Electroacoustic coeficients of crystals of gadolinium and terbium molybdates in the vicinity of phase transition

To clarify the nature of the phase transition and connection of electric and elastic subsystems of ferroelectric crystals investigated has been the behaviour of electroacoustic coefficients in the vicinity of the phase transformation in Gd2(MoO4)3 and Tb2(MoO4)3 crystals. The propagation rate changes of elastic waves under the action of a permanent external electric field has been measured at the elastic wave frequency of 30 MHz using a method of impulse interference. The measurements have been made for longitudinal elastic waves propagating along the x,y axes in the ferrophase and along the X axis in the paraphase when the field is directed along the Z axis. The strong changes of the wave rate having hysteresis character are observed in the case of the elastic wave propagation along the x and y axes of the ferrophase. The temperature dependencies of electroacoustic coefficients in the temperature range from room one to 200 deg C have been measured. The calculation of electroacoustic coefficients has been made in the framework of the phenomenological Landau-Khalatnikov theory taking into account the double component nature of the order parameter. It has been shown that the low temperature phase is well described in the framework of the theory and the account of the fluctuation mechanism is necessary above the transition point