Single-crystal structure refinements and Debye temperatures of Ir2S3 kashinite and Rh2S3 bowieite.

Single crystals of Ir2S3 (diiridium trisulfide) and Rh2S3 (dirhodium trisulfide) were grown in evacuated silica-glass tubes using a chemical transport method and their crystal structures were determined by single-crystal X-ray diffraction analysis. These compounds have a unique sesquisulfide structure in which pairs of face-sharing octahedra are linked into a three-dimensional structure by further edge- and vertex-sharing. Ir2S3 and Rh2S3 had similar unit-cell parameters and bond distances. The atomic displacement parameter (MSD: mean-square displacement) of each atom in Ir2S3 was considerably smaller than that in Rh2S3. The Debye temperatures (ΘD) estimated from the observed MSDs for the Ir, S1 and S2 sites in Ir2S3 were 259, 576 and 546 K, respectively, and those for Rh, S1 and S2 in Rh2S3 were 337, 533 and 530 K, respectively. The bulk Debye temperature for Ir2S3 kashinite (576 K) was found to rank among the higher values reported for many known sulfides. The bulk Debye temperature for Rh2S3 bowieite (533 K) was lower than that for Ir2S3 kashinite, which crystallizes in the early sequences of mineral crystallization differentiation from the primitive magma in the Earth's mantle.

Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au-Te and Te-Te bonds in Au1-xAgxTe2 group minerals.

The structure refinement and XANES study of two gold-silver-tellurides [Au1+xAgxTe2, krennerite (x = 0.11-0.13) and sylvanite (x = 0.29-0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08-0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te-Te bonds (bond distances < 2.9 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non-bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te-Te bond in the [Te3]2- anion is not entirely homopolar. Both tellurides can therefore be described as Madelung-type compounds, despite the presence of Te-Te in both structures.

Crystal structure refinements of stoichiometric Ni3Se2 and NiSe.

Single crystals of Ni3Se2 (trinickel diselenide) and NiSe (nickel selenide) with stoichiometric chemical compositions were grown in evacuated silica-glass tubes. The chemical compositions of the single crystals of Ni3Se2 and NiSe were determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). The crystal structures of Ni3Se2 [rhombohedral, space group R32, a = 6.02813 (13), c = 7.24883 (16) Å, Z = 3] and NiSe [hexagonal, space group P63/mmc, a = 3.66147 (10), c = 5.35766 (16) Å, Z = 2] were analyzed by single-crystal X-ray diffraction and refined to yield R values of 0.020 and 0.018 for 117 and 85 unique reflections, respectively, with Fo > 4σ(Fo). R32 is a Sohncke type of space group where enantiomeric structures can exist; the single-domain structure obtained by the refinement was confirmed to be correct by a Flack parameter of -0.05 (2). The existence of Ni-Ni bonds was confirmed in both compounds, in addition to the Ni-Se bonds. The value of the atomic displacement parameter (mean-square displacement) of each atom in NiSe was larger than that in Ni3Se2. The larger amplitude of the atoms in NiSe corresponds to longer Ni-Se and Ni-Ni bond lengths in NiSe than in Ni3Se2. The Debye temperatures, θD, estimated from observed mean-square displacements for Ni and Se in Ni3Se2, were 322 and 298 K, respectively, while those for Ni and Se in NiSe were 246 and 241 K, respectively. The existence of large cavities in the structure and the weak bonding force are likely responsible for the brittle and soft nature of the NiSe crystal.