Mixed valence oxide ((Sb,As) 2O)Mo 45+Mo 66+O 30: synthesis, crystal structure and physical properties

Abstract

Substitution of arsenic for antimony in the Sb 2MoO 6 and Sb 2Mo 10O 31 catalysts was undertaken. Taking as starting point the composition of biehlite Sb 1.79As 0.21MoO 6, different nominal Sb 2− x As x MoO 6 (0⩽ x⩽1) compositions were studied. All attempts led to mixtures of orange (Sb 2MoO 6) and black ((Sb,As) 2Mo 10O 31) crystals, indicating that arsenic can only substitute for antimony in the Sb 2Mo 10O 31 framework. The crystal structure of Sb 1.82As 0.18Mo 10O 31 was determined by single-crystal X-ray diffraction technique to be isostructural with undoped orthorhombic Sb 2Mo 10O 31 (CCD data, R=0.0394 for 2657 observed reflections). The structure consists of a three-dimensional framework of corner-sharing MoO 6 octahedra, with channels accommodating SbO(Sb,As) 4+ groups which firmly hold together by edge-sharing pairs of MO 6 octahedra belonging to columns parallel to b axis. The lone pairs of electrons of each pnictogen atom face each other inside the tunnels, at a distance similar to that of an oxygen–oxygen contact. The magnetic and the electric properties correspond to a mixed valence compound.