High-temperature phases of NaNbO3 and NaTaO3

The high-temperature phases of the perovskites sodium niobate, NaNbO3, and sodium tantalate, NaTaO3, have been re-examined using the high-resolution powder diffractometer HRPD at the ISIS neutron spallation source; the two materials show the same sequence of phases with tilted octahedra. Diffraction patterns were measured every 5 K allowing structural changes with temperature within a single phase to be determined for the first time. Previous structure determinations within one phase had been performed at a single temperature only. The octahedra are tilted about pseudocubic [100] directions and are also deformed; the magnitude of the deformation is shown to be proportional to the square of the angle of tilt as expected from a phenomenological theory applied to such transitions. The structures of NaNbO3 between 753 and 793 K and of NaTaO3 below 758 K are not as reported in the literature.