Abstract

Crystals of cobalt doped LiGa₅O₈ have a deep blue colour and the absorption spectrum is characteristic of Co²⁺ in tetrahedral sites. The cobalt ions enter tetrahedral gallium sites in this material. The strong absorption in the visible is due to the ⁴A₂→ ⁴A₁ (⁴P) transition which peaks at around 16,000 cm^-1. Luminescence occurs in the red and near infrared. In this material the ⁴A₁ (⁴P) level lies below the ²E(²G) level and consequently the luminescence occurs from the ⁴A₁ state. These luminescence transitions are thus spin-allowed and, since the tetrahedral site lacks inversion symmetry, the transitions occur by an electric dipole process - the lifetime at 77 K is around 200 ns. At low temperatures the ⁴A₂→ ⁴T₂ (⁴P) luminescence and absorption transitions are characterized by a sharp strong zero-phonon line. At higher temperatures the fine structure is lost, and the luminescence shows up as a broad band.

© 1984 Optical Society of America