Investigating Stable Low-Energy Gallium Oxide (Ga2O3) Polytypes: Insights into Electronic and Optical Properties from First Principles

This study provides a comprehensive analysis of the electronic and optical properties of low-energy gallium oxide (Ga2O3) polytypes not considered earlier. Among these polytypes, the monoclinic structure (β-Ga2O3) holds significant relevance for both research and practical applications due to its superior stability under typical conditions. The primary aim of this research is to identify new and stable Ga2O3 polytypes that may exist under zero-temperature and zero-pressure conditions. To achieve this objective, we employ the VASP code to investigate electrical and optical properties, as well as stability assessments. Additionally, we examine phonon and thermal properties, including heat capacity, for all polytypes. This study also encompasses the computation of full elastic tensors and elastic moduli for all polytypes at 0 K, with Poisson's and Pugh's ratios confirming their ductile nature. Furthermore, we present the first ever report on the Raman- and infrared (IR)-active modes of these stable Ga2O3 polytypes. Our findings reveal that these mechanically and dynamically stable Ga2O3 polytypes exhibit semiconductive properties, as evidenced by electronic band structure investigations. This research offers valuable insights into the optical characteristics of Ga2O3 polytypes with potential applications spanning various fields.

First-Principles Insights into the Relative Stability, Physical Properties, and Chemical Properties of MoSe2

A fascinating transition-metal dichalcogenide (TMDC) compound, MoSe₂, has attracted a lot of interest in electrochemical, photocatalytic, and optoelectronic systems. However, detailed studies on the structural stability of the various MoSe₂ polymorphs are still lacking. For the first time, the relative stability of 11 different MoSe₂ polymorphs (1H, 2H, 3H_a, 3H_b, 2T, 4T, 2R₁, 1T₁, 1T₂, 3T, and 2R₂) is proposed, and a detailed analysis of these polymorphs is carried out by employing the first-principles calculations based on density functional theory (DFT). We computed the physical properties of the polymorphs such as band structure, phonon, and elastic constants to examine the viability for real-world applications. The electronic properties of the involved polymorphs were calculated by employing the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06). The energy band gap of the polymorphs (1H, 2H, 3H_a, 3H_b, 2T, 4T, and 2R₁) is in the range of 1.6-1.8 eV, coinciding with the experimental value for the polymorph 2H. The covalent bonding nature of MoSe₂ is analyzed from the charge density, charge transfer, and electron localization function. Among the 11 polymorphs, 1H, 2H, 2T, and 3H_b polymorphs are predicted as stable polymorphs based on the calculation of the mechanical and dynamical properties. Even though the 4T and 3H_a polymorphs' phonons are stable, they are mechanically unstable; hence, they are considered to be under a metastable condition. Additionally, we computed the direction-dependent elastic moduli and isotropic factors for both mechanically and dynamically stable polymorphs. Stable polymorphs are analyzed spectroscopically using IR and Raman spectra. The thermal stability of the polymorphs is also studied.

Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations

Materials made of indium oxide (In₂O₃) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In₂O₃ polytypes. The cubic structure In₂O₃ is majorly used for many of the In₂O₃-based transparent conducting oxides. The objective of this study is to explore other new stable In₂O₃ polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In₂O₃ polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson's and Pugh's ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

First-Principles Exploration into the Physical and Chemical Properties of Certain Newly Identified SnO2 Polymorphs

Tin dioxide (SnO₂) is one of the transparent conductive oxides that has aroused the interest of researchers due to its wide range of applications. SnO₂ exists in a variety of polymorphs with different atomic structures and Sn-O connectivity. However, there are no comprehensive studies on the physical and chemical properties of SnO₂ polymorphs. For the first time, we investigated the structural stability and ground-state properties of 20 polymorphs in the sequence of experimental structures determined by density functional theory. We used a systematic analytical method to determine the viability of polymorphs for practical applications. Among the structurally stable polymorphs, Fm3̅m, I4₁/amd, and Pnma-II are dynamically unstable. As far as we know, no previous research has investigated the electronic properties of SnO₂ polymorphs from the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06) except P4₂/mnm, with calculated band gap values ranging from 2.15 to 3.35 eV. The dielectric properties of the polymorphs have been reported, suggesting that SnO₂ polymorphs are also suitable for energy storage applications. The bonding nature of the global minimum rutile structure is analyzed from charge density, charge transfer, and electron localization function. The Imma-SnO₂ polymorph is mechanically unstable, while the remaining polymorphs met all stability criteria. Further, we calculated Raman and IR spectra, elastic moduli, anisotropic factors, and the direction-dependent elastic moduli of stable polymorphs. Although there are many polymorphic forms of SnO₂, rutile is a promising candidate for many applications; however, we investigated the feasibility of the remaining polymorphs for practical applications.