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

(M2) and (M3) VO2 high pressure phases have been obtained at 65kbars by Cha mberland. Both phases crystallize in the monoclinic system, (M2) with the s pace group C2/m and unit cell dimensions a = 9.083 Angstrom, b = 5.763 Angs trom c = 4.532 Angstrom, beta = 90.3 degrees whereas (M3) belongs to P2/m w ith a = 4.506 Angstrom, b = 2.899 Angstrom, c = 4.617 Angstrom and beta = 9 1.79 degrees, Based on this data ab initio structures have been elaborated and adjusted to fit their experimental powder patterns. (M2) structure exhi bits two crystallographically different infinite parallel [VO4](n)(4n-) str ings of VO6 edge shared octahedra interconnected by apices alike in the rut ile structure but the oxygens are hexagonally close packed, The (M3) variet y shows also two different [VO4](n)(4n-) strings but the general network no w is rutile like slightly distorted, Vanadium atoms are situated in distort ed oxygen octahedra, the V-O bond lengths ranging from 1.66 Angstrom to 2.1 5 Angstrom with the V4+-V4+ pairing, V1-V1 = 2.70 Angstrom and V2-V2 = 2.89 Angstrom in (M2). In (M3) the V-O distances range from 1.75 Angstrom to 2. 10 Angstrom and V1-V1 = V2-V2 = 2.90 Angstrom. The homopolar V4+ pairs evid enced in the (M2) form and the general unsymmetrical arrangement of oxygen about V4+ are in excellent agreement with the unusual physical properties o f these two high pressure varieties.