Acoustic properties of paraelectric and incommensurate phases of monoclinic piezoelectric crystal NH4HSeO4

There in the paper are given the results of ultrasonic studies of the phase transitions succession: initial-incommensurate-commensurate polar phases in the monoclinic piezoelectric crystal NH4HSeO4 at frequency of 10 MHz. It is shown that NH4HSeO4 crystal is a strong piezoelectric. A large contribution of piezoeffect into elastic properties of crystal is revealed. In accounting the piezoeffect, the elastic moduli CEij are calculated. The anomalous behaviour of acoustic properties due to peculiarities of interaction between elastic waves and normal modes of incommensurate phase is discussed.