DEFECT CLUSTERING IN THE SUPERIONIC CONDUCTOR LITHIUM GERMANIUM VANADATE

The defect structure of Li3.5Ge0.5V0.5O4 has been determined at 298 and 573 K using high-resolution powder neutron and synchrotron X-ray diffraction techniques. The compound is one member (x = 0.5) of an extensive solid-solution range with the general formula Li(3 + x)Ge(x)V(1 - x)O4 and belongs to the family of Li+-ion-conducting solids known as the gamma-phases. The structure was refined in the orthorhombic space group Pnma (No. 62), Z = 4, with cell dimensions obtained from neutron data at 298 K of a = 10.8714 (2), b = 6.2606 (1) and c = 5.1258 (1) angstrom, V = 348.87 angstrom 3, M(r) = 150.05, D(x) = 2.858 g cm-3, and at 573 K of a = 10.9326 (2), b = 6.3016 (1) and c = 5.1593 (1) angstrom, V = 355.44 angstrom 3, D(x) = 2.805 g cm-3. The final R factors for the neutron data at 298 K were R(wp) = 5.33, R(ex) = 2.87% and at 573 K R(wp) = 4.27, R(ex) = 4.17%. The lithium-rich defects present are based on two structural elements. At 298 K these seem to be combined to form one defect cluster while at 573 K at least two defect clusters must be present. The defects differ from those discussed in previous studies of another gamma-solid-solution LISICON, Li3Zn0.5GeO4.