Crystal field levels and fine structure of the ground orbital state for high spin Fe²⁺ and Fe⁴⁺ ions in YBa2(Cu1-xFex)3O7- δ

Authors:

• Czesław Zygmunt Rudowicz,
• Maolu Du,
• Yau-Yuen Yeung

Abstract

Crystal field energy levels within the whole 3d⁶ and 3d⁴ configuration and the fine structure of the ground orbital for high spin Fe²⁺ and Fe⁴⁺ ions at several possible sites in YBa₂(Cu_1-xFe_x)₃O_{7- δ} [YBCO] have been investigated. The calculations for three cases, namely ⁵D approximation, (⁵D and ³Γ_i) approximation and full diagonalization, show that the contribution to the splitting of the ground state due to spin-singlets (¹Γ_i) is negligible but that due to spin-triplets (³Γ_i) is not negligible. Especially, in the case of Fe²⁺ in Cu(1) octahedral site, the spin-triplets contribution induces a change of the spin-ground state from Γ'₄ to Γ'₁. The dependence of the fine structure splittings on Fe concentration x in YBCO for Cu(1) octahedral and Cu(2) pyramidal sites is also investigated. The phase transition from orthorhombic to tetragonal structure at Fe concentration x ≊ 0.04 results in a degeneration of levels in the case of Fe²⁺ and Fe⁴⁺ in Cu(1) octahedral site at x > 0.04. In the case of Fe²⁺ and Fe⁴⁺ in Cu(2) pyramidal site a large variation of the ground state fine structure splittings is observed with increasing Fe concentration from low values to x = 0.04, but the splittings are almost constant at x greater than 0.04. The results corroborate our previous findings from the microscopic spin-Hamiltonian analysis within the ⁵D term on the large zero-field splitting for the Fe²⁺ and Fe⁴⁺ ions in YBCO. The present approach offers an improvement over the ⁵D approximation, which turns out to be inadequate for some ion/site cases because of strong contribution to the zero-field splitting due to the spin-triplet states. © 1993.