A theoretical study of the relative importance of chemical and geometric effects for ce-based scintillation in La and Y aluminum perovskites

The purpose of this work is to understand, for Ce doped materials, how the effect of the local crystal environment of Ce compared to the character of the host cation, influences the scintillation and fluorescence of different materials. In the case of YAP (YAlO₃) and LAP (LaAlO₃) while very similar materials, YAP is a moderately bright Ce activated scintillator while LAP shows no Ce activation. It is known from experimental work that in the case of LAP the Ce 5d states lie in the conduction band of the host, quenching any scintillation. Since YAP and LAP have different crystal structures it is not possible to know from experiment if the change in the local environment of the Ce or the different host cation are responsible for the very different scintillation properties. To isolate the different effects of local environment compared to host cation, two theoretically synthesized systems obtained by swapping La and Y in LAP and YAP geometries were created. In this way we artificially create a La version of YAP (and Y version of LAP) where the local environment of Ce is the same as YAP and only the next nearest neighbour cation differs. We then perform first principles density functional based calculations for all four systems to determine the positions of the 4f and 5d Ce states relative t o the conduction and valence bands of the host. Our preliminary results show that LAP in the YAP geometry still has the Ce 5d in the conduction band, quenching scintillation while YAP in the LAP geometry still has the 5d below the conduction band. This suggests the cation character, which typically determines the conduction band character and position, is more important in determining Ce activation properties than the local dopant environment for these materials.