Low- and high-temperature crystal structures of image

Temperature-dependent, single-crystal X-ray diffraction of TiI3TiI3 has indicated that this compound undergoes a first-order phase transition at View the MathML sourceTc=323±2K. Accurate structural parameters are reported for the high-temperature crystal structure at View the MathML sourceT=326K (hexagonal cell, View the MathML sourcea=7.1416(5)A˚, View the MathML sourcec=6.5102(4)A˚, Z=2Z=2, space group P63/mcmP63/mcm) and for the low-temperature structures at both 273 and 100 K (orthorhombic symmetry, space group PmnmPmnm, Z=4Z=4, lattice parameters at 273 K: View the MathML sourcea=12.3609(7)A˚, View the MathML sourceb=7.1365(5)A˚, View the MathML sourcec=6.5083(4)A˚ and at 100 K: View the MathML sourcea=12.2728(7)A˚, View the MathML sourceb=7.0857(5)A˚, View the MathML sourcec=6.4817(4)A˚). Above TcTc, TiI3TiI3 possesses the TiI3TiI3 structure type containing chains of equidistant metal atoms. A twofold superstructure develops below TcTc, resulting in the RuBr3RuBr3 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 TiI3TiI3 towards the RuBr3RuBr3 structure type, despite the fact that metal–metal bonds are weaker in iodides than in chlorides or bromides.