OPTICAL SPECTRA OF 3D TRANSITION METAL IONS IN MGO.3.5AL203 SPINEL

Large gem‐quality boules of MgO·3.5A12, O3 spinel, both pure and incorporating controlled amounts of Ti3, V3+, Cr3+, Mn2, Fe2+, Co2+, or Ni2+ were synthesized by a modified verneuil technique. Each crystal was characterized by measuring the lattice parameter, chemical composition, and density. Using thin polished slices as specimens, the absorption spectrum of each crystal was measured from 400 to 50, 000 cm−1, and band centers and order‐of‐magnitude oscillator strengths were calculated from the spectrum for each specimen. For these well characterized crystals, the absorption spectra were interpreted satisfactorily in terms of crystal field theory when the complete individual spectrum as well as the spectrum of the host matrix was considered. On this basis, the oxidation state and coordination of the 3d ion could be determined. Covalent bonding effects were of definite significance in these spinels and caused an inexact correspondence between observed band centers and those predicted by the Orgel diagrams. The ratio Dq (tetrahedral)/Dg (octahedral) was 0.67 in the 1:3.5 spinel and verified the theoretical relation expressing this ratio in terms of a constant, 4/9, times the fifth power of the ratio of the site radii. Coordination preferences of the incorporated transition metal ions are also discussed. Copyright © 1969, Wiley Blackwell. All rights reserved