Gallium preference for the occupation of tetrahedral sites in Lu3(Al5-xGax)O12 multicomponent garnet scintillators according to solid-state nuclear magnetic resonance and density functional theory calculations

Abstract

In this study, the distributions of aluminum and gallium atoms over the tetrahedral and octahedral sites in the garnet structure were investigated in mixed Lu3Al5-xGaxO12 crystals by using 27Al and 71Ga magic angle spinning nuclear magnetic resonance (NMR) and single crystal 71Ga NMR. The experimental study was supported by theoretical calculations based on density functional theory (DFT) in order to predict the trends in terms of the substitutions of Al by Ga in the mixed garnets. Both the experimental and theoretical results indicated the non-uniform distribution of Al and Ga over the tetrahedral and octahedral sites in the garnet structure, with a strong preference for Ga occupying the tetrahedral sites in the garnet structure at all Ga concentrations, despite Ga having a larger ionic radius than Al and tetrahedrons being smaller than octahedrons. The Ga occupation preference is primarily related to the involvement of Ga 3d10 electrons in interactions, and due to the different nature of the chemical bonds formed by Al and Ga when located in the tetrahedral and octahedral environments. The quadrupole coupling constants and chemical shift parameters for Al and Ga nuclei were determined for all of the compounds considered, and the electric field gradients at the Al and Ga nuclei were calculated in the DFT framework. Our results also showed that the structural relaxation after Al substitution with the larger Ga mainly occurs via deformation of the octahedrons, while leaving the tetrahedrons relatively undeformed.