Temperature Dependence of the Absorption Spectrum of Nickel(II)-Doped KMgCl3 and the Crystal Structure of KMgCl3

Temperature dependence from 80° to 763°K is reported for the intraconfigurational 3d8↔3d8 transitions of octahedrally coordinated [NiCl6]4− in single crystals of K(Mg1−x, Nix)Cl3 with small x. The crystal structure of the host material, KMgCl3, is reported at 298°, 383°, and 448°K. Band energies are treated in terms of the Liehr—Ballhausen model of d2,8 electronic systems with Dq, B, C, and λ used as empirical parameters. At all temperatures the data are satisfactorily represented by B=850 cm−1, C/B=3.986 (free-ion value), and λ=−275 cm−1. Thus, B, C, and λ are all reduced to about 82% of their free-ion values. The parameter λ is estimated from band splittings at 80°K. The parameter Dq varies between −570 cm−1 at 80° and −500 cm−1 at 763°K. Oscillator strengths of the 3A2g→3T1g(F) and 3A2g→3T1g(P) transitions increase by a large factor between 80° and 763°K, while that of 3A2g→3T2g increases by a small factor. The compound KMgCl3 has a cubic perovskite-type structure at 448°K. At lower temperatures it has an orthorhombically distorted perovskite-type structure in which the distortion is small and magnesium retains approximately Oh coordination.