Electronic structure and dynamics of low symmetry Cu2+ complexes in kainite-type crystal KZnClSO4·3H2O: EPR and ESE studies

EPR measurements at X-band were performed in the temperature range 4.2-300 K with angular dependence measurements at 77 K for Cu2+ in KZnClSO4·3H2O. Rigid lattice spin - Hamiltonian parameters are: gz = 2.4247, gy = 2.0331, gx = 2.1535, Az = −103 × 10−4 cm−1, 63 × 10−4 cm−1, and −31 × 10−4 cm−1. The parameters were analyzed using MO-theory with the dx2-y2 ground state containing admixture of the dz2-state in the rhombic symmetry D2h. The analysis consistently explained unusual g-factor sequence and relatively small hyperfine splitting anisotropy as the consequence of the mixing and spin density delocalization via excited orbital states. We assigned that Cu2+ ions substituting host Zn2+ prefer one of the four structurally different zinc sites where they are coordinated by four water molecules and two SO4 groups in an distorted octahedron elongated along SO4-Cu-SO4 direction. The distortion is due to the Jahn-Teller effect which is static at low temperatures but becomes dynamic above 20 K with jumps of the Cu2+ complex between two lowest potential wells. The jumps produce continuous g-factor and hyperfine splitting averaging when temperature increases. This process is discussed in terms of two motional averaging theories: classical theory based on generalized Bloch equations and Silver-Getz model. Their limitations are discussed. Importance of the difference in the g-factors of the averaged line is explained and a new expression for calculation of jump frequency from the line shift is proposed. The jumps are described as phonon induced tunneling via excited vibrational level of energy 76 (±6) cm−1. This process is not effective enough at low temperatures and Boltzmann population of the two lowest energy potential wells is reached above 110 K. From electron spin-lattice relaxation measurements by electron spin echo methods the Debye temperature was determined as ΘD = 172 K. Fourier Transform of strongly modulated spin echo decay gives pseudo-ENDOR spectrum with peaks from 1H and 35Cl nuclei. From splitting of the peaks into doublets we determined the distance to the modulating nuclei and confirmed the position of the site where Cu2+ ion is located.