The pressure dependence of the lattice and elastic constants of the orthorhombic YBa ${}_2$ Cu ${}_3$ O ${}_7$ are firstly investigated using the first principles calculations based on the density functional theory. The calculated lattice parameters at 0 GPa are in agreement with the available experimental data. By the elastic stability criteria under isotropic pressure, it is predicted that YBa ${}_2$ Cu ${}_3$ O ${}_7$ with and orthorhombic structure is mechanically stable under pressure up to 100 GPa. On the basis of the elastic constants, Pugh’s modulus ratio, Poisson’s ratio, elastic anisotropy, Debye temperature, and the minimum thermal conductivity of YBa ${}_2$ Cu ${}_3$ O ${}_7$ under pressure up to 100 GPa are further investigated. It is found that its ductility, Debye temperature, and minimum thermal conductivity increase with pressure.

The effects of pressure on the structural and elastic properties of orthorhombic TiAl are investigated using first-principles calculations based on density functional theory within the projector augmented wave method. The calculated lattice parameters at 0 GPa are in good agreement with the available experimental data. The pressure dependence of the normalized lattice parameters and the single crystal elastic constants are investigated. By the elastic stability criteria under pressure, it is found that orthorhombic TiAl is mechanically stable under pressure up to 100 GPa. The elastic moduli and Poisson’s ratio under pressure up to 100 GPa are calculated using the Hill average method. The ductility/brittleness under pressure are evaluated, and a critical pressure for brittle-to-ductile transition is found to be 40 GPa. The elastic anisotropy and Debye temperature under different pressure are estimated from the calculations.

The lattice parameters and elastic constants of the tetragonal LiFeAs and NaFeAs under different pressures have been investigated by using the first‐principles calculations. It is found that their lattice parameters at 0 GPa are in agreement with the available experimental data. By the elastic stability criteria under isotropic pressure, it is found that LiFeAs and NaFeAs with the tetragonal structure are not mechanically stable above 16 GPa and 18 GPa, respectively. Besides, Pugh’s modulus ratio, Poisson’s ratio, Vickers hardness, and elastic anisotropy factors of LiFeAs in the pressure range of 0–16 GPa and NaFeAs in the pressure range of 0–18 GPa are systematically investigated. It is shown that their ductilities increase with increasing pressure, and the ductility of NaFeAs is superior to that of LiFeAs under different pressures.

Hydrogen energy from solar water-splitting is known as an ideal method with which to address the energy crisis and global environmental pollution. Herein, the first-principles calculations are carried out to study the photocatalytic water-splitting performance of single-layer GaInSe₃ under biaxial strains from −2% to +2%. Calculations reveal that single-layer GaInSe₃ under various biaxial strains has electronic bandgaps ranging from 1.11 to 1.28 eV under biaxial strain from −2% to +2%, as well as a completely separated valence band maximum and conduction band minimum. Meanwhile, the appropriate band edges for water-splitting and visible optical absorption up to ~3 × 10⁵ cm⁻¹ are obtained under biaxial strains from −2% to 0%. More impressively, the solar conversion efficiency of single-layer GaInSe₃ under biaxial strains from −2% to 0% reaches over 30%. The OER of unstrained single-layer GaInSe₃ can proceed without co-catalysts. These demonstrate that single-layer GaInSe₃ is a viable material for solar water-splitting.

The pressure effects on the lattice parameters and elastic constants of the tetragonal RNi ${}_2$ B ${}_2$ C (R=Y, Lu) are investigated by means of the first principles. The predicted lattice constants and elastic constants of YNi ${}_2$ B ${}_2$ C and LuNi ${}_2$ B ${}_2$ C at 0 GPa agree well with the available data. By the elastic stability criteria under isotropic pressure, it is predicted that YNi ${}_2$ B ${}_2$ C and LuNi ${}_2$ B ${}_2$ C with tetragonal structure are not mechanically stable above 93 GPa and 50 GPa, respectively. Pugh’s modulus ratio, Poisson’s ratio, Vickers hardness, elastic anisotropy and Debye temperature of YNi ${}_2$ B ${}_2$ C in the pressure range of 0–100 GPa and LuNi ${}_2$ B ${}_2$ C in the pressure range of 0-60 GPa are further investigated. It is shown that the ductility and Debye temperature of tetragonal RNi ${}_2$ B ${}_2$ C (R=Y, Lu) increase with increasing pressure, and LuNi ${}_2$ B ${}_2$ C is more ductile and lower Debye temperature than YNi ${}_2$ B ${}_2$ C under different pressures.

The volume versus temperature relations for Ni ${}_3$ Si and Ni ${}_3$ Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized stacking fault energies and generalized planar fault energies of Ni ${}_3$ Si and Ni ${}_3$ Ge are investigated by first principles calculations. The elastic constants, antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies and twinning energy decrease with increasing temperature. The twinnability of Ni ${}_3$ Si and Ni ${}_3$ Ge are examined using the twinnability criteria. It is found that their twinnability decrease with increasing temperature. Furthermore, Ni ${}_3$ Si has better twinnability than Ni ${}_3$ Ge at different temperatures.