NUCLEATION AND SUPPRESSION OF TRANSFORMATION TWINNING IN BARIUM SODIUM NIOBATE

It has already been established that the microtwinning associated with the tetragonal to orthorhombic phase transition in barium sodium niobate can be suppressed by cooling from above the transformation temperature under an applied stress. The work reported here indicates that the quotation of specific stress values for this process is misleading since, for any specimen, there is an optimum range of applied stress over which complete suppression of the microtwinning is achieved. This range is shown to be dependent upon the particular twin density and distribution which is determined by the presence of other crystal defects and by the composition of the material. Means of reducing the twin density are also discussed. An anomalous expansion behaviour in the [001] crystal direction, detected by a dilatometric technique, is consistent with the observed tendency to crack as the material is cooled through the Curie point. © 1969 Chapman and Hall Ltd.