Magnetic single-ion anisotropy and zero-field splittings of Mn²⁺ ions in some low-dimensional chlorides

The zero-field splitting parameter D of the ⁶S_5/2 ground state of Mn²⁺ ions with Cl^- ligands is discussed. Ab initio calculations of D, based on the relativistic spin-spin and Blume-Orbach splitting mechanisms, and an electrostatic model of the crystal field, are in poor agreement with experimental measurements for Mn²⁺ ions in a representative group of six chloride host compounds: CdCl₂, MgCl₂, CsMgCl₃, CsZnCl₅, TMCC and (CH₂NH₃)₂CdCl₄. In contrast, the empirical superposition model gives a consistent account of the experimental data for six-fold coordinated Mn²⁺ ions in this group of compounds. The dependence of the intrinsic parameter b₂ on ligand distance R is found to be b₂=-0.235 (R-R₀) cm^-1, with R₀=2.692 AA, suggesting that the zero-field splitting involves a cancellation effect between two competing mechanisms. The superposition model is used to predict zero-field splittings of D=+0.019 and +0.036 cm^-1 in the low-dimensional antiferromagnets TMMC and (CH₃NH₃)₂ MnCl₄ respectively.