Optical features of A2BX4 ferroic nanocomposites with the incommensurate phase

In this paper, we report on the experimental studies of optical birefringence of A(2)BX(4) (K2ZnCl4, Rb2ZnCl4, and K2SeO4) type of crystals in a wide temperature range which show para-electric, ferroelectric properties with incommensurate phases for the 633 nm wavelength. We found that with the decrease of the nanocrystallite sizes, temperature range of the incommensurate phase is enhanced. At the same time the ferroelectricity disappeared at crystallite sizes below 8 nm due to nano-confinement effects which change the electron-phonon anharmonic interactions which are responsible for such kinds of phase transitions. Additional superposition of the external electric field leads to drastic changes in the relative amount of the ferroelectrics and incommensurate phases. The effect is explained within the Landau-Ginzburg phenomenological approach with Lifshitz invariant. Additional confirmation of the observed effects was obtained by optical second harmonic generation. During the decrease of the nanocrystallite sizes below 12 nm there occurs some harmonic soliton waves which are propagated in several directions parallel to the crystallographic axes. Despite the absence of the long-range disorder there occurs some non-centrosymmetry. (c) 2012 Elsevier B.V. All rights reserved.