Ab initio structures of (M2) and (M3) VO2 high pressure phases

Abstract (M2) and (M3) VO2 high pressure phases have been obtained at 65kbars by Chamberland. Both phases crystallize in the monoclinic system, (M2) with the space group C2/m and unit cell dimensions a = 9.083A, b = 5.763A, c = 4.532A, β = 90.3 ° whereas (M3) belongs to P2/m with a = 4.506A, b = 2.899A, c = 4.617A and β = 91.79 °. Based on this data ab initio structures have been elaborated and adjusted to fit their experimental powder patterns. (M2) structure exhibits two crystallographically different infinite parallel [VO4]n4n− strings of VO6 edge shared octahedra interconnected by apices alike in the rutile structure but the oxygens are hexagonally close packed. The (M3) variety shows also two different [VO4]n4n− strings but the general network now is rutile like slightly distorted. Vanadium atoms are situated in distorted oxygen octahedra, the V O bond lengths ranging from 1.66A to 2.15A with the V4+-V4+ pairing, V1-V1 = 2.70A and V2-V2 = 2.89A in (M2). In (M3) the V O distances range from 1.75A to 2.10A and V1-V1 = V2-V2 = 2.90A. The homopolar V4+ pairs evidenced in the (M2) form and the general unsymmetrical arrangement of oxygen about V4+ are in excellent agreement with the unusual physical properties of these two high pressure varieties.