Investigations on the defect structure and the spin Hamiltonian parameters for LaSrGa_0.995Cu_0.005O₄ crystal

The defect structure and the spin Hamiltonian parameters (g factors g_||, g_⊥, the hyperfine structure constants A_|| and A_⊥ of Cu²⁺ and the superhyperfine parameters A_||′ and A_⊥′ of Ga³⁺) for LaSrGa_0.995Cu_0.005O₄ crystal are systematically investigated on the basis of the perturbation calculations for these quantities of a tetragonally elongated octahedral 3d⁹ complex. Arising from the Jahn–Teller effect, the impurity Cu²⁺ center indicates the slightly larger relative tetragonal elongation ratio (≈4.0(1)%) than that (≈3.40(4)%) of the host Ga³⁺ site. The computed values of the spin Hamiltonian parameters agree well with the measured results. To the best of our knowledge, the superhyperfine parameters of Ga³⁺ are theoretically studied for the first time. The unusual superhyperfine anisotropy (A_||′⊥′) for the present Ga³⁺ ligand with respect to the conventional rule (A_||′>A_⊥′) for the common anion ligands is ascribed to the opposite signs of the anisotropic terms for the [CuGa₄]¹⁴⁺ cluster. The present studies reveal significant Cu²⁺–O²⁻ orbital hybridization and local tetragonal elongation around Cu²⁺, reflecting, to a certain extent, similar behaviors of the CuO₂ planes in oxide cuprates high T_c superconductors