Order–order transition structural state in titanium monoxide TiO$_{1.0}$

A new class of defect structures in which point defects of a crystal lattice simultaneously occupy sites of two different superstructures is proposed. The formation of these structural modifications is due to a second-order order–order phase transition that does not occur to the end. The allowable relation between the long-range order parameters in the structural modification formed by a combination of the monoclinic $(M_{5}X_{5})_{\operatorname{mon}}$ (space group $C$2/$m$ ($A$2/$m$)) and the cubic $(M_{5}X_{5})_{\operatorname{cub}}$ (space group $Pm$3$m$) of the superstructures is studied using an atom–vacancy ordering in titanium monoxide TiO$_{1.0}$. The thermodynamic calculations show that the proposed structural modification is equilibrium and must form instead of assumed high-temperature cubic phase (Ti$_{5}$O$_{5}$)$_{\operatorname{cub}}$.