Dephasing relaxation of the electron spin echo of the vibronic Cu(H2O)6 complexes in Tutton salt crystals at low temperatures

Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T-M) were performed in a series of Tutton salt crystals (M2MII)-M-I(SO4)(2).6X(2)O (M-I = NH4, K, Cs; M-II = Zn, Mg; X = H, D) weakly doped with Cu2+ ions (c approximate to 10(18) ions/cm(3)) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2 tau) = V-0 exp(-b tau - m tau (2)) with the m tau (2) term being temperature independent and negligible above 20 K. Various mechanisms leading to the tau- or tau (2)-type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T-M(0) times were found to be close one to another within the crystal family with average values: 17.5 mus (NSD protons), 200 mus (NSD deuterons), 8 mus (SD), and 5 mus (ID). The ID dominates but the calculated effective T-M(0) is longer than the experimental T-M(0) = 2 mus. This is due to a nonuniform distribution of the Cu2+ ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T-M with temperature is due to the mechanisms producing exp(-b tau) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy Delta leading to the temperature dependence 1/T-M = B exp(-Delta /kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the Delta values being on the order of 100 cm(-1), the scheme of vibronic levels in the Tutton salts is presented, and the independence of the Delta on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H2O)(6) complexes is not affected by temperature below 65 K. (C) 2001 Academic Press.