Crystal field levels and zero-field splitting parameters of Cr²⁺ in the mixed system Rb₂MnₓCr₁₋ₓCl₄

The crystal field (CF) energy levels and the fine structure of the orbital ground state as well as the zero-field splitting (ZFS) parameters are investigated for Cr²⁺ in the mixed system Rb₂MnₓCr₁₋ₓCl₄. The CF parameters from the earlier superposition model analysis are used as input for a newly developed computer package to diagonalize the orthorhombic CF and spin-orbit Hamiltonian for two cases, namely, the ⁵D approximation and the ⁵D-³Γᵢ one. The results show that the spin-triplets (³Γᵢ) contributions to the fine structure of the orbital ground state and the ZFS parameters are not negligible, although all ³Γᵢ states lie above the ⁵D states. The theoretical CF levels for different Cr²⁺ concentrations (x) are in good agreement with experimental spectroscopic data. For the axial ZFS parameter b⁰₂, the theoretical result is consistent with the experimental magnetic anisotropy data. A change in the sign of the orthorhombic ZFS parameter b²₂ with Cr²⁺ concentration x increasing from x = 0 to x = 0.01 is predicted. This is consistent with the crystal structure data which indicate a ‘structural phase transition’ with x at low temperatures. EPR studies would help to solve the question of the sign of b²₂ unambiguously. Copyright © 1993 Published by Elsevier B.V.