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We show that the structure, properties, and concentration of vacancies in crystals can be studied by ultrasonic experiments previously employed for impurity centres only. Measurements of the temperature dependence of attenuation and phase velocities of ultrasonic shear waves of 52 MHz propagating along the crystallographic axis [110] of nominally pure ZnSe single crystals (grown by the seeded physical vapour transport method) show strong anomalies which are typical for relaxation processes in system with isolated Jahn–Teller (JT) centres. The observed JT distortion mode is trigonal, subject to a threefold orbitally degenerate T‐term interaction with trigonal and tetragonal nuclear displacements. In the absence of sufficiently high concentrations of impurity atoms with such properties we attributed the observed JT centres to zinc vacancies. The temperature dependence of the isothermal and adiabatic forms of the appropriate elastic modulus and the relaxation time show that the relaxation mechanism changes from thermal activation at higher temperatures to tunnelling through a potential energy barrier below 18 K. We provide an estimate of the magnitude of the potential barrier, as well as the pseudorotation frequency and concentration of vacancies. Also we determine the extremum points of the adiabatic potential energy surface of the vacancy centre.

Transition-metal-doped semiconductor GaAs crystals are used as model systems for spintronic research, as well as in other applications. To explore the structure and properties of such impurities, we extended the methodology of ultrasonic investigation of the Jahn-Teller effect in dielectric impurity-centers to the study of significantly different semiconductor impurities using the GaAs:Cu crystal as an example. Phase velocity and attenuation of ultrasound in this system were measured in the temperature interval of 1.9–80 K at 52 MHz and 156 MHz. The anomaly in the velocity and a peak of attenuation found for the longitudinal and slow shear waves indicate the presence of the Jahn-Teller effect with the e-type local distortions of the CuGa4As impurity complex. The temperature dependence of the elastic modulus and relaxation time shows that below 5 K, the thermal activation mechanism of relaxation is possibly replaced by resonance type transitions. The main parameters of the Jahn-Teller effect, stabilization energy, minima positions and the barrier between them, frequency of pseudorotation of the distortions, and the tunneling splitting of the ground state energy, as well as the constant of exchange interaction between the two holes in Cu2+ centers and the concentration of the centers were estimated.

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