Adiabatic potential energy surface of the Jahn-Teller complexes in CaF2:Ni²⁺ crystal determined from experiment on ultrasonic attenuation

In CaF2 crystal doped with Ni2+ ions, attenuation of all the normal ultrasonic modes with the wave vector k||⟨110⟩ were investigated at 14–161 MHz in the temperature region of 5–120 K. The observed peaks of relaxation origin were interpreted as manifestation of the Jahn-Teller effect subject to the T1g⊗(eg+t2g) problem. The experimental data made it possible to estimate the parameters of adiabatic potential energy surface: the Jahn-Teller stabilization energies, the linear and quadratic vibronic coupling constants, location of the saddle points and the global minima defined in the 5-dimensional space of symmetrized (tetragonal and trigonal) coordinates.