Electronic structure, Jahn–Teller dynamics and electron spin relaxation of two types of octahedral Cu(ii) complexes in cadmium formate dihydrate Cd(HCOO)2·2H2O. EPR and ESE studies

Abstract

Cd(HCOO)₂·2H₂O single crystals weakly doped with Cu(II) ions have been studied by cw-EPR (4.2–300 K) and by electron spin echo – ESE (4.2–60 K). Copper(II) ions substitute Cd(II) in two different sites forming Cu(HCOO)₆–Cu_f and Cu(HCOO)₂(H₂O)₄–Cu_w octahedral complexes with strong preference to Cu_f as shown by the intensity ratio of the EPR spectra (up to 20∶1). Despite different molecular structures both complexes have nearly identical EPR parameters at rigid lattice limit with g_z = 2.429, g_y = 2.092, g_x = 2.064, A_z = 120, A_y = 32 and A_x = 12 × 10⁻⁴ cm⁻¹ for Cu_f. This fact as well as strong axial deformation of the crystal field at Cu(II) sites indicate that the strong Jahn–Teller effect operates producing three wells in the potential surface with one having much lower energy than the others. In the Cu_f complex the dynamic J–T-effect has been observed as a vibronic averaging of the two g and A parameters (along z and y axes). It indicates that only one of the higher energy wells is thermally accessible and the Silver–Getz model leads to the average energy difference between the two lowest energy wells δ₁₂ = 500(60) cm⁻¹. The δ₁₂ is temperature dependent. For Cu_w complex no vibronic effects were observed in EPR spectra indicating that higher energy wells are not populated up to 300 K. The spin–lattice relaxation time T₁ and phase memory time T_M were measured up to 60 K only, because for higher temperatures the ESE decay was too fast. Spin–lattice relaxation is governed by two-phonon Raman processes which allow one to determine the Debye temperature of the crystal as Θ_D = 193 K. The ESE decay was described as V(2τ) = V₀exp(−τ/b − mτ²) indicating the contribution of the spectral diffusion (quadratic term). The ESE dephasing rate 1/T_M is governed by spectral diffusion below 15 K. For higher temperatures the T₁-processes and excitations to the higher vibronic levels of energy Δ = 166 cm⁻¹ give comparable contributions.