Tunneling Relaxation Mechanisms of the Jahn–Teller Complexes in a CaF2:Cr2+ Crystal

The temperature dependences of attenuation and the velocity of ultrasonic waves at frequencies of 26–158 MHz in a CaF2 fluorite crystal at the substitution of Jahn–Teller Cr2+ centers for calcium ions have been studied. An abnormally high relaxation rate, which is two orders of magnitude higher than the relaxation rate in other previously studied CaF2:Ni2+ and SrF2:Cr2+ fluorites, has been found in the system of Jahn–Teller complexes in the low-temperature region. It has been shown that the global minima of the adiabatic potential energy surface of the Cr2+ $${\text{F}}_{8}^{ - }$$ complexes in the CaF2:Cr2+ crystal also have orthorhombic symmetry but are separated by significantly lower potential energy barriers than in CaF2:Ni2+ and SrF2:Cr2+ crystals. It has been found that tunneling relaxation mechanisms (direct and two-phonon transitions) are dominant, rather than thermal activation, in CaF2:Cr2+ in the temperature range where the Jahn–Teller effect is manifested in an ultrasonic experiment. The parameters characterizing these relaxation mechanisms have been determined.