A new colour centre in calcium oxide: possible identification of the FA+centre

Abstract

A new colour centre has been observed in neutron-irradiated crystals of calcium oxide doped with magnesium. It has an absorption band peaking at 374 nm and a complementary emission band at 398 nm. A zero-phonon line appears at 384.5 nm in absorption and emission. The value of the Huang-Rhys factor for both bands is S approximately=4. The defect cannot be detected in absorption in irradiated pure crystals, but the zero-phonon line has been detected weakly in emission. The line is shifted to the blue and broadened in the doped crystals compared with the pure crystals. A similar shift and broadening occurs for the F⁺ centre zero-phonon line. These effects can be understood in terms of internal strains caused by the magnesium impurities. Uniaxial stress experiments show that the new centre has tetragonal symmetry, the absorption and emission bands having electric dipole transition moments parallel to the fourfold axis. It is proposed that this defect is an F_A⁺ centre, an interpretation supported by a comparison of its properties with those of the F⁺ centre.