Ultrasonic investigations of the Jahn–Teller effect in a $ZnSe:Fe2+$ crystal

The temperature dependence of the absorption and velocity of transverse ultrasonic waves propagating in the [110] direction in a $ZnSe:Fe2+$ crystal with the iron impurity of $n=2.2×1019cm−3$ is investigated in the temperature interval $1.4–100K$ at a frequency of $53MHz$⁠. For the slow transverse mode an absorption peak at $7.3K$ and a decrease of the phase velocity below $40K$ are found, while no such features are observed for the fast mode. The anomalies are interpreted as manifestations of the Jahn–Teller effect. The softening of the elastic modulus $(C11–C12)∕2$ indicates that the local distortions in $ZnSe:Fe2+$ are tetragonal. Under the assumption that the absorption maximum is of a relaxational nature, the temperature dependence of the relaxation time is recovered from the data, and the values of the potential barrier height and vibronic frequency are obtained.