Study of the elastic properties of K2SeO4at the phase transition to the incommensurate and the commensurate phase by ultrasonic techniques and Brillouin spectroscopy

Using the ultrasonic pulse-overlap technique and multipass Brillouin spectroscopy the complete elastic stiffness tensor was determined in the temperature range from 300 to 30K. The transition to the incommensurate phase (at T2=127K) is reflected in the elastic stiffness functions c11, c22, c33, c44, c55, c13 and c22. The lock-in transition into the commensurate phase (at T3=93K) has no significant effect on all the measured stiffness functions apart from c55, which experiences large variations around T3. The observed effects are interpreted in terms of coupling to energy-density fluctuations in the case of c33, T>T2. In the incommensurate phase the stiffnesses c22, c33 and c44 are augmented by a contribution from quartic anharmonic interaction with the onset of incommensurate ordering. Another type of quartic anharmonicity is responsible for the decrease in c55 in the commensurate phase. The gradual transition at the lower end of the incommensurate phase is an indication of soliton-like discommensurations.