Refinement of the Structure of Mg3(VO4)2

Abstract

Magnesium orthovanadate crystallizes in the orthorhombic space group Cmca with a = 6.053(3), b = 11.442(6), c = 8.330(3) Å, and Z = 4. The structure was refined by full matrix least square methods to a final R value of 0.047 using 1697 reflections measured by a quarter circle General Electric automatic diffractometer. The structure was assumed to be isomorphous with that of Ni3(VO4)2, reported by Durif and Bertaut (2), and the refinement started using the same fractional atomic coordinates. The structure consists of nearly cubic closest packing of oxygen atom layers roughly normal to the [Formula: see text] direction with the Mg ions in two types of octahedral sites and the V ion in a tetrahedral site. The VO4 group has a crystallographic mirror plane. The cations lying on sites of Cm symmetry show an average Mg—O bond length of 2.092 Å, compared to 2.098 Å for cations lying at sites of C2h symmetry. The anion deviates significantly and substantially from regularity with the three oxygen atoms shared with only two cations showing V—O bond lengths within 0.015 Å of their mean, 1.70 Å, whereas the remaining oxygen atom, which is shared with three Mg ions, lies 1.809 Å from the V ion. The structure has characteristics similar to those of spinel.