Transformation of the crystal structure of Ni_0.5TiSe₂ under in situ heating

The crystal structure of the Ni_0.5TiSe₂ compound has been studied in situ in the temperature range of 25-1000 °C using synchrotron radiation X-ray diffraction. The previously known order-disorder transition in the Ni sublattice at ∼100 °C was found to be a second-order phase transition and belongs to the 3D Ising universality class. Reversible extraction of nickel selenides was observed in the temperature range of 275-975 °C. It was explained in terms of Ni extraction from Ni_0.5TiSe₂ due to the thermal widening of the Ni 3d/Ti 3d/Se 4p impurity band. The Ni-TiSe₂ phase diagram follows the typical pattern of MTiSe₂ (M - transition metal) compounds.