A theoretical investigation of electronic and magnetic properties has been performed on the new members of Heusler alloys M₂NiZ (M = Sc, Ti, and V; Z = Tl and Pb) that crystallize in an inverse Heusler XA structure. The overall electronic properties and magnetic moments are predominated by M atoms, where the total magnetic moment varies linearly with the number of valence electrons, following the Slater–Pauling rule with ferro- or ferri-magnetic ground states. Their Curie temperatures are well above the room temperature and are comparable to analogous compounds. Among the sample alloys, Sc₂NiTl, Ti₂NiTl, and Ti₂NiPb are half-metals, whereas V₂NiTl has a nearly half-metallic profile that can be tuned into novel half-metal under uniform strain. Their thermodynamic, mechanical, and dynamical stabilities are also verified from their formation energy, elastic constants, and phonon spectra, respectively. A strong correlation between the directional elastic sound velocities and spatial dependence of elastic moduli is also observed. The evaluated Poisson's ratio from elastic constants and charge density plots predicts the partial ionic nature of Ti₂NiTl and Ti₂NiPb and the covalent nature of Sc₂NiTl and V₂NiTl. The optical phonon modes are found to be both Raman and infrared active, whereas the reststrahlen band is observed in the far-infrared region.

A first-principle computational method has been used to investigate the effects of Ru dopants on the electronic and optical absorption properties of marcasite FeS₂. In addition, we have also revealed a new marcasite phase in RuS₂, unlike most studied pyrite structures. The new phase has fulfilled all the necessary criteria of structural stability and its practical existence. The transition pressure of 8 GPa drives the structural change from pyrite to orthorhombic phase in RuS₂. From the thermodynamical calculation, we have reported the stability of new-phase under various ranges of applied pressure and temperature. Further, from the results of phonon dispersion calculated at Zero Point Energy, pyrite structure exhibits ground state stability and the marcasite phase has all modes of frequencies positive. The newly proposed phase is a semiconductor with a band gap comparable to its pyrite counterpart but vary in optical absorption by around 10⁶ cm⁻¹. The various Ru doped structures have also shown similar optical absorption spectra in the same order of magnitude. We have used crystal field theory to explain high optical absorption which is due to the involvement of different electronic states in formation of electronic and optical band gaps. Lӧwdin charge analysis is used over the customarily Mulliken charges to predict 89% of covalence in the compound. Our results indicate the importance of new phase to enhance the efficiency of photovoltaic materials for practical applications.

Ab initio study on the family of ternary copper chalcogenides Cu₃TaX₄ (X = S, Se, and Te) is performed to investigate the suitability of these compounds to applications as photovoltaic absorber materials. The density functional theory based full potential linearized augmented plane wave method (FP-LAPW method) is employed for computational purposes. The electronic structure and optical properties are determined including electron-electron interaction and spin-orbit coupling (SOC), within the generalized gradient approximation plus Hubbard U (GGA+U) and GGA+U+SOC approximation. The large optical band gaps of Cu₃TaS₄ and Cu₃TaSe₄ considered ineffective for absorber materials, and also the hole effective mass has been modulated through applied pressure. These materials show extreme resistance to external pressure, and are found to be stable up to a pressure range of 10 GPa, investigated using phonon dispersion calculations. The observed optical properties and the absorption coefficients within the visible-light spectrum make these compounds promising materials for photovoltaic applications. The calculated energy and optical band gaps are consistent with the available literature and are compared with the experimental results where available.