Investigation of structural, mechanical, dynamic, electronic and optical properties of seleno-germanates A[formula omitted]GeSe[formula omitted] (A = Mg, Ca and [formula omitted]-Sr) from first principles

Abstract

In this work we systematically investigated the structural, mechanical, electronic, dynamical and optical properties of A2GeSe4 (A = Mg, Ca and Sr) seleno-germanate compounds by density functional calculations. Synthesis of Mg2GeSe4 and Sr2GeSe4 have been reported, while Ca2GeSe4 is a hypothetical compound. The lattice parameters, equilibrium volume, lattice constants, cohesive and formation energies and bulk moduli were calculated. Our calculated lattice parameters and equilibrium volumes are in reasonable agreement with available experimental data. Elastic constants satisfy the Born stability criteria, confirming mechanical stability of all three compounds. Vibrational properties, studied using a finite displacement method, revealed no negative phonon frequencies across the Brillouin zone and therefore the compounds are dynamically stable. Hybrid functional and many-body perturbation theory were employed for the study of the electronic band structure and optical properties. Band structure results suggest that Mg2GeSe4 and Ca2GeSe2 are indirect band gap and Sr2GeSe4 a direct band gap semiconductor. The Bethe–Salpeter equation (BSE) was solved to include excitonic effect for an accurate description of optical properties. Optical absorption spectra show significant optical anisotropy. Band gaps estimated from BSE are 2.58, 2.30 and 2.86 eV for Mg2GeSe4, Ca2GeSe4 and Sr2GeSe4 respectively. The calculated band gaps suggests that Mg2GeSe4, Ca2GeSe4 and Sr2GeSe4 are semiconductors with potential to absorbed light in the ultra-violet and upper visible regions. The computed band edges from a Mulliken's analysis suggest that the A2GeSe4 compounds have suitable conduction band minima (CBM) potentials of −0.97, −1.45 and −1.86 V, respectively, versus the normal hydrogen electrode (NHE) for water reduction. The calculated BSE gaps and CBM minima of A2GeSe4 suggest that they are suitable candidates that can be exploited for applications in non-linear optics, photovoltaic (multi-junction solar cells) and visible response photo-catalytic materials.