Crystal structures, order-disorder transition and twinning of the Jahn-Teller system (NO)2VCl6

Abstract

Abstract As in the case of (NO)2TiCl6 the octahedral-shaped crystals of dinitrosonium hexachlorovanadate(IV) are tetragonal at room temperature with space group P4/mnc and Z = 2. An X-ray structure determination at 248 K yields lattice parameters of a = 667.8(1) and c = 1023.8(1) pm, V–Cl distances for the ‘octahedral’ VCl6 2- ion of slightly different length [2 × 233.2(1), 4 × 229.4(1) pm], and a fourfold orientational disorder for the species NO+. The phase change at T c = 236.5 ± 0.2 K is very sharp and clearly 1st order leading to a monoclinic unit cell with a = 657.1(3), b = 662.6(2), c = 1027.5(2) pm, and β = 91.82(1)° at T = 156 K in space group P21/n with Z = 2. As expected for vanadium in the oxidation state +4 the paramagnetism of the black compound measured above and below T c confirms the spin-only value for a d1 system. The compressive distortion of the VCl6 2- units to approximate D 4h symmetry [2 × 234.3(1), 2 × 233.7(1), 2 × 224.1(1) pm at 156 K] favours the ordering process of the NO+ cations and vice versa. For the ordered NO+ the atomic distance is 107.0(3) pm. Repetition of the structure analysis at 231 K demonstrates that the static manifestation of the Jahn-Teller effect is coupled to the tilt of the VCl6 2- groups. In accordance with the symmetry reduction in formally two steps of type t2, the formation of quadruplet domains is observed, usually arranged around [001] as the common axis giving a fourfold rotation twin with c tetr parallel c mon and a mon slightly inclined against the tetragonal a–b plane. If the crystal is clamped along c the twin law changes to the corresponding reflection twin (001).