Dimensionality Effect of the “Free-Oxygen” Motif on the Crystal Field Splitting in Rare-Earth Crystals

Abstract

Crystal field theory is one of the fundamental theories to study the electron level splitting and corresponding optical properties of transition metal and lanthanide compounds. The magnitude of energy level splitting is determined by its crystal field intensity; however, a quantitative dependence on the constituent motif is still absent in rare-earth crystals. Herein, we build a modified point-charge model with ligand electro-dipole polarizability and bond length involved, for crystal field strength analysis. A special “free-oxygen” motif is verified to improve the crystal field splitting significantly, due to its large ligand polarizability and short bond length. Meanwhile, experimental spectroscopies demonstrate a large energy splitting of the 2F7/2 manifold of more than 950 cm–1 in Yb:YCaO(BO3), Yb:Na2Y2O(BO3)2, and Yb:LiY6O5(BO3)3 crystals. More importantly, an interesting relation between energy level splitting and the dimensionality of the “free-oxygen” motif is discovered in a 2F5/2-2F7/2 barycenter diagram. Finally, this dimensionality effect of the “free-oxygen” motif can be extended in other systems, such as the Yb:Y2O(SiO4) signal crystal with a large splitting of 1030 cm–1. This work identifies the “free oxygen” as a feasible unit to increase crystal field intensity and provides an opportunity for designing new materials with desirable photoluminescence and potential laser applications.