Preferential Eu Site Occupation and Its Consequences in the Ternary Luminescent Halides AB2I5 ∶ Eu2+ (A = Li–Cs; B = Sr, Ba)

Abstract

Several rare-earth-doped, heavy-metal halides have recently been identified as potential next-generation luminescent materials with high efficiency at low cost. AB2I5 ∶ Eu2+ (A = Li–Cs; B = Sr, Ba) is one such family of halides. Its members, such as CsBa2I5 ∶ Eu2+ and KSr2I5 ∶ Eu2+, are currently being investigated as high-performance scintillators with improved sensitivity, light yield, and energy resolution less than 3% at 662 keV. Within the AB2I5 family, our first-principles-based calculations reveal two remarkably different trends in Eu site occupation. The substitutional Eu ions occupy both eightfold-coordinated B1 (VIII) and the sevenfold-coordinated B2 (VII) sites in the Sr-containing compounds. However, in the Ba-containing crystals, Eu ions strongly prefer the B2 (VII) sites. This random versus preferential distribution of Eu affects their electronic properties. The calculations also suggest that in the Ba-containing compounds one can expect the formation of Eu-rich domains. These results provide atomistic insight into recent experimental observations about the concentration and temperature effects in Eu-doped CsBa2I5. We discuss the implications of our results with respect to luminescent properties and applications. We also hypothesize Sr, Ba-mixed quaternary iodides ABa VIII Sr VII I5 ∶ Eu as scintillators having enhanced homogeneity and electronic properties.