The Crystal Structure of Defect KBB’O 6 Pyrochlores (B,B’: Nb,W,Sb,Te) Revisited from Neutron Diffraction Data

摘要

Three defect pyrochlores KNbWO 6 · x H 2 O, KNbTeO 6 and KSbWO 6 were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K 2 C 2 O 4 , Sb 2 O 3 , Nb 2 O 5 , WO 3 and 20% excess TeO 2 . A neutron powder diffraction (NPD) data analysis allowed unveiling some structural features. They are all defined in the cubic F d 3 ˉ m space group symmetry, with α = 10.5068(1) ?, 10.2466(1) ? and 10.2377(1) ?, respectively. Difference Fourier synthesis for KNbWO 6 ·xH 2 O clearly showed the presence of crystallization water, with extra O’ oxygen and H + atoms that were located from NPD data. These O’ oxygen atoms are placed at 32 e Wyckoff sites, conforming a K 2 O’ sublattice interpenetrated with the covalent framework constituted by (Nb,W)O 6 octahedra. The H + ions coordinate the O’ atoms at partially occupied 96 g Wyckoff sites while K + ions shift also along 32 e sites, but closer to the 16 c special site (0,0,0). By contrast, extra H 2 O molecules are absent in the other two pyrochlores: in KNbTeO 6 and KSbWO 6 K + ions are shifted along 32 e (x,x,x) sites further away from the origin than for the previous material, and the higher covalency of the octahedral network determines more compact structures, with shorter B –O distances and narrower B –O– B angles in the proposed AB 2 O 6 defect pyrochlore structure.