Synthesis of new high-pressure columbite phases containing pentavalent vanadium

Abstract

A series of MV 2O 6 compounds with M = Ni, Mg, Co, Zn, Mn, and Cd having the orthorhombic columbite-type structure has been prepared at pressures of 50–80 kbar and temperatures of 800–1200°C. CuV 2O 6II was found to have a monoclinic distortion with probable space group P2 1/ c and a = 4.824(1), b = 13.483(2), c = 5.652(1)A˚and β = 91.02(2). The transition from the brannerite to columbite-type structures involves an increase of V 5+ coordination from 5 + 1 → 6, a change in oxygen packing from cubic close-packed to hexagonal close-packed, a retention of part of the structure of the VO 6 sheets and a collapse of the MO 6 rutile-like chanins into αPbO 2 chains. The relative stabilities of the trirutile, columbite, and PbSb 2O 6 structures are discussed in terms of electrostatic repulsion forces, polarization, and covalency. It is concluded that minimization of cation-cation repulsion for d 10 ions cannot be responsible for the stability of the trirutile structure, since the d 10 ions As 5+ and Sb 5+ are frequently found in the PbSb 2O 6 structure in which each Sb 5+ or As 5+ ion has 3 Sb 5+ or As 5+ neighbors. In plots of r M 3 vs unit cell volumes for MV 2O 6 and MNb 2O 6 compounds, the volume of the Mg-containing compound always lies above the line connecting the other compositions. This deviation is attributed to the relatively greater covalence and consequent shortening of M O bonds where M = Ni, Co, Zn, Mn, and Cd.