The results of the comprehensive ultrasonic research of high quality single crystals of FeSe are presented. Absolute values of sound velocities and their temperature dependences were measured; elastic constants and Debye temperature were calculated. The elastic C 11 -C 12 and C 11 constants undergo significant softening under the structural tetra-ortho transformation. The significant influence of the superconducting transition on the velocity and attenuation of sound was revealed and the value of the superconducting energy gap was estimated.

The ultrasonic properties of La1−xCaxMnO3 (x ≈ 0.25) with the Curie temperature TC about 200 K are studied. Temperature dependences of longitudinal and transverse sound velocities were measured in zero magnetic field and for different constant magnetic fields as well. The ultrasonic study is supported by magnetic, resistive, magnetoresistive, structural and other measurements of the sample that facilitate interpretation of the results obtained. The magnetic field influence on sound properties found in this study presents some new features of the interplay between the elastic and magnetic properties of these compounds. It is shown that the paramagnetic-ferromagnetic transition in the sample studied is first order, but can become second order under the influence of applied magnetic field.

The behavior of the elastic moduli and sound absorption in a PrFe3(BO3)4 single crystal at low temperatures is studied. A transition of the magnetic subsystem into an antiferromagnetically ordered state is manifested in the temperature behavior of the sound velocities and absorption. The characteristic behavior of the elastic properties of PrFe3(BO3)4 in an external magnetic field is observed. A phenomenological theory that gives a qualitative description of the observed features is constructed. It is proposed that a weak magnetic moment exists in the crystal.

A new version of the phase method of determining the sound velocity is 55proposed and implemented. It utilizes the "Nonius" measurement technique and can give acceptable accuracy (≤ 1%) in samples of submillimeter size. Measurements of the sound velocity are made in single-crystal samples of the borocarbides RNi2B2C (R = Y, Lu, Ho). The elastic constants and the Debye temperature are calculated.