SYNTHESIS AND STRUCTURAL EVOLUTION OF THE SOLID-SOLUTION THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-2.5)(XO20 (O)

A new solid solution of general formula Bi(Bi12-xTexO14) Mo4-xV1+xO20 with x up to 2.5 has been prepared by simultaneous substitution of the couple Te4+-V5+ for Bi3+-Mo6+ in Bi13Mo4VO34. These materials have be en studied by X-ray powder diffraction and thermal analysis, showing a structural evolution with changing x values. Crystal growth of these phases has been carried out; the single crystals have been studied by transmission electron microscopy and their structure determined by X-r ay single crystal diffraction techniques performed for the x = 1 and x = 2 samples, Both materials crystallize in a monoclinic unit cell, sp ace groups P2 for Bi(Bi11TeO14) Mo3V2O20 (x = 1) and P2/c for Bi(Bi10T e2O14) Mo2V3O14 (x = 2), with respective unit-cell parameters a = 11.7 04(9), b = 5.820(1), c 12.16(1) Angstrom, beta = 100.90(2)degrees (Z = 1) and a = 11.642(2), b = 5.771(1), c 24.22(1) Angstrom, beta = 101.1 6(4)degrees (Z = 2), The structures keep the basic framework of ''colu mns'' present in the parent oxide Bi(Bi12O14)Mo4VO20; they are built u p by infinite (Bi12-xTexO14,) columns along the [010] direction and su rrounded by independent (Mo, V) O-4 tetrahedra organized in layers par allel to (100) and (001) planes. The extra Bi cations are located at t he intersection of these layers. The x = 1 compound exhibits original structural features due to its acentric structure. The observed struct ural evolution can be correlated to the different charge balance betwe en columns (Bi12-xTexO14)((8+x)+) and tetrahedral environment, as well as to the lower stereochemical activity of the 6s(2) lone pair of ele ctrons of Bi3+ relative to the 5s(2) electrons of Te4+ cation. (C) 199 8 Academic Press.