Sk. Hoffmann et al., VIBRONIC BEHAVIOR AND ELECTRON-SPIN RELAXATION OF JAHN-TELLER COMPLEXCU(H2O)(6)(2-DOT-6H(2)O SINGLE-CRYSTAL() IN (NH4)(2)MG(SO4)(2)CENTER), The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(10), 1998, pp. 1697-1707
Temperature dependences of the CW-EPR spectrum as well as the electron
spin-lattice relaxation time T-1 and phase memory time T-M determined
by electron spin echo were measured for Cu(2+) ions in (NH4)(2)Mg(SO4
)(2) . 6H(2)O Single crystal. The dependences are dominated by vibroni
c behavior of the Cu(H2O)(6)(2+) complex and connected to the dynamic
Jahn-Teller effect producing reorientations between two lowest energy
wells of the adiabatic potential surface. Below 70 K the static Jahn-T
eller effect is observed, and the system is strongly localized, by the
local strains, in the deepest potential well leaving higher wells not
populated. Above this temperature the second well becomes progressive
ly populated, and a rapid averaging of the g(z) and g(y) factors as we
ll as corresponding hyperfine splittings appears. The Boltzmann popula
tion of these two wells is achieved at 160 K. Simultaneously with g fa
ctors averaging a continuous broadening of the hyperfine lines is obse
rved with line shape transformed from Gaussian at 70 K to Lorentzian a
t 160 K. The averaging and broadening processes are thermally activate
d with energy barrier delta(12) = 108 +/- 3 cm(-1) = 156 K = 1.26 kJ/m
ol being the energy difference between the two deepest potential wells
. Electron spin relaxation was measured below 50 K where electron spin
-echo signal was detectable. Spin-lattice relaxation is driven by the
direct and Raman processes with relaxation rate 1/T-1 = aT + bT(5) as
expected for dynamic Jahn-Teller systems. Spin-spin phase relaxation d
escribed by the phase memory time T-M depends on temperature as 1/T-M
= a + bT + c exp(-Delta lkT) with Delta = 102 +/- 2 cm(-1) At low temp
eratures the higher energy well is not populated; thus, a can be assig
ned as the energy of the first excited vibronic level in the deepest w
ell. The Delta and delta(12) are temperature-independent, indicating t
hat adiabatic potential surface is not affected by temperature. We sug
gest that the deviations of experimental data from theoretically predi
cted vibronic g-factors averaging observed for Cu2+ in many Tutton sal
t type crystals are not due to temperature variations of the local str
ains or barrier height but are due to the fact that Boltzmann populati
on of the potential wells cannot exist at low temperatures. This effec
t is especially pronounced for Cu2+ ions in (NH4)(2)Mg(SO4)(2) . 6H(2)
O since the energy Delta of the first vibronic level is lower than the
energy difference delta(12), between adjacent wells. In such case the
phonon-assisted tunneling jumps between the energy wells induced by t
wo-phonon Raman processes via virtual state of energy delta(12) become
to be effective when kT greater than or equal to delta(12)/2, and the
Boltzmann population of the second well is achieved via direct phonon
process when thermal phonons of energy kT greater than or equal to de
lta(12) are available.