Electron spin relaxation of vibronic Cu(H2O)(6) complexes in K2Zn(SO4)(2)center dot 6H(2)O single crystals

The low temperature (6 K) EPR spectrum of Cu2+ in K2Zn(SO4)(2). 6H(2)O is c haracteristic for the ground state a\x(2) - y(2)] - b\z(2)] with essentiall y the \x(2) - y(2)] state and 4.5% admixture of the \z(2)] state due to the zero-point motions. Temperature variations of the g and A parameters are c haracteristic for two-well vibronic dynamics and the vibronic averaging of the g-factor is well described with the simple Silver-Getz model above 60 K with energy difference delta (12) = 68 cm(-1) between the wells. However, this model does not reproduce the A(T) dependence, which seems to be influe nced by temperature induced changes in the unpaired electron delocalization onto ligands. Electron spin relaxation was measured at low temperatures up to 55 K where the electron spin echo signal was detectable. At low tempera tures, In the static Jahn-Teller limit, the Cu(H2O)(6) complexes are strong ly localized in the deepest potential well and the slow vibronic dynamics i s overdominated by two-phonon Raman processes in electron spin-lattice rela xation. The relaxation rate is described by 1/T-1 = aT(9)I(8)(Theta (D)/T) with transport integral I-8 and the Debye temperature Theta (D) = 170 K. El ectron spin echo decay is strongly modulated by dipolar coupling with H-1 a nd K-39 nuclei and the decay function is V(2 tau) = V(0)exp(-a tau - m tau (2)). The quadratic term dominates in the static Jahn-Teller limit (below 1 8 K) and describes the decay produced by the nuclear spectral diffusion. Fo r higher temperatures the exp(-a tau) term dominates and describes an effec t of the intrawell excitations with phase relaxation rate 1/T-M = a + bexp( -Delta /kT) with Delta = 67 cm(-1) being the energy of the first excited vi bronic level. The excitations produce a clear broadening of the Fourier tra nsform electron spin echo (FT-ESE) spectra (at 18 K), where peaks from pota ssium and hydrogen nuclei have been identified.