Structural, electronic, and spectroscopic effects of Ga codoping on Ce-doped yttrium aluminum garnet: First-principles study

Periodic-boundary-conditions density-functional theory and embedded cluster wave-function theory calculations performed on Ga-doped and Ce,Ga-codoped yttrium aluminum garnet (YAG) Y3 Al5 O12, allowed for the determination of the atomistic structures of these materials when Ga substitutes for Al in octahedral and tetrahedral sites and Ce substitutes for Y, as well as for the shifts of the local excited states of main character Ce4 f1, Ce5 d1, and Ce6 s1 induced by Ga codoping. The experimental blueshift experienced by the lowest Ce4f→5d absorption upon Ga codoping has been reproduced and it has been found to be caused by the reduction in the effective ligand splitting of the 5 d1 manifold, which is due to Ga forcing an anisotropic expansion of the surroundings of Ce. The effects of Ga on the energy centroids of the 4 f1 and 5 d1 configurations are negligible. The direct electronic effects of Ga are insignificant and all effects of Ga codoping are a consequence of the geometrical distortions it causes. This picture corresponds to a simple model under use and it contrasts with the case of La codoping, where the direct electronic effects of La and the centroid energy shift are responsible for the redshift. The reason for such a different behavior could lie in the distance between the dopant and the Ce impurity, which is shorter for Ce,La:YAG than for Ce,Ga:YAG.