Absorption Spectra Evidence of the Decomposition of the Ground Term of Nd ^{+ + +} Ion due to Crystalline Fields

Abstract

PARAMAGNETIC ions in crystals are subject to electrostatic crystalline potentials the symmetry properties of which depend mainly on the local arrangement of other atoms around the ion. For example, six oxygen atoms grouped octahedrally around the ion, as in NdCl₃.6H₂O and Nd₂(SO₄)₃.8H₂O, give rise to a field of cubic symmetry of the type D(x⁴ + y⁴ + z⁴). A general method for determining the decomposition of the energy levels of the ion in such a field has been worked out by Bethe¹, and Van Vleck² has utilized this theory to account for the broad features of the temperature variation of the magnetic susceptibility of crystals. Penney and Schlapp³ have considered the rare earth salts in detail. From the temperature variation of the mean susceptibility, as measured by Gorter and de Haas, they find that the level ⁴I_9/2 of the free ion is split into three groups of levels at 0, 244 and 610 cm.^-1. Spedding⁴, by direct spectroscopic measurements of the absorption spectra of this crystal, finds levels at 0, 77 and 260 cm.^-1. From these results the conclusion is drawn that the levels predicted by Schlapp and Penney are not found, but the levels actually observed have the same relative, but only one third overall, separation. Determining the value of the constants in Penney and Sehlapp's equations, Spedding finds that his results are in better agreement with the susceptibility measurements of Zernicke and James, and of Selwood.