POWDER X-RAY CRYSTAL-STRUCTURE OF VO2(A)

VO2(A), a metastable phase of vanadium dioxide, was synthesized by hydrothermal treatment of VO(OH)2. The powder X-ray structure analysis of VO2(A) has been performed by the use of EXAFS and the Rietveld method; possible structure models based on the EXAFS results were examined by the Rietveld analysis. VO2(A) has a tetragonal symmetry with a space group P42 nmc; a = 8.4336(7) Å, c = 7.6782(7) Å, and Z = 16. The structure consists of a three-dimensional framework of VO6 octahedra. Four units of two edge-sharing octahedra are linked to each other by sharing corners forming a 2 × 2 square block in the c-plane. The blocks are piled up along the c-axis by sharing edges of the octahedra resulting in zigzag chains of V ions running along the c-axis. Taking the revealed structure into consideration, a comparison with the structure of VO2(B) is made and a tentative mechanism of the phase transition in VO2(A) is proposed. © 1990.