VALENT STATE AND COORDINATION OF COBALT IONS IN BERYL AND CHRYSOBERYL CRYSTALS

Abstract

We have studied the polarized optical absorption and EPR spectra of Co-doped beryls grown by hydrothermal, flux, and gas-transport methods, and chrysoberyl grown by the Czochralski method. In beryls three groups of bands, belonging to three various Co centers, were distinguished by analysis of the absorption band intensities. The first group, bands with maxima at 22 220 (E c), 17 730 (E c), and 9090 (E c), 7520 (E c) cm–1 are due to Co2+ in octahedral site of Al3+. The second group is bands at 18 940, 18 250, 17 700 (E c), 18 300, 17 700, 17 000 (E c) and 8830 (E c), 7350 (E c) cm–1 and 5320 (E c), 3880 (E c) cm–1, which are caused by Co2+ in tetrahedral site of Be2+. A weak wide band in flux and gas-transport beryl in the region of 12 500–8300 cm–1 (E , c) is related to Co3+ in octahedral Al3+ site. In hydrothermal beryl, bands 13 200 (E c), 10 900 (E c), and 8500 (E c) cm–1 are caused by an uncontrolled impurity of Cu2+ ions. For Co-doped chrysoberyl one type of center of Co has been established: Co2+ in the octahedral site of Al3+. In the approximation of the trigonal field with regard to Trees correction, the energy levels of Co2+ have been calculated in octahedral and tetrahedral coordination. There is good agreement between the obtained experimental and calculated data. The polarization dependence of the optical absorption bands is explained well in terms of the spin-orbit interaction.