LOW-FREQUENCY ELASTIC PROPERTIES OF THE INCOMMENSURATE FERROELASTIC [N(CH3)(4)]2CUCL4

The low-frequency elastic properties of incommensurate ferroelastic cr ystals [N(CH3)(4)]2CuCl4 are studied in the vicinity of the incommensu rate phase transitions using parallel-plate-stress and three-point-ben ding methods. The temperature dependences of the effective elastic con stants (relative Young's modulus) along all principal crystal directio ns and the shear elastic compliance S-55=1/C-55 were obtained. Similar ly to the elastic properties at ultrasonic frequencies (10 MHz), the e lastic anomalies at low frequencies (5 Hz) near the paraelastic-incomm ensurate transition (at T-i) are well explained in the framework of th e plane-wave approximation. In the transition region from the incommen surate to the commensurate improper ferroelastic phase we apply solito n theory. From the experimental data it follows that the anomalous beh avior of S-55 near the phase transition from incommensurate to commens urate improper ferroelastic phase has the same features as the dielect ric constant in the vicinity of incommensurate-Commensurate improper f erroelectric phase transition, including the Curie-Weiss-type anomaly for S-55(T) near T-c, thermal hysteresis phenomena, and similar change s in the character of the anomaly under applied static stress sigma(5) . The last results are explained within the framework of soliton theor y proposed by Holakovsky and Dvorak [J.Phys. C 21, 5449 (1988)].