Extensive LAPW frozen phonon calculations were performed in order to understand the origin of ferroelectricity in LiTaO 3 and LiNbO 3 . Displacement of the Li atoms alone results in an anharmonic single well, whereas displacements of oxygen and lithium together result in deep double wells, much deeper than the transition temperatures, T C . This is contrary to current theories which model the underlying potential as a triple well potential for the lithium atoms. Our results support an order-disorder model for the oxygen atoms as the driving mechanism for the ferroelectric instability. Oxygen displacements alone against the transition metal atoms result in shallower double wells as a result of oxygen-lithium overlap so that the lithium and oxygen displacements are strongly coupled . We find large hybridization between the oxygens and the transition metal atoms. Thus ferroelectricity in the Li(Nb,Ta)O 3 system is similar in origin to ferroelectricity in the perovskites. We also find that the electronic structures of LiTaO 3 and LiNbO 3 are very similar and hardly 1 change during the phase transition.

Using the non-empirical Variational Induced Breathing (VIB) model, the thermal properties of periclase (MgO) under high pressures and temperatures are investigated using molecular dynamics, which includes all anharmonic effects. Equations of state for temperatures up to 3000K and pressures up to 310 GPa were calculated. Bulk modulus, thermal expansivity, Anderson-Griineisen parameter, thermal pressure, Gr/ineisen parameter and their pressure and temperature dependencies are studied in order to better understand high pressure effects on thermal properties. The results agree very well with experiments and show that the thermal expansivity decreases with pressure up to about 100GPa (•-0.73), and is almost pressure and temperature independent above this compression. It is also effected by anharmonicity at zero pressure and temperatures above 2500K. The thermal pressure changes very little with increasing pressures and temperatures, and the Griineisen parameter is temperature independent and decreases slightly with pressure.

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In order to better understand the origin of the ferroelectric instability in LiTaO, and LiNbO,, a set of LAPW frozen phonon calculations were performed. Deep double wells, much deeper than the transition temperatures, Tc., are found as a result of the oxygen displacements against the transition metal atoms, whereas displacement of the Li atoms alone results in an anharmonic single well. This supports an order-disorder character for the oxygen atoms, contrary to current theories emphasizing the orderdisorder character of the Li atoms as the mechanism for the ferroelectric phase transformation. Also, we find large hybridization between the transition metal atoms and the oxygens. The Li(Nb,Ta)O, system is very similar to the perovskite ferroelectrics where the hybridization plays a major role in the transition to a ferroelectric phase.