL. Furthermore, a series of weak C-H...O hydrogen bonds are identified by means of the existence of bond critical points (BCPs) in the multipole electron density. It is proposed that these weak interactions might be important for defining the tertiary structure and activity of HEWL. The deprotonated state of Glu35 prevents a distinction between the Phillips and Koshland mechanisms.
[Show abstract][Hide abstract] ABSTRACT: Single crystals of Cu3TaSe4 were obtained by chemical vapor transport with iodine as transport agent. Cu3TaSe4 crystallizes in the sulvanite structure type with space group P3m and a = 5.6613(3) Å at room temperature. Structure refinements against single-crystal X-ray diffraction data result in a Cu–Se distance slightly longer than the Ta–Se distance, whereas previous Rietveld refinements obtained a Cu–Se distance shorter than the Ta–Se distance [G. E. Delgado et al., J. Alloys Comp. 2007, 439, 346]. The structural variations of sulvanite type compounds Cu3MX4 (M = V, Nb, Ta; X = S, Se, Te) are discussed in view of small distortions of CuX4 and MX4 tetrahedra and the atomic valences.
Zeitschrift für anorganische Chemie 02/2014; · 1.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Customized metal nanoparticles are highly relevant in industrial processes, where they are used as catalysts and therefore needed on a large scale. An extremely economically and environmentally friendly way to produce metal nanoparticles is microbial biosynthesis, meaning the biosorption and bioreduction of diluted metal ions to zero valent (metal) nanoparticles. To maintain the key advantage of biosynthesis, including eco friendliness, a bioreactor (e.g., bacteria) has to be harmless by itself. Here, the ability of the cyanobacteria Anabaena sp. (SAG 12.82) is shown to fulfill both needs: bioreduction of Au3+ ions to Au0 and the subsequent formation of crystalline Au0-nanoparticles as well as absence of the release of toxic substances (e.g., anatoxin-a). The time-dependent growth of the nanoparticles is recorded by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) over a range of several days. Formation of nanoparticles starts within the first minutes at the heterocyst polysaccharide layer (HEP). After 4 h, the dominating amount of nanoparticles is found in the vegetative cells. The bioproduced nanoparticles are found in both cell types, mainly located along the thylakoid membranes of the vegetative cells and have a final average size of 9 nm within the examined timescale of a few days.
Journal of Nanoparticle Research 01/2014; 16(4):1-14. · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polycrystalline material of a sulfate apatite with chemical composition Na6Ca4(SO4)6F2 or (Na2Ca4)Na4(SO4)6F2 has been synthesized by solid state reactions. Basic crystallographic data are as follows: hexagonal symmetry, a = 9.3976(1) Å, c = 6.8956(1) Å, V = 527.39(1) Å3, Z = 1, space group P63/m. For structural investigations the Rietveld method was employed. Thermal expansion has been studied between 25 and 600 °C. High temperature (HT) powder diffraction data as well as thermal analysis indicate that the apatite-type compound undergoes a reconstructive phase transition in the range between 610 and 630 °C. Single-crystals of the HT-polymorph were directly grown from the melt. Structural investigations based on single-crystal diffraction data of the quenched crystals performed at −100 °C showed orthorhombic symmetry (space group Pna21) with a = 12.7560(8) Å, b = 8.6930(4) Å, c = 9.8980(5) Å, V = 1097.57(10) Å3 and Z = 2. Unit cell parameters for a quenched polycrystalline sample of the HT-form obtained at ambient conditions from a LeBail-fit are as follows: a = 12.7875(1) Å, b = 8.7255(1) Å, c = 9.9261(1) Å, V = 1107.53(2) Å3. The lattice parameters of both modifications are related by the following approximate relationships: a HT ≈ 2c RT, b HT ≈ -(½a RT + b RT), c HT ≈ a RT. The HT-modification is isotypic with the corresponding potassium compound K6Ca4(SO4)6F2. The pronounced disorder of the sulphate group even at low temperatures has been studied by maximum entropy calculations. Despite the first-order character of the transformation clusters of sulfate groups surrounding the fluorine anions can be identified in both polymorphs. Each of the three next neighbor SO4-tetrahedra within a cluster is in turn surrounded by 8–9 M-cations (M: Na,Ca) defining cage-like units. However, in the apatite structure the corresponding three tricapped trigonal prisms are symmetry equivalent. Furthermore, the central fluorine atom of each cluster is coordinated by three next M-neighbors (FM3-triangles), whereas in the HT-polymorph a four-fold coordination is observed (FM4-tetrahedra).
Mineralogy and Petrology 01/2014; · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A combined experimental and theoretical study of one oxaphosphinane derivative was made on the basis of a topological analysis of its electron density distributions. The electron density was determined from a high-resolution X-ray diffraction data set measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using density functional theory (DFT) methods at the B3LYP\6-311++G(3df,3pd) level of approximation. The charge-density distribution and analysis of topological properties revealed that the P-O bond is of the transit closed-shell type. The crystal structure possesses one intra- and several intermolecular hydrogen bonds. They were characterized quantitatively by topological properties using Bader's Atoms in Molecules theory. All hydrogen bonds were classified as weak. Further analysis of the experimental electron density by the source function allowed the intramolecular hydrogen bond to be characterized as an isolated hydrogen bond, in contrast to the resonance-assisted hydrogen bond in related molecules, such as chromone derivatives.
[Show abstract][Hide abstract] ABSTRACT: On the basis of single-crystal x-ray diffraction we show that TiPO4 undergoes a spin-Peierls distortion below 74.5(5) K, with a dimerization of the Ti chains along the c axis. Between 74.5(5) and 111.6(3) K, TiPO4 develops an incommensurate (IC) phase with temperature-dependent q vector (σ1,0,0). Density functional calculations strongly suggest that the IC phase results from a frustration of the lock-in spin-Peierls transition due to the competition of three energetically almost degenerate crystal structures and elastic coupling of the Ti chains via the bridging PO4 units. The phase transition into the IC phase is of second order, but the lock-in transition into the spin-Peierls distortion below 74.5 K is of weak first order in nature.
Physical Review B 11/2013; 88:184420. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dynamic and static electron densities (EDs) based on the independent spherical atom model (IAM) and multipole (MP) models of crambin were successfully computed, holding no series-termination effects. The densities are compared to EDs of small biological molecules at diverse temperatures. It is outlined that proteins exhibit an intrinsic flexibility, present as frozen disorder at 100 K, in contrast to small molecules. The flexibility of the proteins is reflected by atomic displacement parameters (B-factors), which are considerably larger than for small molecules at 298 K. Thus, an optimal deconvolution of deformation density and thermal motion is not guaranteed, which prevents a free refinement of MP parameters but allows an application of transferable, fixed MP parameters. The analysis of the topological properties, such as the density at bond critical points (BCPs) and the Laplacian, reveals systematic differences between static and dynamic EDs. Zero-point-vibrations, yet present in dynamic EDs at low temperature, affect but marginally the EDs of small molecules. The zero-point-vibrations cause a smearing of the ED, which becomes more pronounced with increasing temperature. Topological properties, primarily the Laplacian, of covalent bonds appear to be more sensitive to effects by temperature and the polarity of the bonds. However, dynamic EDs at ca. 20 K based on MP models provide a good characterization of chemical bonding. Both the density at BCPs and the Laplacian of hydrogen bonds constitute similar values from static and dynamic EDs for all studied temperatures. Deformation densities demonstrate the necessity of the employment of MP parameters in order to comprise the nature of covalent bonds. The character of hydrogen bonds can be roughly pictured by IAM, whereas MP parameters are recommended for a classification of hydrogen bonds beyond a solely interpretation of topological properties.
Zeitschrift für anorganische Chemie 09/2013; 639(11). · 1.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electron density of the α form of boron has been obtained by multipole refinement against high-resolution, single-crystal x-ray diffraction data measured on a high-quality single crystal at a temperature of 100 K. Topological properties of this density have been used to show that all chemical bonds between B 12 clusters in α-B 12 are formed due to one orbital on each boron atom that is oriented perpendicular to the surface of the cluster. It is shown that the same orbital order on B 12 clusters persists in both α-B 12 and γ -B 28 polymorphs and in several dodecaboranes, despite the fact that in every case the B 12 clusters participate in entirely different kinds of exocluster bonds. It is likely that the same orbital order of B 12 clusters can explain bonding in other boron polymorphs and boron-rich solids.
[Show abstract][Hide abstract] ABSTRACT: Single-crystal X-ray diffraction in a diamond anvil cell has proven to be a powerful tool for studying matter at extreme conditions. Displacive zone-boundary phase transitions between a high-symmetry low pressure structure and a low-symmetry high pressure superstructure represent a significant part of pressure-induced processes. However, the analysis of such superstructures is usually complicated by low angular resolution and limited reciprocal space coverage. This leads to unfavorable data/parameter ratio during the structure solution and refinement. The superspace approach is an established method to describe commensurately modulated structures, however it is not often applied to high pressure phase transitions. Here, we discuss the advantages of the superspace approach for the description and crystal-chemical analysis of high pressure superstructures. The main ideas of this approach are illustrated by the examples of isostructural FeOCl and CrOCl, forming different superstructures above 15 GPa.
High Pressure Research 08/2013; 33(3):501-510. · 0.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A pressure-induced phase transition of FeOCl is discovered to occur at Pc=15±1 GPa. It is preceded by extremely anisotropic lattice compression, which is explained by a gradual collapse of the van der Waals gap between the chlorine atoms on the borders of the slabs of this layered compound. Single-crystal x-ray diffraction in a diamond anvil cell is used to show that the high-pressure phase can be described as a fourfold superstructure with monoclinic lattice distortion, described by a 4a×1b×2c supercell with space group B21/m (b unique). The high-pressure crystal structures have been used to uncover the mechanism of the phase transition, as the formation of alternating regions of increased packing density of chlorine atoms within a single layer and regions of interpenetrating layers. Raman and Mössbauer spectroscopies indicate that the phase transition is not related to electronic effects or magnetic order.
Physical Review B 07/2013; 88(1):014110. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electron density of the α form of boron has been obtained by
multipole refinement against high-resolution, single-crystal x-ray
diffraction data measured on a high-quality single crystal at a
temperature of 100 K. Topological properties of this density have been
used to show that all chemical bonds between B12 clusters in
α-B12 are formed due to one orbital on each boron atom
that is oriented perpendicular to the surface of the cluster. It is
shown that the same orbital order on B12 clusters persists in
both α-B12 and γ-B28 polymorphs and in
several dodecaboranes, despite the fact that in every case the
B12 clusters participate in entirely different kinds of
exocluster bonds. It is likely that the same orbital order of
B12 clusters can explain bonding in other boron polymorphs
and boron-rich solids.
[Show abstract][Hide abstract] ABSTRACT: Dynamic model densities according to Mondal et al. [(2012), Acta Cryst. A68, 568-581] are presented for independent atom models (IAM), IAMs after high-order refinements (IAM-HO), invariom (INV) models and multipole (MP) models of α-glycine, DL-serine, L-alanine and Ala-Tyr-Ala at T ≃ 20 K. Each dynamic model density is used as prior in the calculation of electron density according to the maximum entropy method (MEM). We show that at the bond-critical points (BCPs) of covalent C-C and C-N bonds the IAM-HO and INV priors produce reliable MEM density maps, including reliable values for the density and its Laplacian. The agreement between these MEM density maps and dynamic MP density maps is less good for polar C-O bonds, which is explained by the large spread of values of topological descriptors of C-O bonds in static MP densities. The density and Laplacian at BCPs of hydrogen bonds have similar values in MEM density maps obtained with all four kinds of prior densities. This feature is related to the smaller spatial variation of the densities in these regions, as expressed by small magnitudes of the Laplacians and the densities. It is concluded that the use of the IAM-HO prior instead of the IAM prior leads to improved MEM density maps. This observation shows interesting parallels to MP refinements, where the use of the IAM-HO as an initial model is the accepted procedure for solving MP parameters. A deconvolution of thermal motion and static density that is better than the deconvolution of the IAM appears to be necessary in order to arrive at the best MP models as well as at the best MEM densities.
[Show abstract][Hide abstract] ABSTRACT: Crystal structures are reported for two perovskite-related compounds with nominal compositions La5(Ti0.8Fe0.2)5O17 and La6(Ti0.67Fe0.33)6O20 at seven different temperatures between 90 and 350 K. For both compounds no evidence of a structural phase transition in the investigated range of temperatures was found. The thermal expansions are found to be anisotropic, with the largest thermal expansion along a direction parallel to the slabs of these layered compounds. The origin of this anisotropy is proposed to be a temperature dependence of tilts of the octahedral (Ti,Fe)O6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds AnBnO3n + 2. The crystal structures have revealed partial chemical order of Ti/Fe over the B sites, with iron concentrated towards the centers of the slabs. Local charge compensation is proposed as the driving force for the chemical order, where the highest-valent cation moves to sites near the oxygen-rich borders of the slabs. A linear dependence on the site occupation fraction by Fe of the computed valences leads to extrapolated valence values close to the formal valence of Ti(4+) for sites fully occupied by Ti, and of Fe(3+) for sites fully occupied by Fe. These results demonstrate the power of the bond-valence method, and they show that refined oxygen positions are the weighted average of oxygen positions in TiO6 and FeO6 octahedral groups.
[Show abstract][Hide abstract] ABSTRACT: Al23Co15Cr23Cu8Fe15Ni16 and Al8Co17Cr17Cu8Fe17Ni33 high entropy alloys were investigated by scanning electron microscopy, transmission electron microscopy and three dimensional atom probe. While the brittle Al23Co15Cr23Cu8Fe15Ni16 alloy decomposes during solidification mainly into two body centred cubic phases, the ductile Al8Co17Cr17Cu8Fe17Ni33 shows two prominent face-centred cubic phases. Al23Co15Cr23Cu8Fe15Ni16 consists of Fe–Cr-rich and Cu-rich intermetallic phases embedded in an Al–Ni-rich solid solution matrix. The microstructure of Al8Co17Cr17Cu8Fe17Ni33 is mainly characterized by two phases. Nano-sized precipitates enriched mainly in Ni are embedded in a matrix depleted in Al and Cu. The phases observed in both high entropy alloys are compared with the equilibrium phases predicted by Thermo-Calc simulation.
Journal of Alloys and Compounds 03/2013; 552:430–436. · 2.73 Impact Factor