STUDY OF THE ELECTRICAL-FIELD STRENGTH FIELD DIRECTION PHASE-DIAGRAM

The dc field-induced room temperature Pbma --> P2(1) ma phase transiti on in the sodium niobate crystal has been studied by variation of the electric-field direction with the respect to crystallographic axes. Th e phase transition voltage across an interelectrode gap generally tend s to increase with increasing deviation of the midgap electric field f rom the effective niobium shift direction. There exist threshold devia tion angles beyond which drastic changes in the phase-transition behav ior are observed. The proportion of niobium Nb ions engaged in the str uctural phase-transition process, as well as the extent and direction of their displacement are functions of the midgap electric held direct ion.