HYDROTHERMAL SYNTHESIS, STRUCTURE, AND MAGNETIC-PROPERTIES OF A NEW POLYMORPH OF LITHIUM VANADYL(IV) ORTHOPHOSPHATE - BETA-LIVOPO4

A new polymorph of lithium vanadyl(IV) orthophosphate, β-LiVOPO4, has been synthesized hydrothermally in a sealed quartz glass tube under autogeneous pressure by slowly cooling from 450°C a mixture of Li2O, VO2, and 7.25 M H3PO4. This metastable compound has been characterized through a combination of techniques including single-crystal X-ray diffraction, differential thermal analysis, electron paramagnetic resonance, and magnetic susceptibility measurements. It crystallizes in the orthorhombic space group Pnma with a = 7.444(2), b = 6.300(1), c = 7.174(2) A ̊, V = 336.4(2) A ̊3, Z = 4, and R = 0.0248. Its framework consists of infinite chains of corner-shared VO6 octahedra, parallel to the a-axis, cross-linked by corner-shared PO4 tetrahedra. These infinite chains have alternating short and long VO bonds and are similar to those found in β-VOPO4. The Li atom has six close oxygen neighbors. The β → α-LiVOPO4 phase transition is reconstructive and occurs at ∼750°C. The magnetic susceptibility is analyzed considering the expression of Bonner and Fisher for S = 1 2 ions isotropically coupled in antiferromagnetic linear chains. © 1991.