X-ray powder diffraction and electron diffraction studies of the thortveitite-related L phase, (Zn,Mn)2V2O7

The phase designated gamma-Zn3(VO4)2 reported as a minor second phase in zinc oxide-based varistor materials doped with vanadium oxide and manganese oxide is shown to be the L phase, (Zn(1-x)Mn(x))2V2O7 (0.188 < x < 0.538), in the pseudo-binary Mn2V2O7-Zn2V2O7 system. Analysis of X-ray powder diffraction patterns and electron diffraction patterns of this phase shows that the previously published a, c and beta values for this thortveitite-related phase are incorrect. Instead, Rietveld refinement of the X-ray powder pattern of the L phase shows that it has a monoclinic C lattice with Z = 6, with a = 10.3791 (1), b = 8.5557 (1), c = 9.3539 (1) A and beta = 98.467 (1) degrees. Although prior convergent-beam electron diffraction work of 'gamma-Zn3(VO4)2' confirmed the C Bravais lattice, the space group was found to be Cm rather than C2/m, the difference perhaps arising from the inability of the X-rays to detect small displacements of oxygen. Attempts to refine the structure in Cm did not produce improved R factors. The relationship between the crystal structure of the L phase and the high-temperature C2/m beta'-Zn2V2O7 thortveitite-type solid solution is discussed.