The lattice parameters of NaNbO3 (which has a perovskite structure and is antiferroelectric at room temperature) have been measured by single-crystal X-ray methods from room temperature to 800°C. While the measurements are in substantial agreement with earlier work, the greater resolution shows up new details, and the interpretation of the earlier work has to be very seriously revised, particularly as concerns the symmetry of the high-temperature phases.Below the 360°C transition a second very similar phase Q can sometimes replace the normal roomtemperature phase P. Q is believed to be the 'forced ferroelectric' known to be produced from P by the action of an electric field; the field-free P-Q transitions are influenced by temperature but have not a reproducible transition temperature.Above 360°C there are a large number of transitions characterized by very different features. The facts from this and earlier work are critically reviewed. While the effects cannot yet be explained in detail, the general reasons for expecting such behaviour are discussed. No detailed structural work is reported, but there is evidence for the persistence of Nb displacements above 360 °C, which is therefore not the paraelectric transition.The transitions in a pseudosymmetric structure cannot be explained or predicted in terms of conventional or macroscopic theories which disregard structural details. A new empirical approach is needed, for which this system provides useful material.

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