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In this work, we have employed the first-principles calculations to investigate the phase stability and mechanical and optoelectronic characteristics of Hf2AX (A=Al, Si and X=C, N) MAX phases. Phase stabilities of Hf2AX compounds have been evaluated by the formation enthalpy computations and phonon dispersion curves, which indicate that all studied compounds are structurally and dynamically stable. The mechanical stability has been determined by elastic stiffness constants, which confirms that the studiedMAX phases aremechanically stable. The computed results for Pugh's and Poisson ratios indicate the brittle nature of Hf2AX MAX phases. It is interesting to note that Si-based MAX phases possess high values of B/G, indicating that they are harder than Al-based compounds. The obtained band structures and partial density of states indicate the metallic character of all Hf2AX compounds. The strong hybridization of d-orbitals of Hf with p-orbitals of N and the comparatively weaker hybridization of p-orbitals (Hf) with Al/Si p-orbitals is observed. The presence of pseudogaps near the Fermi energy level emerges due to the orbital hybridization involving Hf, Al/Si, and C/N atoms. The analysis of charge density difference maps reveals the presence of a strong covalent bond between the Hf and C/N atoms, whereas a relatively weaker covalent bond is seen between the Hf and Al/Si atoms. Furthermore, numerous optical characteristics have been investigated to account for the behavior of the Hf2AX compounds to those of impinging electromagnetic rays. The highest absorptivity is noticed within the energy range of 7.5-12.5 eV. The optical spectra in the range of 1.7 eV (IR) to 9 eV ultraviolet (UV) have been observed for the investigated MAX phases, predicting their suitability as proficient energy absorbers within the UV region. The Intriguing properties of Hf2AX compounds are anticipated to be appropriate materials for a variety of applications. [ABSTRACT FROM AUTHOR]

Hydrogen storage is a crucial step in commercializing hydrogen‐based energy production. Solid‐state hydrogen storage has gained much attention from researchers and needs extensive research. In the present study, we investigate the structural, mechanical, and optoelectronic properties of lithium‐based LiAH3 (A=Mn, Fe, Co) metal hydrides to elucidate their potential for solid‐state hydrogen storage. First, we evaluate the structure stability of LiAH3 hydrides using formation enthalpies calculations. Then, the mechanical stability is determined by elastic stiffness constants, which reveal that LiAH3 hydrides are stable mechanically as they meet the Born stability requirements. Electronic band structure calculations manifest that all LiAH3 hydrides possess a metallic character. Several optical properties have been discussed in detail. The gravimetric hydrogen storage capacities of LiMnH3, LiFeH3 and LiCoH3 hydrides are 4.65, 4.60 and 4.39 wt%, respectively, achieving the target of US‐DOE for rechargeable equipment. Additionally, we have determined the volumetric hydrogen storage capacities (Cv) for all LiAH3 hydrides. It is worth mentioning that the highest Cv values have been obtained to be 180.80, 188.18 and 177.25gH2l−1 for LiMnH3, LiFeH3 and LiCoH3 hydrides, respectively, which have achieved the US‐DOE target set for 2025. Our investigation predicts the applicability of lithium‐based hydrides as promising solid‐state hydrogen storage materials. [ABSTRACT FROM AUTHOR]

Monolayer transition metal oxides have emerged as an exciting area of study of two-dimensional materials. It is crucial to comprehend the structural stability and physical characteristics of various dielectric materials at the ultrathin level due to the diminishing size of microelectronic equipment. In this regard, we have studied the structural, optoelectronic and thermodynamical characteristics of the 1T phase of transition metal oxides TMO2 (TM = Zr and Hf) using first-principles computations. The structural stability of both monolayers has been assessed by formation enthalpy computations, which suggest that both monolayers are thermodynamically stable and can be synthesized experimentally. The optimized lattice parameters for 1T-ZrO2 and 1T-HfO2 are found to be 3.15 and 3.17 Å, respectively. The phonon calculations assure the dynamical stability of both monolayers as there are no imaginary frequencies present in the phonon dispersion plots. In order to assess the thermodynamical stabilities of TMO2 monolayers, various thermodynamical characteristics have been determined for 0 to 1000 K. Notably, the TMO2 monolayers depict their thermal stability as both monolayers possess negative free energies up to 1000 K. The maximum calculated formation enthalpy value for 1T-ZrO2 and 1T-HfO2 is − 0.86 and − 0.85 eV, respectively, at 1000 K. Furthermore, the 1T-ZrO2 show the high thermal stability than 1T-HfO2 because it has a lower Helmholtz free energy value. Electronic properties have been analyzed through density of states and band structures calculations. The calculated band gaps for 1T-ZrO2 and 1T-HfO2 monolayer are 4.30 and 5.41 eV, respectively. The difference in the band gaps develops due to the varying energy levels by the valence orbitals of Zr (5s2, 4d2) and Hf (6s2, 5d2). The indicated difference acts as the main distinguishing feature among the studied metal oxides. Moreover, the valence bands in both metal oxide monolayers are mostly obtained from the p-states of Oxygen, and d-states of Zr and Hf. Furthermore, numerous optical characteristics for both under-study monolayers have been computed and compared. The first critical edge for 1T-ZrO2 and 1T-HfO2 are found to be 2.13 and 1.95, respectively. The highest absorption peaks for 1T-ZrO2 and 1T-HfO2 are noted at 5.43 and 6.24 eV. It is found that the absorptivity of 1T-HfO2 is 2.91% higher as compared to 1T-ZrO2. The high value of dielectric constants, efficient thermodynamical stability and wide band gaps of under-investigation TMO2 (TM = Zr and Hf) make them the potential contenders for 2D FETs. In short, our findings suggested that monolayers TMO2 are ideal choices for a range of applications and could help in developing efficient optoelectronic devices. [ABSTRACT FROM AUTHOR]