AN APPROACH TO THE STUDY OF THE ELECTRICAL-FIELD STRENGTH FIELD DIRECTION PHASE-DIAGRAM

Investigation of the ''electrical'' phase diagram for a crystal as a f unction of the dc field direction has been originally attempted. The f ield-induced room-temperature Pbma --> P2(1) ma phase transition in th e sodium niobate crystal has been studied for the sake of example. The electric-field direction was varied by varying orientation of planar electrodes. It has been found that: (1) the phase transition voltage a cross an interelectrode gap generally tends to increase with increasin g deviation of the midgap electric field from the effective niobium sh ift direction; (ii) the dependence of the transition-field strength on the deviation angles is nonmonotonous; (iii) there exist threshold de viation angles beyond which drastic changes in the phase-transition be havior are observed; (iv) the proportion of Nb ions engaged in the str uctural phase-transition process, as well as the extent and direction of their displacement are function of the midgap electric field direct ion.