Electronic structure and dynamics of low symmetry Cu 2+ complexes in kainite-type crystal KZnClSO 4·3H 2O: EPR and ESE studies

Abstract

EPR measurements at X-band were performed in the temperature range 4.2–300 K with angular dependence measurements at 77 K for Cu 2+ in KZnClSO 4·3H 2O. Rigid lattice spin – Hamiltonian parameters are: g z = 2.4247, g y = 2.0331, g x = 2.1535, A z = −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 d x 2 - y 2 ground state containing admixture of the d z 2 -state in the rhombic symmetry D 2h. 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 Cu 2+ ions substituting host Zn 2+ prefer one of the four structurally different zinc sites where they are coordinated by four water molecules and two SO 4 groups in an distorted octahedron elongated along SO 4–Cu–SO 4 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 Cu 2+ 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 Θ D = 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 Cu 2+ ion is located.