X-ray diffuse scattering from NaNbO₃ as a function of temperature

A qualitative description of the X-ray diffuse scattering from NaNbO₃ single crystals as a function of temperature up to 800°C is given. Two types of distinct diffuse scattering where observed in {100} reciprocal planes and on (100) reciprocal axes. While the diffuse scattering in reciprocal planes shows no critical behaviour at the different phase transitions and can be attributed to a `linear disorder' similar to that suggested earlier for KNbO₃, the diffuse scattering on reciprocal axes is critical in the vicinity of the 641°C phase transition. The atomic displacements involved with the linear disorder that persists up to 800°C are attributed to the niobium atoms; the critical planar disorder in the cubic phase is attributed to rotations of oxygen octahedra similar to those suggested for SrTiO₃ and KMnF₃. Both types of disorder exist in the cubic paraelectric phase.