Hydrated structures and dehydration processes of lanthanoid(III) chloranilates studied by EXAFS

The local structures of lanthanoid(III) chloranilate complexes of Pr(III), Nd(III), Tb(III) and Er(III) have been studied by EXAFS (extended X-ray absorption fine structure). Hydrated structures of the lanthanoid(III) ions in these complexes have been investigated with respect to their coordination numbers and interatomic distances. Six or four water molecules coordinate to the lanthanoid(III) ion of Pr(III) or Nd(III), respectively, just after preparation of the complexes. The temperature dependence of the first coordinated structures has been studied in order to reveal the behavior of the coordinated water molecules in dehydration process. The coordination number around the central lanthanoid(III) ion decreases stepwise as temperature increases, depending on the type of central lanthanoid(III) ion present. The interatomic distance between the central lanthanoid(III) ion and oxygen atoms in the first shell decreases, accompanying the decrease of the coordination numbers. A parameter representing proportion shows the reduction of interatomic distance as one coordinated water molecule removes from the central ion, depending on the type of lanthanoid(III) ions.     

Thermodynamische untersuchung von legierungen der systeme kobalt—indium und nickel—indium

By means of solution calorimetry at high temperatures with tin as solvent, the formation enthalpies of the intermetallic phases in the cobalt—indium and nickel—indium systems have been determined. Also, the mixing enthalpies of liquid indium-rich nickel—indium alloys have been measured. The resulting formation enthalpies and the mixing enthalpies are negative. The entropy of formation of the compound CoIn₂ is also consistent with strong interatomic interactions. The mixing enthalpies of the liquid cobalt—indium and iron—indium alloys calculated from the phase equilibria, however, are distinctly positive. The energy data of the systems investigated here depend on individual bonding conditions and are scarcely influenced by the atomic structures of the phases.     

Analysis of liquid structure without construction of any structure models by the X-ray scattering method.

A simple approach for determining a liquid structure using X-ray scattering data, in which a liquid structure is uniquely evaluated without construction of any plausible structure models, has been applied to liquid acetonitrile, acetone and cyclohexane. For a pair of molecules, a given point within a molecule is located at the origin with a given molecular orientation. The site of the given point of another molecule is defined by the polar coordinates and the molecular orientation is treated by three Eulerian angles. These parameters are optimized by a non-linear least-squares calculation applied to X-ray scattering data. The reliability of the method was examined by determining the liquid structure of polar acetonitrile and the obtained intermolecular interatomic distances are in good agreement with the previously reported values. The method was then successfully applied to the determination of the liquid structure of acetone and cyclohexane. Especially for nonpolar cyclohexane, the construction of a variety of plausible structural models is very difficult. It was revealed that acetone has an ordered liquid arrangement similar to that found in its crystal, although the intermolecular distances in liquid acetone are different from those in the crystal. On the other hand, the liquid structure of cyclohexane is disordered.     

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO₄ under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO₄ beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO₄ as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.     

Effect of doping and temperature on the crystal structure of (V1−xMox)2O3 above and below the metal/insulator transition

The crystal structure of new molybdenum-doped vanadium sesquioxides (V_1−xMo_x)₂O₃ (0?x?0.20) has been studied at low temperature (10 K) and up to room temperature, through neutron and X-ray powder diffraction. The transition from insulating I- to metallic M-type phases, either by doping or thermally driven, is accompanied by an abrupt decrease of all interatomic distances. Within each structural type however, at 10 K, the effect of doping is essentially the same as at room temperature: it increases cation-oxygen distances, and decreases cation-cation distances, making the cationic coordination octahedra more regular. Thermal effects differ for each phase type: all interatomic distances normally increase in the M-type phase (but with different octahedral modifications depending on doping), but they decrease or remain constant in the I-type phase. This produces an unusual negative thermal expansion coefficient up to 5% at low temperature for the doped compounds.     

Über ternäre Thalliumchalkogenide mit Thalliumselenidstruktur

On Ternary Thallium Chalcogenides with Thallium Selenide Structure. By means of single crystal investigations, it has been confirmed that the compounds TlGaTe₂, TIInSe₂, and TIInTe₂ crystallize in the TISe type (B37). As an univalent cation thallium is surrounded by eight chalcogen atoms. Gallium and indium are trivalent with tetrahedral coordination and covalent bonds. The interatomic distances as well as coordination and bonding resulting from them are discussed.     

A pressure-induced nonlayered structure of indium monoselenide

An X-ray diffraction study was made on a new polymorphic phase of InSe obtained by heating a layered crystal of the 3R(γ) polytype under pressures of 40 and 50 kbar and by quenching to ambient conditions. An analysis of the diffraction intensity data collected on precession and Debye photographs gave a monoclinic structure (space group C¹_2h) with four molecules in a unit cell. Upon transformation, new, short interatomic distances characteristic of covalent bonding are formed between indium and selenium atoms that were originally located in the adjacent layers. The structure is regarded as a modified version of the nonlayered InS-type structure. In the real structure of the high-pressure phase, however, there is a small amount of disorder in the orientation of the InIn bonds. Discussion is given on the reconstructive, layer-nonlayer transformation of InSe.     

Small Tin Oxide Grains: Structural and Electronic Properties Evaluated using the Density Functional Theory

This study is motivated by the existence of many phases of tin oxide with a bulk crystalline structure and its purpose is the assessment if this property also applies to the clustered state of this material. Therefore small grains of a columnar and a spherical shape with a rutile lattice and a size up to 120 atoms have been considered and their structural and electronic properties are evaluated applying the Density Functional Theory. The central result of the calculations is that the rutile skeleton is retained starting from the size 20–40 atoms. The main factors contributing to this behaviour are the large interatomic distances in the rutile lattice and the irregular nature of the interatomic forces. Due to these effects, most of the structural and electronic parameters have a bulk-like behaviour with a nearly constant value. These values, however, have a clear dependence on the grain size and shape and significantly diverge from the bulk ones so that the grains have to be regarded as a new phase of the tin oxide materials.     

蒙特卡洛模拟退火与距离限制相结合的方法——在多肽溶液构象分析中的应用

Distance geometry and molecular dynamics are currently employed in determining molecular structures with interatomic distances from NMR NOESY experiment. Because of the flexibility of peptide, distances obtained from NMR are usually not sufficient to confine its structure. Both distance geometry and molecular dynamics will bias in the conformational space at this circumstance. Constraint Monte Carlo simulated annealing was established to solve this problem. Distance constraints were included into the ECEPP/2 force field by introducing a harmonic energy term. Conformational analysis of a pentapeptide with eight interatomic distances from NMR was carried out as a test. By comparison of the 100 conformers obtained from constraint simulated annealing and the 100 conformers from distance geometry calculation, it was found that constraint simulated annealing can cover the outcomes of distance geometry and at the same time give more con-formers fitting to the experimental data. The result shows that constraint Monte-Carlo simulated annealing is more valid in constructing peptide structures from NMR distances than currently employed methods when no sufficient distances from NMR are available.     

The Absolute Thermoelectric Power of Some Dilute Liquid Sodium Alloys

Abstract

The absolute thermoelectric powers of liquid sodium and of dilute liquid alloys of sodium with silver, cadmium, indium and tin have been measured within the temperature range 100–460°C. The thermopower of liquid sodium is decreased by the addition of cadmium, indium or tin, with cadmium having the smallest and indium the greatest effect, whereas it is increased slightly on the addition of 1 at. % of silver. Values of the dimensionless thermopower parameter, ?, derived from the experimental data, are compared with those predicted by the Faber-Ziman theory; the agreement is quite good for Na-Ag and Na-Cd but less satisfactory for Na-In and Na-Sn. The discrepancies are discussed in terms of the assumptions involved in the calculations, namely the use of only one structure factor and local dielectric screening of the ionic pseudopotentials.     

Preparation and crystal structure analysis of CeCuOS

A single phase of CeCuOS was prepared by sulfurization of starting materials, CeO₂+Cu₂S, and subsequent post-annealing in evacuated silica tubes. The crystal structure of the CeCuOS single phase was examined by the Rietveld analysis. The shrinkage of the unit-cell volume for CeCuOS was obviously observed in the series of LnCuOS (Ln=La-Nd) oxysulfides. The examination of interatomic distances revealed that Ce-S distances in CeCuOS are shorter than those predicted from the lanthanide contraction and this short Ce-S bond length is responsible for the unit-cell-volume shrinkage of CeCuOS. The origin of the shrinkage is discussed in the viewpoints of the Ce valence and interatomic distances along with the results of magnetic measurements.     

Molecular dynamics simulation of amorphous SiO2 nanoparticles

Molecular dynamics simulation of amorphous SiO2 spherical nanoparticles has been carried out in a model with different sizes, 2, 4, and 6 nm, under non-periodic boundary conditions. We use the pair interatomic potentials which have weak Coulomb interaction and Morse type short-range interaction. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. Structural properties of amorphous nanoparticles obtained at 350 K have been studied via partial radial distribution functions (PRDFs), mean interatomic distances, coordination numbers, and bond-angle distributions, which are compared with those observed in the bulk. Calculations of the radial density profile in nanoparticles show the tendency of oxygen to concentrate at the surface as observed previously in other amorphous clusters or thin films. Size effects on structure of nanosized models are significant. The calculations show that if the size is larger than 4 nm, amorphous SiO2 nanoparticles have a distorted tetrahedral network structure with the mean coordination number ZSi-O approximately 4.0 and ZO-Si approximately 2.0 like those observed in the bulk. Surface structure, surface energy, and glass transition temperature of SiO2 nanoparticles have been obtained and presented.     

Theoretical investigation of small MgO, CaO and NaCl clusters

We study the size dependence of the electronic and structural properties of small MgO, CaO and NaCl clusters (up to 12 atoms), thanks to a selfconsistent tight binding approach in which we treat electrostatic and covalent effects on the same level. We discuss the conditions under which the ionic charges get reduced and the interatomic distances are contracted or dimerized.     

Structure flexibility of iridium metal aggregates. Comparison with platinum and rhodium

The atomic structure of iridium aggregates occluded in zeolite has been studied by radial electron distribution from X-ray scattering data taken under different atmospheres. The interatomic distances in 1–2 nm naked aggregates are contracted with respect to the bulk distances. The adsorption of hydrogen produces a relaxation but a residual contraction remains unlike in the case of platinum aggregates. The carbon monoxide adsorption produces little effect on the position of surface iridium atoms unlike on platinum and rhodium. The lack of structure flexibility of iridium aggregates is probably due to the higher cohesive energy of iridium.     

TiOBr; Darstellung,Eigenschaften und Struktur*

TiOBr was prepared by reaction of Ti with TiO₂ and Br₂. The compound forms fiat reddish-brown needles and shows a temperature independent weak paramagnetism. It crystallises in the orthorhombic system (Pmmn; a = 3.787, b = 3.487, c = 8.529 Å) with the FeOCl type of structure. The interatomic distances are Ti?O = 1.952; 2.245 Å (2X) and Ti?Br = 2.544 Å (2X).     

Asymmetry of H2O Molecules in the Liquid State and Its Consequences

The correlations between the geometrical parameters of the O–H...O hydrogen bridge and the stretching frequency _OH are refined by using neutron diffraction and vibrational spectroscopy data. The distribution functions of the interatomic distances r _OH and intermolecular distances R _O...O in water in the range from –40 to 100°C were calculated from the Raman spectra of HOD. The extent of asymmetry of H₂O molecules in the liquid state, caused by fluctuations of the local environment of two OH groups, and its manifestations in the structure and vibrational spectra of water are analyzed.     

Dependence of energies on the bond lengths in molecules and crystals

The dependence of the binding energies in molecules and crystals on the interatomic distances near the equilibrium position was established in the framework of the Mie equation using the force constants of molecules and the elasticity moduli of solids. To evaluate the repulsion forces, we used the Morse function for molecules and the Born-Landé approach for ionic crystals.