Optical Spectra of Transition-Metal Ions in Corundum

The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point-charge model and p-d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational-electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal-field parameters have been calculated as convergent lattice sums. The observed trigonal-field parameter has the opposite sign from that calculated by the point-charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 Å displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

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