Crystal Structures and 3d Electron Configurations for the LixV2(PO4)3 Insertion Electrode System with a Semi-Double-Electron Reaction

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The crystal structures and electronic properties of the LixV2(PO4)3 or LixV2P3O12 insertion electrode system that exhibits a semi-double-electron reaction are explored through measurements of x-ray four-circle diffraction and magnetization. The structure for the monoclinic crystal with composition x = 3 is refined precisely and those for the orthorhombic crystals with x = 2.42, 2.22, and 2 are determined. The monoclinic--orthorhombic phase transition in the range of 2.5 \leq x \leq 2.7 is accompanied by the disordered site of Li, which vanishes at x = 2. This may be responsible for a step-like structure for the charge process. The effective valences estimated for 2 \leq x \leq 3 are consistent with the values expected on the basis of ionic crystals, where the V ions exhibit an average valence. The electron configurations of V ions for 0.9 \leq x \leq 3 are considered in the framework of intermediate pseudo-trigonal crystal field approach. For x = 3 and 1 with respective valences of V3+ and V4+, the trigonal field splittings are of the order of 10^{3} cm-1 and the covalent bonding for the direction normal to the pseudo-trigonal axis is significant, which may be considered a common nature for the vanadium-phosphate insertion-electrode system.
2012-09-15