Crystal Spectrum of the Rare-Earth Hexahalide Complexes, MCl63−: The Absorption Spectrum of [(C6H5)3PH]3NdCl6

The site symmetry of Nd³⁺ in single‐crystal [(C₆H₅)₃PH]₃NdCl₆ is predicted by considerations of molar absorptivity, vibronic spectra, and stoichiometry. A calculation of the crystalline‐field splitting of ${\text{4f}}^{3}J$ levels appears to predict a sixfold octahedral site symmetry with a slight distortion along one of the axes, in agreement with the observed spectrum. In addition, the observed spectrum indicates the occurrence of two slightly inequivalent sites of Nd³⁺. The effects of the small distortion away from octahedral symmetry and the spectrum due to two inequivalent sites are sufficiently small (5% of the average J‐manifold crystalline‐field splitting), however, as not to affect a first‐order crystal‐field analysis of the dominant portion of the spectrum. The crystal‐field parameters based on octahedral symmetry, $B_4\text{\hspace{0.17em}=\hspace{0.17em}282}{\mathrm{cm}}^{\text{−1}}$ and $B_6\text{\hspace{0.17em}=\hspace{0.17em}74.7}{\mathrm{cm}}^{\text{−1}}$ , predict a splitting of crystal quantum states in good agreement with experiment.