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In this work, the electronic structure and magnetic properties of the Mn-Al alloys with different Mn and Al concentrations in the β-Mn-type structure have been theoretically investigated. The structural stability of different types of aluminum and manganese atoms occupations of the 12d and 8c site positions are compared. For the Mn3Al with 5Al in the 12d positions we obtained the most energetically preferable arrangement of Mn and Al ions in this alloy. Further occupation of this type of position by the Al atoms might be responsible for the lower effective magnetic moment estimated from the experimental data. Our theoretical calculations demonstrate the complex character of the magnetic properties and electronic structure of the Mn-Al alloys.

A new structural model of the high-temperature ordered titanium monoxide phase β-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 α-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.