New M(2 + δ1)X(2 + δ2) Superstructure of Ordered Nanocrystalline Titanium Monoxide and the Problem of Transition States

Abstract

The results of experiments on the vacancy ordering in nanocrystalline nonstoichiometric titanium monoxide TiOy (space group Fm $$\bar {3}$$ m), which is accompanied by the formation of the orthorhombic Ti9O10 phase (space group Immm), are considered. The vacancy distribution in the ordered phase is described by a two-sublattice-ordering M(2 + δ1)X(2 + δ2) superstructure. The disorder–order (TiOy–Ti(2 + δ1)O(2 + δ2)) phase transition is accompanied by symmetry distortions along non-Lifshitz star {k4}. When ordering, vacancies concentrate in every third (110) plane of the basic B1-type structure. The deviation of stoichiometry is caused by different vacancy concentrations in the defect planes in the metallic and nonmetallic sublattices (parameters δ1 and δ2, respectively). The new class of M(n + δ1)X(n + δ2) superstructures is shown to generate numerous order–disorder transition state models, which cannot be described in terms of the mean-field approximation. At δ1 = δ2 = 0.5, the orthorhombic M(2 + δ1)X(2 + δ2) structure is a particular case of the partly disordered monoclinic M5X5 superstructure (space group C2/m(A2/m)), which is characteristic of a coarse-grained state.