Cr 3+ ions under very weak crystal fields

Abstract

The crystal field strength and the covalence effect in the bonds to the Cr 3+ ions, in the crystalline solutions Na 1+xCr x Zr 2−x(PO 4) 3 and Ca 0.5+x/2Cr x Zr 2−x(PO 4) 3 with x=0.06→ 1 have been determined from diffuse reflectance spectra. Chromium is located in the octahedral sites of the structure under a weak crystal field which varies with composition. The measured crystal field strength increases with the degree of substitution, contrary to the effect that corresponds to similar substitutions in corundum (ruby). These calculated Δ parameters are the lowest found for Cr 3+ under octahedral oxide polyhedron and an empirical correlation with the CrO distance is shown. The forbidden transitions to the 2E and 2T 1 levels are remarkable stronger than those in other oxide compounds. The deconvolution of the diffuse reflectance spectra into their gaussian components shows a big distortion in the coordination polyhedra, but this is not responsible for the appearance of the forbidden transitions, that it is better correlated to the mixing of the energy levels.