The structure of sodium niobate (T2) at 600°C, and the cubic-tetragonal transition in relation to soft-phonon modes

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Abstract
The structure of NaNbO3, phase T2 (immediately below the cubic phase), has been determined from lattice parameters and difference reflections. It is tetragonal C-face-centred, with a= 7·8698(8) Å, c=3·9428(4) Å at 600°c. It corresponds to condensation of an M3 soft-phonon mode from the cubic phase, in contrast to SrTiO3 where the condensing mode is Γ25. The anomalous changes in mean square values of atomic displacements have been derived from changes in lattice parameters. Hence the intensities of x-ray scattering are calculated and compared with direct measurements of diffuse scattering by other workers, with satisfactory agreement. A mechanism for the transition is postulated.