Effects of lattice and local dynamics in EPR spectra and electron spin relaxation of vibronic Cu(imidazole)6 complexes in Zn(imidazole)6Cl2·4H2O crystals

Cu(im)₆ complexes in Zn(im)₆Cl₂·4H₂O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm⁻¹ between the wells and energy 300 cm⁻¹ of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm⁻¹ at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)₆ complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T₁ relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature Θ_D=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm⁻¹. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm⁻¹. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating ¹⁴N atom of the imidazole rings and the peak from double quantum transition ν_dq. We show that the amplitude of the ν_dq transition can be used to determine the number of non-coordinating nitrogen atoms.