FERROELECTRIC STRUCTURE OF KNBO3 AND KTAO3 FROM 1ST-PRINCIPLES CALCULATIONS

Based on the results of total-energy calculations u sing the full-potential linear muffin-tin orbital method, the equilibrium ground-state structure is determined in perovskite-type KNbO3 and KTaO3 complex oxides. The first compound is found to have a ferroelectric zero-temperature ground state, induced by atomic displacements along [111], as is consistent with the experimentally determined low-temperature crystal structure. The displacements along [001] give rise to another ferroelectric structure which is stable in the constrained tetragonal symmetry; this structure, however, corresponds to a saddle point on a more general path connecting two adjacent [111]-type displacements. KTaO3 was found to be stable in the undistorted cubic phase at zero temperature, exhibiting a soft phonon mode, but it can be driven to a ferroelectric state by negative pressure.