Electron spin relaxation of vibronic Cu(H2O)6 complexes inK2Zn(SO4)2·6H2O single crystals

Abstract

The low temperature (6 K) EPR spectrum of Cu2+ inK2Zn(SO4)2·6H2O is characteristic for the ground statea|x2-y2⟩-b|z2⟩ with essentially the |x2-y2⟩ stateand 4.5% admixture of the |z2⟩ state due to the zero-point motions.Temperature variations of the g and A parameters are characteristic fortwo-well vibronic dynamics and the vibronic averaging of the g-factor iswell described with the simple Silver-Getz model above 60 K with energydifference δ12 = 68 cm-1 between the wells. However, thismodel does not reproduce the A(T) dependence, which seems to be influencedby temperature induced changes in the unpaired electron delocalization ontoligands. Electron spin relaxation was measured at low temperatures up to 55 Kwhere the electron spin echo signal was detectable. At low temperatures, inthe static Jahn-Teller limit, the Cu(H2O)6 complexes are stronglylocalized in the deepest potential well and the slow vibronic dynamics isoverdominated by two-phonon Raman processes in electron spin-latticerelaxation. The relaxation rate is described by 1/T1 = aT9I8(ΘD/T)with transport integral I8 and the Debye temperature ΘD = 170 K.Electron spin echo decay is strongly modulated by dipolar coupling with 1Hand 39K nuclei and the decay function is V(2τ) = V0exp (-aτ-mτ2). The quadratic term dominates in the staticJahn-Teller limit (below 18 K) and describes the decay produced by thenuclear spectral diffusion. For higher temperatures the exp (-aτ) termdominates and describes an effect of the intrawell excitations with phaserelaxation rate 1/TM = a + bexp (-Δ/kT) with Δ = 67 cm-1being the energy of the first excited vibronic level. The excitations producea clear broadening of the Fourier transform electron spin echo (FT-ESE)spectra (at 18 K), where peaks from potassium and hydrogen nuclei have beenidentified.