Structural, elastic, electronic, optical and anisotropy properties of newly quaternary Tl2HgGeSe4 via DFPT predictions associated to XPES and RS experiments

In the present work, we report on theoretical studies of thermodynamic properties, structural and dynamic stabilities, dependence of unit-cell parameters and elastic constants upon hydrostatic pressure, charge carrier effective masses, electronic and optical properties, contributions of interband transitions in the Brillouin zone of the novel Tl₂HgGeSe₄ crystal. The theoretical calculations within the framework of the density-functional perturbation theory (DFPT) are carried out employing different approaches to gain the best correspondence to the experimental data. The present theoretical data indicate the dynamical stability of the title crystal and they reveal that, under hydrostatic pressure, it is much more compressible along the a-axis than along the c-axis. Strikingly, the charge effective mass values ( \(m_{e}^{{^{*} }}\) $m_e^{_{}^{\ast }}$ and \(m_{h}^{{^{*} }}\) $m_h^{_{}^{\ast }}$ ) vary considerably when the high symmetry direction changes indicating a relative anisotropy of the charge-carrier’s mobility. Furthermore, the Young modulus and compressibility are characterized by the maximum and minimum values ( \(E^{max}\) $E^{\mathit{max}}$ and \(E^{\min }\) $E^{min}$ ) and ( \(\beta^{max}\) ${\beta }^{\mathit{max}}$ and \(\beta^{\min }\) ${\beta }^{min}$ ) that are equal to (62.032 and 28.812) GPa and (13.672 and 6.7175) TPa^–1, respectively. Additionally, we have performed calculations of the Raman spectra (RS) and reached a good correspondence with the experimental RS spectra of the Tl₂HgGeSe₄ crystal. The XPES associated to RS constitutes powerful techniques to explore the oxidized states of Se and Ge in Tl₂HgGeSe₄ system.