New structural model of β-TiO: Superposition of M5X5 superstructures

A new structural model of the high-temperature ordered titanium monoxide phase beta-TiO is proposed. This phase has monoclinic symmetry and belongs to the space group P1m1. Its unit cell is a three-fold unit cell of the basis crystal structure B1 of titanium monoxide. The proposed structural model is a superposition of M5X5 ((mon)) and M5X5 ((cub)) superstructures, the former of which corresponds to the structure of the low-temperature ordered phase alpha-TiO and the latter is an additional structure increasing the contribution from the configuration entropy to the free energy with a rise in temperature. In comparison with the familiar simple structural models (M5X5 ((mon)) and M5X5 ((cub)) superstructures), the new model describes more accurately the behavior of structural vacancies at elevated temperatures and explains the reason of the misfit between the experimental and theoretical X-ray diffraction patterns of the high-temperature ordered phase.