Linear birefringence and ultrasonic studies of phase transition in (Ch3)2NH2H2PO4 crystal

Linear birefringence (LB) and ultrasonic wave (USW) velocity measurements were performed for (CH3)(2)NH2H2PO4 single crystal in the temperature range 300-230 K. The observed anomalies of LB (Delta n(i)) and USW velocity (v(ii)) are characteristic for continuous phase transition and they are explained within a framework of phenomenological approach. The birefringence increments delta(Delta n(i)) and ultrasound velocity increment delta V-6 are proportional to the square of spontaneous polarisation. Critical exponent obtained from both methods has practically the same value beta = 0.43.