Off-centre dynamic Jahn-Teller effect studied by electron spin relaxation of Cu2+ ions in SrF2 crystal

Temperature cw-EPR and pulsed EPR electron spin echo experiments were performed for a low concentration of Cu2+ ions in cubic SrF2 crystals. The well resolved EPR spectrum at low temperatures (below 30 K) with parameters g(parallel to) = 2.493, g(perpendicular to) = 2.083, A(parallel to) = 121, A(perpendicular to) = 8.7, A(parallel to)(F-19) = 135, A(parallel to)(F-19) = 33.0 (A-values in 10(-4) cm(-1)) is transformed continuously into a single broad line above 225 K on heating, due to the g-factor shift and EPR line broadening. These data along with the angular variation EPR data are described in terms of a pseudo-Jahn-Teller effect of (T-2g + A(2u)) x (a(1g) + e(g) + t(1u)) type producing six off-centre positions of the Cu2+ ion in the fluorine cube. Above 30 K a two-step averaging g-factor process occurs and is governed by vibronic dynamics between potential wells of the off-centre positions. This dynamics governs the electron spin relaxation in the whole temperature range. The electron spin-lattice relaxation rate 1/T-1 grows rapidly by six orders of magnitude in the temperature range 30-100 K and is determined by the Orbach-type process with excitations to two excited vibronic levels of energy 83 and 174 cm(-1). For higher temperatures the relaxation is dominated by overbarrier jumps leading to the isotropic EPR spectrum above 225 K. The phase memory time T-M has the rigid lattice value 3.5 mu s determined by nuclear spectral diffusion and its temperature variation is governed by the vibronic dynamics indicating that the excitations between vibronic levels produce a dephasing of the electron spin precessional motion.