Acta Cryst. (1999). B55, 24-30 [doi:10.1107/S010876819800963X]
Abstract: 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 
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.
Online 1 February 1999
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