Sound propagation effects near phase transitions in crystals

Studies of sound velocity changes and attenuation changes in the vicinity of phase transitions provide interesting insight into the static and dynamic properties of the system undergoing the transition. In our laboratory, we studied the following cases: 1) magnetic phase transitions in rare earth metals Gd, Tb, Dy, Ho, and in magnetic insulators EuO, RbMnF 3 and MnF 2 ; 2) spin reorientation transitions in ErFeO 3 and Gd; 3) structural transition, induced by an optical phonon instability in SrTiO 3 ; 4) structural transition, induced by an electronic instability (cooperative Jahn-Teller phase transition) in the Ni-Zn-chromite system and in DyVO 4 (in both cases the structural transition occurs at a higher temperature than the magnetic transition); 5) nematic-isotropic transition in the liquid crystal MBBA. In the order-disorder phase transition valuable information can be gained from attenuation and velocity measurements about the relaxation time of the order parameter or of related quantities. In the case of displacive phase transitions, the main effect is a resonant interaction of the sound wave to the soft mode, the latter being the k=0 spin wave mode for spin reorientation transitions, an optical phonon mode for the structural transitions encountered in perovskites and an electronic excitation for the cooperative Jahn-Teller transitions. For additional details see, Digests of the 1972 Intermag Conference (available from IEEE Headquarters).