[Show abstract][Hide abstract] ABSTRACT: The high-pressure behavior of layered CrOCl is shown to be governed by non-bonded interactions between chlorine atoms in relation to a rigid framework composed of Cr and O atoms. The competition between optimizing intra- and interlayer Cl-Cl distances and the general trend towards denser packing defines a novel mechanism for high-pressure phase transitions of inorganic materials. CrOCl possesses an incommensurate phase for 16-51 GPa. Single-crystal x-ray diffraction in a diamond anvil cell provides an accurate description of the evolution of the incommensurate wave with pressure. It thus demonstrates a continuous increase of the amplitude up to 30 GPa, followed by a decrease of the wavelength until a lock-in transition occurs at 51 GPa.
[Show abstract][Hide abstract] ABSTRACT: The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) becomes
ferroelectric upon cooling through the loss of inversion symmetry. Further
cooling results in the development of an incommensurate ferroelectric phase,
followed by a lock-in transition towards a twofold superstructure. Here we
present the incommensurately modulated crystal structure of Phz-H2ca at T =
139 K with a symmetry given by the superspace group P21(1/2 b 1/2)0 and b =
0.5139. The modulation mainly affects the positions of the protons within half of
the intermolecular hydrogen bonds that are responsible for the spontaneous
polarization in all three low-temperature phases. Evidence for proton transfer in
part of the hydrogen bonds is obtained from the correlated dependence on the
phase of the modulation of the lengths of bonds involved in resonance
stabilization of the acidic anion, and much smaller variations of bond lengths of
atoms not involved in the resonance mechanism. Incommensurability is
explained as competition between proton transfer favored for single hydrogen
bonds on the basis of pKa values and avoiding unfavorable Coulomb repulsion
within the lattice of the resulting ionic molecules.
Acta Crystallographica Section B 03/2015; 71(Pt 2):228–234. DOI:10.1107/S2052520615004084
[Show abstract][Hide abstract] ABSTRACT: Several fcc- and hcp-structured Ir–Os alloys were prepared from single-source precursors in hydrogen atmosphere at 873 K. Their atomic volumes measured at ambient conditions using powder X-ray diffraction follow nearly linear dependence as a function of composition. Alloys have been studied up to 30 GPa at room temperature by means of synchrotron-based X-ray powder diffraction in diamond anvil cells. Their bulk moduli increase with increasing osmium content and show a deviation from linearity. Bulk modulus of hcp-Ir0.20Os0.80 is identical to that of pure Os (411 GPa) within experimental errors. Peculiarities on fcc-Ir0.80Os0.20 compressibility curve indicate possible changes of its electronic properties at ∼20 GPa.
Journal of Alloys and Compounds 02/2015; 622:155-161. DOI:10.1016/j.jallcom.2014.09.210 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Single crystals of CuxTa1+ySe2 were grown by chemical vapor transport. Single crystals of different compositions were obtained at slightly different reaction conditions from mixtures of the reactants of the same nominal composition. It is suggested that different diameters of the ampoules imply different contributions of convection and diffusion to the mass transport, and thus are responsible for different ratios of the amount of Cu, Ta, and Se transported. 2H-Cu0.52TaSe2 (x = 0.52, y = 0) is formed in the narrower ampoule (diameter 15 mm). The crystal structure is based on the MoS2 type of stacking of TaSe2 layers. Partial ordering of Cu over the tetrahedral sites is responsible for a 2a0 × 2b0 × c0 superstructure with hexagonal Pm2 symmetry [a0 = 3.468 (1) Å, c0 = 13.568 (3) Å]. 2H-Cu0.16Ta1.08Se2 (x = 0.16, y = 0.08) is formed in the wider ampoule (diameter 18 mm). It possesses a NbS2-type of stacking. A superstructure is not formed, but the presence of Cu and intercalated Ta in alternating van der Waals gaps is responsible for the reduction of symmetry from P63/mmc to Pm1 [a0 = 3.439 (2) Å, c0 = 12.870 (2) Å]. Single crystals are formed towards the hotter side of the ampoules up to a temperature of 1168 K in both reactions.
Zeitschrift für anorganische und allgemeine Chemie 02/2015; 641(2):464-469. DOI:10.1002/zaac.201400335 · 1.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The organic compound Λ-Co(sepulchrate) trinitrate, [C(12)H(18)N(8)Co](3+) 3[NO(3)](-), exhibits at room temperature a disordered structure in symmetry P6(3)22 [1,2]. The Co(sepulchrate) cation and two of the nitrate anions, all centred on three fold rotation axes, are linked via dense N--H...O hydrogen bond networks, the third nitrate anion, centred on the intersection of the two fold rotation axes, shows orientational disorder (see Figure). Three phase transitions have been observed upon cooling by means of light microscopy and spectroscopic measurements  and by single crystal neutron diffraction  at T(1) = 133 K, T(2) = 106 K and T(3) = 98 K. These phase transitions are interpreted as ordering of the disordered nitrate anions  and as reduction of symmetry from hexagonal to orthorhombic associated with twinning. The appearance of satellite reflections in the diffraction pattern at T(1) = 133 K indicates a modulated structure; as the positions of those satellite reflections are temperature dependent , the modulation is incommensurate. By single crystal X-ray diffraction at beam lines D3 and F1 of Hasylab (DESY, Hamburg) at different low temperatures we found that all observed peaks are indexable in an hexagonal setting and two q-vectors q(1)=(sigma,sigma,0) and q(2)=(-2sigma,sigma,0) with sigma = 0.0882. This setting is compatible with a three-fold orthorhombic twinning and one q-vector q(orth)=(2sigma,0,0) for each of the three twin domains, which allows to reduce the symmetry. Structure refinement of all three low temperature phases allows to set them into relation to each other.
[Show abstract][Hide abstract] ABSTRACT: The compound TiOCl is a quasi-1-dimensional (1D) quantum magnet (Seidel et al., 2003). Upon cooling, TiOCl undergoes a phase transition at Tc2 = 90 K towards a state with incommensurate magnetic order, followed by a second phase transition at Tc1 = 67 K towards a spin-Peierls state (Seidel et al., 2003; Shaz et al., 2005; van Smaalen et al., 2005). Both low-temperature phases involve structural distortions that have been characterized by x-ray diffraction. The absence of any phase transitions has been reported for scandium-doped TiOCl with doping levels 0.01 < x < 0.1 for ScxTi1-xOCl (Glancy et al., 2008, 2010; Zhang et al., 2010; Aczel et al., 2011). We have synthesized ScxTi1-xOCl for x = 0.005. Based on temperature-dependent x-ray diffraction experiments and specific-hear measurements, we have found that the x = 0.005 compound transforms into incommensurate and spin-Peierls-like phases on cooling. Despite apparent large correlation lengths, these phases lack long-range order. A sluggish transformation is thus found between states of ScxTi1-xOCl that support different kinds of fluctuations.
[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: 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 08/2014; 108(4):1-15. DOI:10.1007/s00710-013-0319-x · 1.35 Impact Factor
[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. DOI:10.1007/s10870-014-0527-1 · 0.50 Impact Factor
[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(1):014415. DOI:10.1103/PhysRevB.90.014415 · 3.74 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.
[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 04/2014; 16(4-4):1-14. DOI:10.1007/s11051-014-2370-x · 2.18 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(18):184420. DOI:10.1103/PhysRevB.88.184420 · 3.74 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.
[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.