Electronic structure and dynamics of low symmetry Cu~(2+) complexes in kainite-type crystal KZnClSO_4·3H_2O: EPR and ESE studies

摘要

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 dx2-y2 ground state containing admixture of the dz2-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 ~(35)Cl 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.