Atomic Positional Parameters Research Articles

Preparation and photocatalytic property of LiCr(WO 4) 2

A new photocatalyst LiCr(WO 4) 2 was synthesized via a polymer complex method and was characterized by X-ray diffraction, BET measurement, UV–vis diffuse reflectance spectra and photocatalytic experiments for H 2 and O 2 evolution. The crystal model of LiCr(WO 4) 2, including lattice constants and atomic position parameters, was obtained by Rietveld refinement of powder X-ray diffraction data. The photocatalytic evaluation measurements showed that LiCr(WO 4) 2 exhibits the ability for H 2 evolution from an aqueous methanol solution and for O 2 evolution from an aqueous AgNO 3 solution under ultraviolet and visible light irradiation. The mechanism for H 2 and O 2 evolution over LiCr(WO 4) 2 photocatalyst was further investigated by density functional theory calculations and was discussed from the view of electronic structure point.

In‐situ investigation of the structural phase transition in kesterite

AbstractA polycrystalline Cu2ZnSnS4 sample (kesterite) was investigated by in‐situ high temperature diffraction using synchrotron X‐rays, revealing for the first time a structural phase transition from the tetragonal kesterite type structure (space group I$ \bar 4 $2m) to the cubic sphalerite type structure (space group F$ \bar 4 $3m). Within 866–883 °C a phase transition region occurs, were a tetragonal phase and a cubic phase coexists. Outside the phase transition region, the lattice parameter of the tetragonal phase increase nearly linearly with increasing temperature, whereas the lattice parameter of the cubic phase show first a negative thermal expansion changing at ∼930 °C to a further linear increase of acub. A cation anti‐site occupancy can only be observed within the phase transition region. On the other hand, the anion position parameters x and z change, which starts at ∼150 °C and lasts up to the phase transition region. Both atomic position parameters reach nearly the ideal value of 1/4, indicating the anion in the middle of the cation tetrahedra. Thus the structural phase transition in Cu2ZnSnS4 from the tetragonal kesterite type to the cubic sphalerite type structure is characterized much more by a displacive behaviour of the anion substructure then by an anti‐site occupancy of the cation substructure. Because the space group I$ \bar 4 $2m) is not a subgroup of I$ \bar 4 $2d) and a group‐subgroup relation is mandatory for displacive phase transitions, the non‐existence of a I$ \bar 4 $2m) → I$ \bar 4 $2d) transition seems plausible. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure refinement of a decagonal quasicrsytal of Al72Ni20Co8 by convergent-beam electron diffraction

Abstract Structure refinement of a water quenched alloy of Al72Ni20Co8, which is known as a highly ordered decagonal quasicrystal, has been carried out by quantitative comparison of experimental and simulated intensities of convergent-beam electron diffraction (CBED) patterns. In the present study, higher-order Laue zone reflections, whose intensities are senstively changed with atom positions, have been used for the refinement of atom position parameters in the quasiperiodic directions. The refined parameters have been compared with the results obtained by X-ray diffraction.

The compressibility and high pressure structure of diopside from first principles simulation

The structure of diopside (CaMgSi2O6) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch–Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa.

Structural and magnetic properties of(Fe0.93Ni0.07)2P

Structural and magnetic properties of(Fe0.93Ni0.07)2P havebeen investigated by means of powder x-ray and neutron diffraction, magnetization and paramagneticsusceptibility measurements over a temperature range of 10–500 K. The system crystallizes in theFe2P type hexagonal structure ( space group, Z = 3) in which the Ni atoms occupy the tetrahedralMI sites with total preference. Refined values of the cell parametersand bond distances are found slightly higher than those forFe2P and the atomic positional parameters are found quite close to those reported for the parent compoundFe2P. The temperature dependence of the magnetization shows a sharp magnetic phasetransition around 298(5) K. However there is difference of the zero-field cooled andfield cooled modes of the magnetization, which is indicative of the formation offerromagnetic clusters. The ferromagnetic to paramagnetic transition shifts towardsthe higher temperature side with increase in the applied magnetic field, whichindicates that the phase transition is a field induced type first-order magneticphase transition. There is no crystallographic structural transition associated withthe magnetic phase transition. The transition is caused by the change in thec/a ratio. The alloy retains the ferromagnetic order ofFe2P with the moments orienting along the [001] direction. The Rhodes–Wohlfarth ratio(µp/µs) of(Fe0.93Ni0.07)2P is foundto be 1.53 (>1), showing itinerant magnetism in this compound. At 297 K the magnetic moment at theMI site is found negligiblebut at the MII site itis ∼0.51 µB. The observednon-linearity above Tc in the χ−1–T curve gives clear evidence of the presence of short range magnetic order aboveTc. The momentsat the MII sites in the paramagnetic state and the exchange interactions are responsible forthe short range one-dimensional ferromagnetic chains along [001] well above theTc in(Fe0.93Ni0.07)2P.

Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs 2Au IAu IIICl 6 under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs 2Au IAu IIICl 6, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/ mmm to Pm3 m was found. The lattice parameters a 0 and c 0, denoted in the tetragonal cell setting, result in the relationship 2 1/2 a 0= c 0, and the superstructure reflections h k l ( l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au IIICl 6] and compressed [Au ICl 6] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl 6] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au I/Au III mixed-valence state to the Au II single-valence state.

Structure and dielectric properties of the Aurivillius phase Bi2.25Ca0.5Na0.25Nb2O9

A new layered perovskite-like bismuth-oxide compound with the Bi2.25Ca0.5Na0.25Nb2O9 stoichiometry (Aurivillius phase) has been synthesized in polycrystalline form. As shown by powder x-ray diffraction, Bi2.25Ca0.5Na0.25Nb2O9 has an orthorhombic structure (sp. gr. A21am, no. 36) with unit-cell parameters a = 5.4845 A, b = 5.4549 A, and c = 24.9195 A. Rietveld analysis has been used to determine the atomic positional parameters in its unit cell and the position of the niobium ions. Nb5+ has been shown to be displaced from the center position in the oxygen octahedra by ∼0.15 A. The Nb K-edge extended x-ray absorption fine structure (EXAFS) spectrum of the new compound has been measured in the range 295 to 960 K. The 295-K EXAFS data have been used to evaluate the Nb-O bond lengths in the octahedra and the corresponding Debye-Waller factors. The bond lengths thus determined agree well with those inferred from powder x-ray data. With increasing temperature, the short Nb-O bonds become longer, while the long bonds become shorter. The dielectric properties of the new compound have been studied between room temperature and 1200 K at frequencies from 1 to 50 kHz. Bi2.25Ca0.5Na0.25Nb2O9 is shown to be a ferroelectric with a Curie temperature of 972 K.

Neutron diffraction study on the magnetic structure of (Fe 0.90Cr 0.03Ni 0.07) 2P

Crystal and magnetic structures of (Fe 0.90Cr 0.03Ni 0.07) 2P have been investigated using neutron powder diffraction measurements over a temperature range of 10–300 K. The system crystallizes in Fe 2P type hexagonal structure ( P 6 ¯ 2 m space group, Z = 3) in which Cr atoms occupy the pyramidal Fe II site and Ni atoms occupy the tetrahedral Fe I site with total preference. Refined values of the cell parameters and atomic positional parameters are quite close to those reported for the parent compound Fe 2P. The substituted alloy retains the ferromagnetic order of Fe 2P with the moments orienting along [0 0 1] direction. There are signatures of the presence of magnetic clusters. The ordered magnetic moments at 10 K are 0.88(2) μ B and 2.16(5) μ B at the two metallic sites Fe I and Fe II, respectively. These values are close to those reported for Fe 2P at 77 K. At 297 K, which is in the region of ferromagnetic to paramagnetic transition, the moment at Fe I site is quite low at 0.14(3) μ B but at Fe II site, it is still fairly large at 0.67(15) μ B.

Structure Parameters of the Au–49.5 at%Cd Martensite at Various Temperatures Approaching the Reverse Transformation Temperature <I>A</I><SUB>s</SUB>

Precursor phenomenon on the martensitic transformation was studied in Au 50.5 Cd 49.5 For the precursor study, single variant of ζ2' martensite was produced and the reverse transformation process was utilized to avoid a superposition of several variants in diffraction experiments through the martensitic transformation, i.e. from parent to martensite phase. Atomic parameters in martensite state were refined with use of single crystal structure determination procedure by X-ray at various temperatures approaching the reverse transformation temperature. Atomic positional parameters of some atoms which are expected to have relatively large displacements from parent phase showed first-order feature.

Thermal expansion and compressibility in superconductingNaxCoO2∙4xD2O(x≈1∕3): Evidence for pressure-induced charge redistribution

We have performed thermal expansion and compressibility measurements on the recently discovered superconducting material NaxCoO2*4xD2O (x=1/3) using neutron powder diffraction over the temperature range 10-295 K and the pressure range 0-0.6 GPa. Pressure measurements were done in a helium-gas pressure cell. Both the thermal expansion and compressibility are very anisotropic, with the largest effects along the c axis, as would be expected for a layered material with weak hydrogen bonding nominally along the c axis. Near room temperature, the anisotropies of the thermal expansion and compressibility of the hexagonal crystal structure are nearly the same [(Dc/c)/(Da/a)=3-4], with a 100 C change in temperature being roughly equivalent to 0.2 GPa pressure. This would imply that changes in atom position parameters are also the same, but this is not the case. While the effects of temperature on the atom positions are essentially what one might expect, the effects of pressure are surprising. With increasing pressure, the thickness of the CoO2 layer increases, due to the combined effects of an increasing Co-O bond length and changes in the O-Co-O angles of the CoO6 octahedra. We conclude that this unusual effect results from pressure-induced strengthening of the hydrogen bonding between the Nax(D2O)4x layers and the CoO2 layers. The strengthening of these hydrogen bonds requires that charge be moved from the somewhere else in the structure; hence, there is a pressure induced charge redistribution that weakens (lengthens) the Co-O bonds and changes the electronic structure of the superconducting CoO2 layers.

Influence of pressure on the structure and electronic properties of the layered superconductorY2C2I2

The structural properties under hydrostatic pressure up to3 GPa of the layered rare earth carbide halide superconductorY2C2I2 are studied by neutron powder diffraction at room temperature. The compressibilities areanisotropic, such that the compressibility perpendicular to the layers is approximatelytwice as large as within the layers. The atomic positional parameters determined from thepowder diffraction patterns as a function of pressure were used as the basis for highlyresolved electronic structure calculations. These reveal the electronic density of states atEF to increase with pressure. As is shown quantitatively, this effect outweighs the pressure inducedlattice stiffening effects and is responsible for the pressure induced increase and saturation ofTc towards 2 GPa.

EPR paramaters and defect structure of the trigonal Ti 2+ center in ZnS

The axial Ti 2+ center in a nearly wholly cubic ZnS crystal is assigned to the Ti 2+ ion on the hexagonal site of wurtzite structure caused by stacking faults. On the ground of the assignment, the EPR parameters (zero-field splitting D, g factor g ∥ and g-anisotropy Δ g = g ∥ − g ⊥) of the axial Ti 2+ center are calculated from the high-order perturbation formulas based on the cluster approach for the EPR parameters of 3d 2 ion in trigonal symmetry. From the calculations, the local atom-position parameter u loc (which is different from the corresponding parameter u in the host wurtzite structure) and hence the defect structure of the Ti 2+ center are estimated. The results (the calculated EPR parameters and the defect structure) are discussed.

Electrical conductivity of M-Ge (M = Al, Sc, Y) films

—A number of tetradymite-like layered compounds in the PbTe−Bi2Te3 and PbTe−Sb2Te3 systems were prepared by a vertical Bridgman process, and their crystal structures were investigated by x-ray diffraction using single-crystal cleaved specimens. The atomic positional parameters and bond distances in PbBi4Te7 were determined. In the pseudobinary system PbTe−Sb2Te3, two new compounds were identified, PbSb2Te4 and PbSb4Te7, isostructural with PbBi2Te4 and PbBi4Te7, respectively. The positional parameters, site occupancies, and bond distances in the structure of PbSb2Te4 were determined. The room-temperature thermoelectric power of the grown ingots was measured along their length. All of the Sb compounds studied were found to be p-type, in contrast to the compounds of the PbTe−Bi2Te3 system.

EPR Parameters and Local Atom-position Parameters for Co2+ Ions in CdS and CdSe Semiconductors

The EPR parameters (zero-field splitting D and g factors gII, g⊥) of Co2+ ions in CdS and CdSe semiconductors are calculated from the high-order perturbation formulas based on the cluster approach for a 3d7 ion in trigonal symmetry. These formulas include the contribution to the EPR parameters from both the spin-orbit coupling parameter of the 3d7 ion and that of the ligand. From the calculations, the local atom-position parameters u (which are different from the corresponding values in the host crystals) for the Co2+ impurity centers in both semiconductors are estimated. The results are discussed

Enantiomorph identification of transition-metal disilicides with the C40 structure (the space group of P6 222 and P6 422) by new convergent-beam electron diffraction method

The new CBED method we proposed recently for enantiomorph identification has been applied to transition-metal disilicides with the C40 structure (the space groups of P6 222 and P6 422). The enantiomorph can be easily identified by simply inspecting the intensity asymmetry of Bijvoet pairs of ZOLZ and/or FOLZ reflections in a CBED pattern taken along an appropriate zone-axis orientation. The way for enantiomorph identification with the present method is altered neither with the changes in the lattice parameters and averaged atomic scattering factors in the range expected for real C40 disilicide crystals of multi-component nor with the atom position parameter for Si atoms in 6j sites. The method is successfully applied to real C40 disilicides, and our single crystals of VSi 2, CrSi 2, TaSi 2 and Mo(Si 0.85Al 0.15) 2 are determined to belong to P6 422 whereas that of NbSi 2 is determined to belong to P6 222. These results are all consistent with those of our previous X-ray diffraction study.

The influence of positional errors on the Debye effects.

The relation between a Gaussian perturbation of the atomic positional parameters and the average squared structure-factor amplitude is presented. Using an error-dependent radial distance distribution of an atomic protein model, it can be shown that the Debye effects diminish exponentially as a function of increasing positional errors. These relations can be used to estimate the quality of an atomic model and the corresponding phases. The limiting case of equal atoms with an infinitely large coordinate error results in the classical Wilson model.

Crystal structure and electrical properties of the new R 2TSi 2 compounds (R = rare earths; T = Fe, Co)

The R 2FeSi 2 compounds with R=Gd, Tb, Dy, Ho, Er, Tm, Y, and R 2CoSi 2 with R=Ho, Er, Tm, were synthesized and studied by X-ray powder diffraction. All phases crystallize in the Sc 2CoSi 2 monoclinic structure type, and the atomic positional and thermal parameters were refined by single crystal data for Tb 2FeSi 2, Er 2FeSi 2 and Er 2CoSi 2. The measurement of the electrical resistance in the 13–300 K temperature range showed for all phases, save the yttrium compounds, a transition point which indicates an order–disorder magnetic change. The scaling of these temperatures with the de Gennes factor gives a good correlation, with some slight deviations probably due to crystal electric field effects.

Molecular Dynamics Simulation of Germanium Nanoparticles

AbstractNanoparticles are known to melt at temperatures well below the bulk melting point. This behavior is being exploited for the recrystallization of Germanium to form large-grain semiconductor thin films on flexible and low temperature substrates. The melting of Ge nanoparticles as a function of size was investigated using the ab-initio Harris functional method of density functional theory (DFT).The DFT code was initially evaluated for its ability to predict the bulk properties of crystalline Ge. A conjugate gradient method was employed for minimizing the multiphase atomic positional parameters of the diamond, BC8 and ST12 structures. The computed lattice constants, bulk moduli, and internal atomic positional parameters were found to agree well with other calculations and with reported experimental results.A constant temperature Nose-Hoover thermostat was added to the DFT code in order to compute thermal properties via molecular dynamics. The simulations were tested on a 13-atom Ge cluster, which was found to melt at 820 K. Further heating resulted in the cluster breaking up into two smaller clusters, which remained stable up to 1300K.

Structural study of ternary mercury chalcogenides at high pressures

The structure of ternary mercury chalcogenides HgSe1-xSx (x=0.3,0.5,0.6) and HgTe1-xSx (x=0.15) was studied by means of powder neutron diffraction at pressures up to 8 GPa. A phase transition from the zinc blende cubic structure to the hexagonal cinnabar structure was observed at P≈1 GPa in HgSe1-xSx systems and at P≈1.6 GPa in HgTe0.85S0.15. A coexistence of these two phases was observed in the vicinity to the transition pressure. Lattice parameters and atomic positional parameters for the high-pressure hexagonal phase were determined.

Ordered Atomic Distribution in the Structure of TiCrP

A TiCrP single crystal was prepared by arc melting. Single-crystal x-ray diffraction data show that TiCrP has a ZrAlNi-type structure (superstructure of the Fe2P type), sp. gr. P6¯2m, a = 0.63998(3) nm, c = 0.32896(1) nm. The atomic positional and thermal parameters were refined to RF = 0.029 and Rw = 0.036 using 133 unique reflections with Fhkl > 4σ(Fhkl ).