Ginzburg-Landau theory of an incommensurate-to-commensurate transition in barium sodium niobate

Features of an incommensurate-to-commensurate transition in barium sodium niobate, which is obtained in a cooling process from a tetragonal phase, have been theoretically examined on the basis of the Ginzburg-Landau theory. The present theory which takes into account the contribution of higher-order distortion waves produced from higher-order harmonics via the umklapp process, well reproduces an experimentally obtained change in an incommensurability δ on cooling. The theory also predicts both a discommensuration with a phase slip of 2π/4 and the incommensurate-to-commensurate transition.