Self-trapped exciton and recombination luminescence in PbCl2, PbBr2 and their mixed crystals

Intrinsic luminescence of PbCl2, PbBr2 and their mixture have been investigated at low temperature. Two kinds of luminescence are observed in the pure crystals. The one is excited only in the excitonic absorption band and appears at 3.78 eV (UV) in PbCl2 and at 2.75 eV (B) in PbBr2. The other is excited only in the band-to-band region and is located at 2.62 eV (BG) in both crystals. In the PbCl2(1-x)-PbBr2, mixed crystals, two additional bands appear at 3.20 eV (V-1: x < 0.1) and at 2.95 eV (V-2: 0.01 < x < 0.5) owing to the coexistence of Cl- and Br-. It is proposed that there are two types of self-trapped excitons (STE): the [Pb+(X-)(3)+hole] STE responsible for the UV-(X = Cl), B-(X = Br) and V1-, V2- (X = Cl,Br) emission, and the [(Pb-2)(3+)hole] STE responsible for the BG-emission.