Ultrasonic investigation of ZnSe:V²⁺ and ZnSe:Mn²⁺: Lattice softening and low-temperature relaxation in crystals with orbitally degenerate states

Temperature dependences of elastic moduli and ultrasonic attenuation were investigated in ZnSe:V²⁺ and ZnSe:Mn²⁺ crystals as functions of temperature in the interval of 1.4–100 K for frequencies of 52–270 MHz. A peak in the attenuation was found in ZnSe:V²⁺ below 4 K for fast shear and longitudinal waves propagating along the [110] crystallographic axis. We interpret it to be a manifestation of a relaxation process related to the Jahn–Teller effect in the 3d electron system of the impurity. Softening of the elastic moduli C₄₄ and C_ℓ=(C₁₁+C₁₂+2C₄₄)∕2 were observed below 40 K. Temperature dependences were deduced for relaxation time and relaxed and unrelaxed C₄₄ and C_ℓ moduli. The temperature dependence of relaxation time shows that ZnSe:V²⁺ has potential barrier V₀=5.6 cm⁻¹. Softening of the C₄₄ modulus indicates that the local distortions have a trigonal character. Neither attenuation anomalies nor softening of the elastic moduli was observed in ZnSe:Mn²⁺, which has a singlet orbital ground state of the 3d impurity.