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

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.