X-ray diffraction study of K3NdSi7O17: a new framework silicate with a linear Si-O-Si bond.
Hydrothermal investigations in the high-silica region of the K(2)O-Nd(2)O(3)-SiO(2) system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound tripotassium neodymium heptasilicate, K(3)NdSi(7)O(17). Single-crystal X-ray methods revealed that K(3)NdSi(7)O(17) crystallizes in space group P3, has lattice constants a = 16.131 (2) and c = 7.7146 (19) A, Z = 4, and 22 atoms in the asymmetric unit. Refinement was carried out to a residual, R(F), of 0.0253 and a weighted residual, wR(F(2)), of 0.0702 using anisotropic displacement parameters for all atoms. The silicate anion forms an interrupted framework, within which both Nd octahedra and K polyhedra are situated. The structure is unusual in that it contains a symmetry-constrained Si-O-Si bond angle of 180 degrees. No isomorphs to K(3)NdSi(7)O(17) are known.
Preferred ion diffusion pathways and activation energies for Ag in the crystal structure of stephanite, Ag5SbS4
Abstract The crystal structure of stephanite from the type locality, Freiberg District, Saxony, Germany, was refined in the space group Cmc21, up to a final R index of 0.0427. Unit-cell parameters are: a 7.8329(6) Å, b 12.458(1) Å, c8.5272(7) Å, V 832.1(1) Å3; Z = 4. The previously reported structural model is confirmed, but a higher-precision refinement was achieved herein by the introduction of third-order non-harmonic Gram-Charlier tensors for one Ag atom. In the structure of stephanite, Sb forms isolated SbS3 pyramids, which typically occur in sulphosalts, and Ag occupies sites with coordination ranging from triangular to almost tetrahedral. Both the Sb-S and Ag-S bond distances closely match the values commonly observed in the structures of other Ag sulphosalts and sulphides. The use of non-harmonic parameters for Ag allowed a better description of the electron density related to Ag, which is usually difficult to refine in good ionic conductors. A careful analysis of the energy barriers between the Ag sites defines preferred ion-diffusion pathways within the crystal structure of stephanite. The diffusion of Ag ions occurs preferentially along the sites Ag1 and Ag2, giving rise to two-dimensional nets of Ag atoms in which the ion conduction probably takes place.
Les pyrochlores AIB2X6: Mise en evidence de l'occupation par le cation AI de nouvelles positions cristallographiques dans le groupe d'espace fd3m
Abstract In A I B 2 X 6 pyrochlores—A I = Pb, Tl; X = O, F— the A I ions occupe 32e positions with a probability of 25% instead of the 8b positions as it was previously described; this results from the observations of very weak X-diffraction peaks provided by planes such as h = 4n, k = 4n, l = 4n + 2, visible on single crystals photographs of RbCoCrF 6 , RbNb 2 O 5 F, TlNb 2 O 5 F; this displacement of A I with respect to the 8b position -about 0,6 A for Tl + and 0,4 A for Rb + - is in agreement with the great size of the 8b cavity -about 1,80 A-. However Cs + , the radius of which is well adapted to this cavity, is not significantly displaced in homologous compounds.