Evidence of three different Eu2+ sites and their luminescence quenching processes in CaAl2O4:Eu2+

Abstract

We report the 5d–4f luminescence of Eu2+ and europium trapped exciton (ETE) luminescence in the different three crystallographic Ca sites in the Eu2+-doped CaAl2O4 and those quenching processes based on the results of the temperature dependence of photoluminescence and photoconductivity. The CaAl2O4:Eu2+ shows the 5d–4f luminescence peaked at 440nm, 540nm and the ETE luminescence peaked at 650nm by UV and blue light excitation at low temperatures. The quenching temperature, at which the luminescence intensity is 50% of the intensity at low temperatures, of 440nm, 540nm and 650nm bands are >300K, 195K and 69K, respectively. The strong quenching for the 650nm band is caused by the thermal ionization process, which was proved by the photoconductivity measurement. We also construct the vacuum referred binding energy diagram in each Ca site and discuss optical properties from the relative energy location between the 5d state, ETE state and conduction band.