The crystal structure of the 42-Å subcell of a layer structure with approximate composition Ba 4Nb 2S 9

Abstract

Platy crystals from the products of a mixture 4 Bas : 2 Nb : 5 S reacted at 1000°C have cell constants a = 13.754(3) Å, c = 83.73(2) Å, R 3m . The reciprocal lattice had a pronounced subcell with dimensions a = 6.877(1) Å, c = 41.84(1) Å, same space group. Three dimensional X-ray diffraction data were collected using monochromatized Mo Kα radiation and of 5051 measured intensities 1892 were considered observed. From the set of observed intensities 611 reflections having all even indices were used to refine the crystal structure of the 42 × 7-Å subcell. The final R = 0.036 and ωR = 0.052 for the 611 observed amplitudes and R = 0.046, ωR = 0.052 for all 711 amplitudes of the subcell. The structure is based on the stacking of hexagonal BaS 3 layers with the sequence D A B A B D B C B C D C A C A D. The D layer denotes a disordered level and occurs at z = 0, 1 3 and 2 3 . The different letters for the ordered layers are based on the Ba positions in that layer. The Nb ions occupy octahedral interstices and form a unit of three face sharing octahedra parallel to c. The column is terminated above and below by disordered levels. The NbNb distances are 3.22 Å, causing displacement of Nb from the centers of the two outside octahedra. One Ba is in the center of a triangular orthobicupola formed by 12 S atoms. The other Ba is in the center of a hexagon of 6 S with 3 additional S above this layer forming 1 2 of a cuboctahedron. The lower half consists of a disordered layer of atoms. The NbS distances are 2.279, 2.433, and 2.683 Å; BaS distances vary between 3.1 and 3.5 Å. The subcell content based on the ordered structure only is Ba 12Nb 9S 36. The placement of disordered Ba and S at z = 0, 1 3 , and 2 3 levels of the subcell leads to the unlikely composition Ba 16.5Nb 9S 42. The ordered structure most likely has a composition Ba 4Nb 2S 9, z = 36, so that the subcell composition should be Ba 18Nb 9S 40.5. The completely ordered structure has not been solved.