The structural, electronic, dipole-induced internal electric field, optical and photocatalytic properties of monolayer GeS and GeSe under external biaxial strain were investigated by using first-principles calculations. The monolayer GeS and GeSe are indirect semiconductors with the band gaps of 3.265 eV and 2.993 eV, respectively. The band alignment of the monolayer GeS and GeSe manifests the photocatalytic activity for water splitting. Especially, it is effective to tune the properties including structures, band gaps, surface potential difference, dipole moment P, dipole-induced internal electric field, absorption and photocatalytic activity of the monolayer GeS and GeSe via biaxial strain. Our results suggest that monolayer GeS and GeSe possess photocatalytic properties for water splitting, and strain engineering, especially tensile strain, can enhance the photocatalytic activity under ultraviolet and visible light.

We present a study of the thermodynamic and physical properties of ${\text{Ta}}_5{\text{Si}}_3$ compounds by means of density-functional theory based calculations. Among the three different structures $\left(\text{D}{8}_{\text{m}},\text{D}{8}_{\text{l}},\text{D}{8}_8\right)$, the $\text{D}{8}_{\text{l}}$ structure (${\text{Cr}}_5{\text{B}}_3$ prototype) is the low-temperature phase with a high formation enthalpy of $-449.20\text{kJ}/\text{mol}$, the $\text{D}{8}_{\text{m}}$ structure (${\text{W}}_5{\text{Si}}_3$ prototype) is the high-temperature phase with a formation enthalpy of $-419.36\text{kJ}/\text{mol}$, and the $\text{D}{8}_8$ structure (${\text{Mn}}_5{\text{Si}}_3$ prototype) is a metastable phase. The optimized lattice constants of the different ${\text{Ta}}_5{\text{Si}}_3$ compounds are also in good agreement with the experimental data. The electronic density of states and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the $\text{D}{8}_{\text{m}}$ and $\text{D}{8}_{\text{l}}$ structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter have also been obtained in the present work. The transformation temperature (2303.7 K) between the $\text{D}{8}_{\text{m}}$ and the $\text{D}{8}_{\text{l}}$ structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7 K) is close to the experimental value (2433.5 K).

We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5\char21{}6 meV, similar to what has been found in Zn4Sb3, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic p-type conductivity of ZnSb.