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A methodology is worked out to retrieve the numerical values of all the mainparameters of the six-dimensional adiabatic potential energy surface (APES) ofa polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) fromultrasound experiments. The method is based on a verified assumption thatultrasound attenuation and speed encounter anomalies when the direction ofpropa- gation and polarization of its wave of strain coincides with thecharacteristic directions of symmetry breaking in the JTE. For the SrF2:Crcrystal, employed as a basic example, we observed anomaly peaks in thetemperature dependence of attenuation of ultrasound at frequencies of 50-160MHz in the temperature interval of 40-60 K for the wave propagating along the[110] direction, for both the longitudinal and shear modes, the latter with twopolarizations along the [001] and [110] axes, respectively. We show that theseanomalies are due to the ultrasound relaxation by the system of non-interactingCr2+ JT centers with orthorhombic local distortions. The interpretation of theex- perimental findings is based on the T2g (eg +t2g) JTE problem including thelinear and quadratic terms of vibronic interactions in the Hamiltonian and thesame-symmetry modes reduced to one interaction mode. Combining the experimentalresults with a theoretical analysis we show that on the complicatedsix-dimensional APES of this system with three tetragonal, four trigonal, andsix orthorhombic extrema points, the latter are global minima, while the formerare saddle points, and we estimate numerically all the main parameters of thissurface, including the linear and quadratic vibronic coupling constants, theprimary force constants, the coordinates of all the extrema points and theirenergies, the energy barrier between the orthorhombic minima, and the tunnelingsplitting of the ground vibrational states.

journal of applied physics

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects  retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr  crystal