Linear birefringence and ultrasonic studies of phase transition in (Ch(3))(2)NH2H2PO4 crystal

Linear birefringence (LB) and ultrasonic wave (USW) velocity measurements w ere 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 explaine d within a framework of phenomenological approach. The birefringence increm ents delta(Delta n(i)) and ultrasound velocity increment delta V-6 are prop ortional to the square of spontaneous polarisation. Critical exponent obtai ned from both methods has practically the same value beta = 0.43.