First-principles investigation of structural, electronic, and optical properties of K$$_3$$GaX$$_6$$ (X = Cl, Br, I) for optoelectronic applications

Abstract

In this study, we conduct a detailed first-principles investigation of the structural, electronic, optical properties, thermodynamic and mechanical stability of K $$_3$$ GaX $$_6$$ (where X = Cl, Br, I) for optoelectronic applications. This investigation is carried out using density functional theory (DFT) within the Quantum ESPRESSO framework, applying the generalized gradient approximation with the Perdew–Burke–Ernzerhof functional. The structural properties were studied by full geometry optimization and the stable lattice parameters were obtained for all the compounds. The charge density distribution shows a strong ionic character and is significantly localized around halide ions, which confirms the bonding nature within the [GaX $$_6$$ ] octahedral units. The calculated negative formation energies and elastic constants satisfying the Born criteria indicate that the investigated compounds are thermodynamically favorable and mechanically stable. For the electronic properties, the band gap of K $$_3$$ GaCl $$_6$$ , K $$_3$$ GaBr $$_6$$ and K $$_3$$ GaI $$_6$$ were calculated. A similar decreasing trend is observed in the optical properties such as absorption coefficient, optical conductivity and plasmonic response from Cl to Br to I. In contrast, the reflectivity is almost invariant over the series, suggesting similar surface interaction behavior despite the compositional variation. The projected density of states reveals that the valence band maximum is predominantly derived from the halogen p orbitals, while the conduction band minimum is mainly composed of Ga s and Ga p orbitals. The Ga-3d states are located at lower energies and do not contribute significantly to the electronic states near the Fermi level. Among the compounds studied, the calculated band-gap values show that the K $$_3$$ GaX $$_6$$ (X = Cl, Br and I) compounds are promising for optoelectronic applications in different spectral regions. K $$_3$$ GaI $$_6$$ with a narrow direct band gap of 0.80 eV is promising for near-infrared (NIR) applications, such as infrared photodetectors and NIR optoelectronic devices. K $$_3$$ GaBr $$_6$$ has an intermediate direct band gap of 2.05 eV, which is in the visible spectral range, and thus, it is a promising candidate for visible-light optoelectronic devices, such as photodetectors, light-emitting devices and optical sensors. On the other hand, K $$_3$$ GaCl $$_6$$ has a large direct band gap of 3.32 eV, suggesting that it can be a good candidate for ultraviolet (UV) and near-UV optoelectronic applications such as UV photodetectors, optical coatings and ultraviolet filtering devices.