Luminescence of CaS:Bi

Luminescence of bismuth activated CaS, CaS:Bi, prepared in sodium polysulfide is studied. Excitation spectrum of CaS:Bi shows a band at 350 nm due to the recombination process between holes in Na⁺(Ca)²⁺ and electrons in conduction bands, in addition to bands at 260 nm from band gap of CaS, and at 320 nm (¹S₀ → ¹P₁) and at 420 nm (¹S₀ → ³P₁) from electronic energy transitions of Bi. Emission band at 450 nm is from ³P₁ → ¹S₀ transition of Bi³⁺, bands at 500 nm and 580 nm correspond to recombinations of electron donors (Bi³⁺(Ca)²⁺ and V(s)^2-) with acceptors (V(Ca)²⁺ and Na⁺(Ca)²⁺). Emission band of ³P₁ → ¹S₀ transition is shifted to longer wavelength from CaS:Bi to BaS:Bi, due to the increase of the Stokes shift by the decrease of the crystal field parameter from CaS:Bi to BaS:Bi.