Abstract
Temperature-dependent, single-crystal X-ray diffraction of TiI 3 has indicated that this compound undergoes a first-order phase transition at T c = 323 ± 2 K . Accurate structural parameters are reported for the high-temperature crystal structure at T = 326 K (hexagonal cell, a = 7.1416 ( 5 ) A ˚ , c = 6.5102 ( 4 ) A ˚ , Z = 2 , space group P 6 3 / 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 T c , TiI 3 possesses the TiI 3 structure type containing chains of equidistant metal atoms. A twofold superstructure develops below T c , resulting in the RuBr 3 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 TiI 3 towards the RuBr 3 structure type, despite the fact that metal–metal bonds are weaker in iodides than in chlorides or bromides.
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