Density Functional Theory Driven Analysis of the Interplay among Structure, Composition, and Oxidation State of Titanium in Hibonite, Spinel, and Perovskite

Hibonite (CaAl12O19), spinel (MgAl2O4), and perovskite (CaTiO3) represent oxides that occur in primitive meteorites and are among some of the first phases to have condensed from the early solar nebula. Hibonite and spinel can contain 3d transition metals such as titanium that substitute at cation sites. The substituted titanium can occur in multiple oxidation states, which are in turn linked to the redox conditions under which the material formed or last equilibrated. Similarly, in perovskite, the oxidation state of titanium can also vary but depends on the extent of nonstoichiometry that can arise because of the presence of oxygen vacancies. Here we present density functional theory-based calculations that provide a fundamental understanding on the interplay among changes in composition, structure, and the oxidation state of titanium in spinel, perovskite, and hibonite, with an added emphasis on characterizing the underlying role of oxygen vacancies. Single-site substitution by titanium for aluminum and coupled substitution in conjunction with divalent cations such as magnesium were examined for hibonite and spinel. In addition, the crystal-chemical effects of a single oxygen vacancy were examined in all three systems. Our results show that in hibonite, the M2 cation site is preferred for single titanium substitution, where the titanium oxidation state corresponds to 3+. For the coupled substitution system, the M4 and M3 sites are energetically favorable for titanium and magnesium, respectively, with the oxidation state of titanium corresponding to 4+. In the case of spinel, the substitution of titanium in any of the octahedral aluminum sites are equivalent, and interestingly, substituted titanium is more reduced in spinel as compared with hibonite for both single as well as coupled substitution cases. Further, the oxidation state and Ti-O bonding of couple-substituted titanium in spinel is similar to the titanium cations in the phase-pure perovskite structure. For hibonite and spinel with an oxygen vacancy, we restricted our examination to mono- and di-coupled substitution with the choice motivated by available experimental data. Our results suggest that the oxidation state and the chemical bonding of the substituted titanium cations is directly influenced by their spatial location with respect to the oxygen vacancy.