Low- and high-temperature crystal structures of TiI3

Temperature-dependent, single-crystal X-ray diffraction of TiI3 has indicated that this compound undergoes a first-order phase transition at Tc=323±2K. Accurate structural parameters are reported for the high-temperature crystal structure at T=326K (hexagonal cell, a=7.1416(5)A, c=6.5102(4)A, Z=2, space group P63/mcm) and for the low-temperature structures at both 273 and 100 K (orthorhombic symmetry, space group Pmnm, Z=4, lattice parameters at 273 K: a=12.3609(7)A, b=7.1365(5)A, c=6.5083(4)A and at 100 K: a=12.2728(7)A, b=7.0857(5)A, c=6.4817(4)A). Above Tc, TiI3 possesses the TiI3 structure type containing chains of equidistant metal atoms. A twofold superstructure develops below Tc, resulting in the RuBr3 structure type that is characterized by a dimerization of the metal chains. The magnitude of the distortion is found to be the largest amongst the known transition metal trihalides. It thus provides an explanation for the inclination of TiI3 towards the RuBr3 structure type, despite the fact that metal-metal bonds are weaker in iodides than in chlorides or bromides. - TiI3 undergoes a first-order phase transition at Tc=323K. The dimerization of chains of metal atoms at low temperatures is larger in TiI3 than in corresponding chlorides and bromides, and thus provides an explanation for the larger inclination of TiI3 towards dimerization, despite metal-metal bonds being weaker than in chlorides and bromides