Abstract :
(B) VO2 crystallizes in the monoclinic system, C2/m space group with a=12.093(1) ?, b=3.7021(2) ?, c=6.4330(5) ?, and beta=106.97(1°) as cell parameters. (A) VO2 with two allotropic varieties crystallizes in the tetragonal system, the low temperature form AL adopts P4/ncc space group with a=8.4403(9) ? and c=7.666(1) ? and AH the space group I4/m with a=8.476(2) ? and c=3.824(2) ?. In these structures, VO6 octahedra are associated in quadruple strings by edge sharing in the former structure whereas they form cross-linked double strings in the latter; both endless string types develop along parameters b or c (~3.8 ? periodicity). In the first association they build the so-called double layers D4 in (B) VO2 and Dz in (A) VO2, parallel to (001) and (110) planes, respectively, and associated, in their perpendicular direction, by corner sharing. These D4 layers exhibit the weakest V–O bonding perpendicular to their medium plane (parallel to (001)) allowing a crystallographic slip Cs–1/3 [100] (001) to occur in the structure, transforming (B) VO2 into (A) VO2. Design of (B) and (A) crystal structures via crystallographic shear mechanisms is related to chemical experimental conditions of formation: temperature, pressure, and stress.
