Ultrasonic study of the elastic and nonlinear acoustic properties of single-crystal triglycine sulphate in the ferroelectric and paraelectric phases

An experimental ultrasonic study has been made of the elastic and nonlinear acoustic behaviour of single-crystal triglycine sulphate (TGS) in both the ferroelectric and paraelectric phases. The ultrasonic wave velocities for 12 modes of vibration propagated along the [100], [010], [001] and [101] di rections have been measured as functions of temperature in the range 293-33 8 K, which covers the Curie temperature T-C, and hydrostatic pressure up to 0.2 GPa at selected temperatures in the same temperature range. The result s provide ten out of the 13 elastic stiffness tensor components C-ij of TGS (the exceptions being C-12, C-23 and C-25) and their hydrostatic-pressure derivatives as a function of temperature. Marked anomalies are observed in the C-ij(T) in the vicinity of T-C, with that for C-33 being the most prono unced. The elastic stiffness tensor component C-22, which determines the te mperature dependence of the velocity of longitudinal ultrasonic waves propa gated along the ferroelectric axis, shows normal behaviour without any anom aly near Tc. The temperature dependences of C-ij are discussed in the frame work of existing theoretical models. The pressure derivatives (partial deri vative C-ij/partial derivative P)(P=0) of the elastic stiffness tensor comp onents are all positive; application of hydrostatic pressure does not induc e acoustic-mode softening. The hydrostatic-pressure derivative (partial der ivative C-22/partial derivative P)(P=0) increases approximately linearly wi th increasing temperature in both the ferroelectric and paraelectric phases without any marked anomaly in the phase transition region. The Values obta ined for the remaining (C-ij/partial derivative P)(P=0) increase slightly w ith increasing temperature in the ferroelectric phase and show a step-like decrease at the phase transition. Using the values determined for C-ij and (partial derivative C-ij/partial derivative P)(P=0), the acoustic-mode Grun eisen parameters have been calculated in the pseudo-orthorhombic approximat ion. All mode Gruneisen parameters are positive and considerably larger in the ferroelectric phase than in the paraelectric phase. With the exception of that determined for the longitudinal mode along the ferroelectric axis, each mode Gruneisen parameter exhibits a stepwise decrease at the phase tra nsition. The results establish that a pronounced change in the anharmonic c oupling takes place between the zone-centre acoustic modes and the spontane ous polarization as the crystal is transformed from the ferroelectric to th e paraelectric phase.