Interpretation of Gd3+spin-Hamiltonian parameters

Evidence for the validity of the superposition model as applied to the n=4 Gd 3+ spin-Hamiltonian parameters is examined. This model is shown to give a consistent interpretation of all available experimental data, provided that the possibility of the single ligand parameter b 4 varying over a wide range of both positive and negative values is accepted. The variation of this parameter is shown to be correlated with the ligand ionicity and provides a measure of the extreme variations in the ionicity of oxygen atoms in crystals.