Modeling Defect States and the Role of 4f Electrons in Gd₃Ga₅O₁₂ Using DFT Methods

Abstract

Gadolinium gallium garnet (Gd₃Ga₅O₁₂, GGG) is an important synthetic crystal widely used in laser systems, magneto-optics, and radiation-resistant electronics. A fundamental understanding of its electronic structure and defect states is essential for optimizing optical and functional properties. This study aims to investigate the role of 4f electrons and oxygen vacancies in shaping the band structure and optical response of GGG using modern density functional theory (DFT) methods. Calculations were performed with the VASP package using GGA-PBE, SCAN, mBJ, and hybrid HSE06 functionals. Comparative analysis shows that Gd³⁺ 4f electrons significantly affect the band gap and density of states: their inclusion creates localized levels near the valence band edge and narrows the band gap, whereas their exclusion overestimates the gap and neglects local effects. Charged [GGG]²⁺ vacancies shift the Fermi level without forming defect levels, while neutral [GGG]⁰ vacancies create localized states within the gap, reducing transparency and deteriorating the optical properties of the crystal.

The results have both fundamental and applied significance. Scientifically, they clarify the role of strongly correlated 4f electrons in garnets and the mechanism of oxygen defect effects. Practically, they provide a basis for predicting radiation resistance and designing next-generation optical materials. This work contributes to the physics of defect states and theoretical modeling of functional crystals.