Crystal structures and phase transitions of (NO)2TiCl6

Abstract

Abstract Solid (NO)2TiCl6 undergoes two phase transitions closely related to the order-disorder behaviour of the NO+ species present in this salt-like compound. Starting from ambient conditions the sequence of space groups is P4/mnc → C2/c → P21/n when the temperature is lowered. At Tc 1 = 199 K the formation of quadruplet domains is observed, while at Tc 2 = 176 K superstructure reflexions appear due to decentring of the unit cell. In the tetragonal high-temperature form the orientational disorder of the nitrosonium groups is fourfold, partial order then starts to develop in the intermediate phase, whereas almost complete order is reached at 123 K in the second monoclinic phase. This two-step symmetry reduction was studied by DSC, powder diffraction, and by single-crystal measurements including complete structure determinations at 291, 203 and 123 K. In P21/n the selection of just one NO+ orientation per site is coupled to a characteristic displacement pattern of the titanium atoms. For electrostatic reasons the latter show a response to short N···Cl contacts by an off-centre shift (ca. 11pm) inside their 'octahedral' chlorine cages. The stereochemical influence of antibonding NO+ orbitals determines the inclination of the cation towards the nearest chlorine neighbours.