Variable Temperature Single Crystal EPR Studies of Cu(II) in Dipotassium Diaquabis(Malonato-κ2O,O′) Nickelate Dihydrate: An Example of Contaminated Ground State

Abstract

Single crystal X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in dipotassium diaquabis(malonato-κ²O,O′) nickelate dihydrate have been performed at 300, 123 and 77 K to understand the nature of Jahn–Teller distortion in the paramagnetic host lattice. The angular variation of the EPR spectra reveals the presence of two sites, with one site not showing hyperfine resolution even at 77 K. The spin-Hamiltonian parameters of this six-coordinated Cu(II) ion, evaluated from EPR spectra at various temperatures, are:

• 300 K: g₁₁ = 2.125, g₂₂ = 2.118, g₃₃ = 2.290, no copper hyperfine resolution

• 123 K: g₁₁ = 2.229, g₂₂ = 2.113, g₃₃ = 2.319 and A₁₁ = 5.02, A₂₂ = 3.82, A₃₃ = 6.87 mT

• 77 K: g₁₁ = 2.224, g₂₂ = 2.114, g₃₃ = 2.324 and A₁₁ = 5.32, A₂₂ = 3.90, A₃₃ = 7.06 mT

respectively. The low value observed for A₃₃ at 123 and 77 K has been explained by assuming a ground state \({\text{d}}_{{x^{2} - y^{2} }}\) wave function for Cu(II) ions, contaminated with the excited state \({\text{d}}_{{z^{2} }}\). From the temperature dependence of the EPR spectra, the Cu(II) ions can be considered as a static Jahn–Teller system, with contaminated ground state. The admixture coefficients and bonding parameters have also been calculated by combining EPR and optical data. The EPR spectrum of powder sample confirms single crystal data.