Dimerization and charge order in hollandite K2V8O16.

The metal-insulator transition occurring in hollandite K2V8O16 has been studied by means of neutron and x-ray diffraction as well as by thermodynamic and electron-spin resonance measurements. The complete analysis of the crystal structure in the distorted phase allows us to identify dimerization as the main distortion element in insulating K2V8O16. At low-temperature, half of the V chains are dimerized perfectly explaining the suppression of magnetic susceptibility due to the formation of spin singlets. The dimerization is accompanied by the segregation of charges into chains.

The role of oxygen stoichiometry on phase stability, structure, and magnetic properties of Sr2CoIrO(6-δ).

The phase stability, crystal structure, and magnetic properties of perovskite-like nonstoichiometric Sr(2)CoIrO(6-δ) were studied. Oxygen deficiency can be well controlled and reversibly varied up to δ = 0.33. A single phase exists at least for partial oxygen pressures between 10(-5) and 1 bar at 1273 K, followed by phase decomposition at higher temperature with the elimination of metallic Ir and the formation of a new phase with approximately Sr(3)CoIrO(6) composition crystallizing in K(4)CdCl(6) structure type. The structural features of Sr(2)CoIrO(6-δ) are dependent on both temperature and oxygen content and were determined by synchrotron and neutron powder diffraction. Both the increasing amount of oxygen vacancies at constant temperature and increasing temperature at constant oxygen content result in the same higher crystal symmetry of Sr(2)CoIrO(6-δ): (1) The oxygen-stoichiometric phase Sr(2)CoIrO(6.00) is monoclinic (I2/m or P2(1)/n) at room temperature but cubic (Fm-3m) for Sr(2)CoIrO(5.67). (2) A sequence of phase transitions [Formula: see text] was observed for Sr(2)CoIrO(6.00) in air. All Sr(2)CoIrO(6-δ) compositions show weak ferromagnetism at low temperature with a canted but predominantly antiferromagnetic ground state. The magnetic ordering temperature decreases monotonously with increasing oxygen deficiency, while pronounced extrema are observed for the paramagnetic moment and the Curie-Weiss temperature at an oxygen deficiency δ ≈ 0.10, which corresponds to the P2(1)/n ↔ I2/m phase transformation.

Crystal structure of ZnWO(4) scintillator material in the range of 3-1423 K.

The behaviour of the crystal structure of ZnWO(4) was investigated by means of synchrotron and neutron powder diffraction in the range of 3-300 K. Thermal analysis showed the sample's melting around 1486 K upon heating and subsequent solidification at 1442 K upon cooling. Therefore, the structure was also investigated at 1423 K by means of neutron diffraction. It is found that the compound adopts the wolframite structure type over the whole temperature range investigated. The lattice parameters and volume of ZnWO(4) at low temperatures were parametrized on the basis of the first order Grüneisen approximation and a Debye model for an internal energy. The expansivities along the a- and b-axes adopt similar values and saturate close to 8 × 10(-6) K(-1), whereas the expansion along the c-axis is much smaller and shows no saturation up to 300 K. The minimum expansivity corresponds to the direction close to the c-axis where edge-sharing linkages of octahedra occur.

Anomalous thermal expansion in rare-earth gallium perovskites: a comprehensive powder diffraction study.

Crystal structures of rare-earth gallium perovskites LaGaO(3), PrGaO(3), NdGaO(3) and Pr(1-x)Nd(x)GaO(3) (x = 0.25, 0.50, 0.75) solid solutions were investigated in the temperature range 12-300 K by high-resolution powder diffraction using synchrotron or neutron radiation. The previously reported negative thermal expansion in the b direction of the PrGaO(3) lattice has been found to be persistent in Pr(1-x)Nd(x)GaO(3) solid solutions and its magnitude has been revealed as proportional to the amount of praseodymium. Evaluation of the obtained temperature evolution of cell dimensions indicated a weak anomalous behaviour of the b lattice parameter in NdGaO(3), and its origin is supposed to be the same as in PrGaO(3), i.e. a coupling of the crystal electric field levels with phonon excitations of about 23-25 meV energy. The performed bond length analysis revealed an anomalous behaviour of both LnO(12) (Ln-rare-earth) and GaO(6) coordination polyhedra, which can be a structural manifestation of anomalous thermal expansion in the considered compounds.

CaCrO3: an anomalous antiferromagnetic metallic oxide.

Combining infrared reflectivity, transport, susceptibility, and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. Local spin density approximation calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizable pd hybridization remarkably strong next-nearest-neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magnetoelastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.

Geometric and electronic structure of lanthanide orthophosphate nanoparticles determined with X-rays.

The evolution of the geometric and electronic structures within the entire series of lanthanide orthophosphate nanoparticles ( approximately 2- approximately 5 nm) has been determined experimentally with X-ray diffraction and near edge X-ray absorption fine structure spectroscopy. In particular, the interplay between electronic structure, crystal morphology, and crystal phase has been systematically studied. A missing local order in the crystal structure accompanied by multiple ion sites in the nanoparticles was revealed to be due to the small crystal size and large surface contribution. All lanthanide ions were found to be in "3+" configuration and accommodated in three different crystallization states: the larger lanthanide ions (La, Ce, Pr, Nd, Sm) in the monoclinic phase, the smaller ones (Er, Tm, Yb, Lu) in the tetragonal phase, and the intermediate lanthanide ions (Eu, Gd, Tb, Dy, Ho) in a "mixed phase" between monoclinic and tetragonal phases.

Lattice vibrations in an α- and ß-AgCuS superionic conductor: experimental time-of-flight inelastic neutron scattering studies.

Inelastic neutron scattering (INS) measurements of a powder specimen of cation conducting AgCuS have been performed in its ß-phase at 298 and 348 K and in its α-phase at 398 K by neutron time-of-flight (TOF) scattering. The neutron-weighted phonon density of states, G(ε), of the superionic and non-superionic phases of AgCuS has been obtained. G(ε) reveals a non-Debye behaviour at low energy that is caused by the presence of low-energy modes at 2.6 meV observed in the dynamic structure factor. The origin of this mode is not clear. The [Formula: see text] phase transition at 366 K is accompanied by considerable softening of several modes and simultaneous broadening of all peaks that result in a large increase of quasi-elastic intensity. Thermodynamic variables have been derived on the basis of G(ε).