Crystal structures and transition mechanism of VO2(A)

Structures of VO2(A) have been redetermined by single-crystal diffractometry for low- (LTP) and high-temperature (HTP) phases at 298 and 473 K, respectively. The LTP adopts the tetragonal system P4/ncc with a = 8.4403(9) Angstrom, c = 7.666(1) Angstrom, and Z=16, whereas the HTP adopts the body-centered tetragonal system I4/m with a=8.476(2)Angstrom, c=3.824(2)Angstrom, and Z= 8, The refinements led to R/R-w = 0.031/0.032 for LTP and 0.012/0.033 for HTP, The structures of both phases consist of edge-sharing VO6 octahedra and exhibit quite similar oxygen frameworks. Through the transition the V4+-V4+ bonding in LTP with a distance of 2.7695(8)Angstrom is dissociated in HTP to a distance of 3.0794(3)Angstrom. The transition occurs with cooperative movements of the V atoms, namely, a rotation around the c axis and a shift along the c axis. Strangely, twinning is induced on the LTP to HTP transition but disappears on the reverse transition. (C) 1998 Academic Press.