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

Abstract

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(Mg_1−x, Ni_x)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 d^2,8 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 ³A_2g→³T_1g(F) and ³A_2g→³T_1g(P) transitions increase by a large factor between 80° and 763°K, while that of ³A_2g→³T_2g 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 O_h coordination.