Electronic Spectra of Weak-Field Fluoride Complexes of Tetravalent Neodymium

Elevated temperature and pressure fluorination of Cs₃NdF₆, mixtures of neodymium oxide and cesium chloride containing 3:1 cesium:neodymium molar ratios, or Nd(III) dissolved in borax glass beads yielded products with absorption and fluorescence spectra characteristic of the ${\text{4f}}^2$ configuration, confirming the presence of Nd(IV). The observed levels were fitted satisfactorily using the Slater Coulomb integrals $F_2{\mathrm{,\; F}}_4{\mathrm{,\; F}}_6$ , and the spin–orbit interaction $\zeta$ as adjustable parameters in an iterative least‐squares free‐ion energy‐level calculation. Comparison of the free‐ion $f^2$ levels of Nd⁴⁺ in intermediate coupling with literature data for Pr³⁺ ,Th²⁺, and U⁴⁺ gave excellent agreement with expected behavior.