TRANSFERABLE POTENTIALS FOR THE TI-O SYSTEM

Transferable potentials have been derived by computer modelling of the series of higher titanium oxides, from Ti2O3 (with Ti3+ ions) to TiO2 (with Ti4+ ions), and including the lower members of the series of ph ases, TinO2n-1, which have mixed oxidation states; some of these are d escribed as crystallographic shear phases. Two potential models have b een developed: one comprises coulombic interactions between charge cen tres, a core-shell model for the oxygen anions, repulsive exponential anion-cation interactions and, in addition, primary antiferromagnetic interactions between Ti3+-Ti3+ ions at short distances (r(ij) < 2.6 An gstrom), and secondary repulsive ferromagnetic interactions at longer distances; the second potential differs only in that it uses oxygen an ions which are rigid. The potentials more-or-less successfully model t he static crystal structures and their corresponding lattice energies for the full group of oxides included in the modelling, as well as for two independent test structures, and are expected to be equally effec tive in modelling still higher members of the shear phases, TinO2n-1. Modelling of the dynamic properties (elastic constant and relative per mittivity) is not reliable. The shell model oxide potential has a larg e ('hard') spring constant, and largely mimics the rigid potential, bu t at the expense of slight instabilities in the modelled structures.