Sk. Hoffmann et Va. Ulanov, Off-centre dynamic Jahn-Teller effect studied by electron spin relaxation of Cu2+ ions in SrF2 crystal, J PHYS-COND, 12(8), 2000, pp. 1855-1866
Temperature cw-EPR and pulsed EPR electron spin echo experiments were perfo
rmed 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(pa
rallel to) = 2.493, g(perpendicular to) = 2.083, A(parallel to) = 121, A(pe
rpendicular to) = 8.7, A(parallel to)(F-19) = 135, A(parallel to)(F-19) = 3
3.0 (A-values in 10(-4) cm(-1)) is transformed continuously into a single b
road line above 225 K on heating, due to the g-factor shift and EPR line br
oadening. These data along with the angular variation EPR data are describe
d 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 fl
uorine 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 po
sitions. This dynamics governs the electron spin relaxation in the whole te
mperature range. The electron spin-lattice relaxation rate 1/T-1 grows rapi
dly by six orders of magnitude in the temperature range 30-100 K and is det
ermined 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 ab
ove 225 K. The phase memory time T-M has the rigid lattice value 3.5 mu s d
etermined by nuclear spectral diffusion and its temperature variation is go
verned by the vibronic dynamics indicating that the excitations between vib
ronic levels produce a dephasing of the electron spin precessional motion.