Crystal structures of TiI3 were determined and refined at selected temperatures between 4 and 340 K against neutron powder diffraction data. On heating, thermal expansion is found to increase abruptly at the phase transition at Tc = 323 K. Most prominently, linear thermal expansion is highly anisotropic and small along c (i.e. along chains of Ti atoms) in the low-temperature phase, while above Tc it is much larger along c. An order parameter is defined by the ratio between the long and the short Ti—Ti distances on the metal chains. The temperature dependence of the order parameter indicates the transition to be of first order.
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