Manifestation of the Jahn–Teller effect subject to quadratic T⊗e+t2 problem in ultrasonic attenuation. Case study of CaF2:Cr crystal with isovalent and aliovalent substitution

Abstract

Abstract A new approach is introduced to obtain information about the adiabatic potential energy surface of the Jahn–Teller (JT) complexes subject to the T ⊗ e + t 2 problem in doped crystals. It employs simulation of the contribution of the JT sub-system to ultrasonic attenuation with account of three mechanisms of relaxation and subsequent comparison with the experimental data on temperature dependence of the attenuation obtained with the use of doped crystal and nominally pure one. The approach has been applied to the CaF2 crystal doped with chromium. The experiment has been carried out at the frequencies of (13–158) × 106 Hz at low temperatures (4–170 K). Peak for all the ultrasonic normal modes propagating along the [110] axis has been observed at about 8 K. It has been considered as the JT effect manifestation by the sub-system of Cr2+ F 8 − complexes. Symmetry properties of the global minima and saddle points have been identified, the JT stabilization energies and coordinates of the extrema points in the five-dimensional space of symmetrized displacements have been evaluated. Besides, a broad shoulder has been found at 15–40 K and attributed to contribution of the Cr3+ F 8 − sub-system.