A first principles study of structural stabilities and electronic, optical and photocatalytic properties of Seleno-Germanates A2GeSe4 (A = Mg, Ca, -Sr and Ba) compounds

Abstract

In this work, we systematically investigated the structural, mechanical, dynamic, electronic, optical and photocatalytic properties of A2GeSe4 (A = Mg, Ca, -Sr and Ba) seleno-germanate compounds by density functional calculations. Synthesis of Mg2GeSe4,

-Sr2GeSe4 and Ba2GeSe4 have been reported, while Ca2GeSe4 is a hypothetical compound. Good structural chemistry and physical properties of seleno-germanates compounds have been reported, their tetrahedral GeSe4 and polyhedral A2Sen coordinations (A = Alkaline earth metals; n = 6 - 9 fold coordination) have a signi cant role in shaping their physical properties. Density functional theory (DFT) and many-body perturbation theory (MBPT) were used in our theoretical calculation of structural, mechanical, dynamic, electronic, optical and photocatalytic properties. The ground-state properties i.e. 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, con rming mechanical stability of all four compounds and the calculated cohesive and formation energies also confirm that Ca2GeSe4 can be formed. 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 was employed for the study of the electronic band structure and optical properties. Band structure results suggest that Mg2GeSe4 and Ca2GeSe2 are indirect band gap whereas -Sr2GeSe4 and Ba2GeSe4 are direct band gap semiconductor respectively. For the study of optical spectra, 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 onset absorption spectra are 2.58, 2.30, 2.86 and 2.05 eV for Mg2GeSe4, Ca2GeSe4, -Sr2GeSe4 and Ba2GeSe4 respectively. The calculated band gaps suggests that Mg2GeSe4, Ca2GeSe4, -Sr2GeSe4 and Ba2GeSe4 are semiconductors with potential to absorb light in the ultra-violet and upper visible regions and are suitable for applications as non-linear optics, photovoltaic (multi-junction solar cell) and photocatalytic materials. Surface energy and electronic properties of 100, 101, 110 and 111 periodic supercell surfaces of A2GeSe4 is calculated using the Generalized Gradient Approximation (GGA) the PBEsol, based on the calculated surface energies; a Wul plot is constructed to analyzed the surface stability. From surface energy calculations and Wul plot, Mg2GeSe4(101) and (110), Ca2GeSe4(100), -Sr2GeSe4(111) and Ba2GeSe4(111) surfaces are identi ed as the most stable surfaces. Surface charge density distribution analysis using Bader charge analysis reveals the presence of charge accumulation at isosurface of A2GeSe2. This change is likely to decrease charge recombination, facilitate easy charge transfer to adsorbate and enhance photocatalytic activities. Comparison of total and projected density of states (TPDOS) from the electronic structure of surface and crystal bulk reveal additional electronic surface state near the valence band and reduced band gap in surface structure with shifting of conduction band minimum towards the valence band maximum. Band edge positions, the valence band maximum (EVBM) and conduction band minimum (ECBM) are calculated on the most stable surface using band gap center (BGC) technique from periodic supercell GGA framework and crystal bulk quasiparticle G0W0+BSE band gap. Our calculated EVBM and ECBM for Mg2GeSe4(101) and (110), Ca2GeSe4(100), -Sr2GeSe4(111) and Ba2GeSe4(111) are -6.017, -6.480, -5.794, -6.009, -4.736 and -3.437, -3.90, -3.824, -3.149 and -2.689 eV respectively. Band edge positions in comparison to redox potential both at pH = 0 and 7 show that A2GeSe4 (A = Mg, Ca and -Sr) thermodynamically have the ability to photocatalytically oxidise and reduce water while Ba2GeSe4 is capable of photocatalytically reduction of water. The absolute Mulliken's electronegativity, the so-called EN-model analysis of EVBM and ECBM compared to redox potential of water versus the normal hydrogen electrode (NHE) shows that the A2GeSe4 (A = Mg, Ca, -Sr and Ba) compound can straddle redox potential of water at pH = 7. Finally, the calculated ground-state properties, electronic and optical properties, the EVBM and ECBM positions in comparison to reiii dox potentials of water revealed that A2GeSe4 (A = Mg, Ca, -Sr and Ba) are stable compounds and suitable candidates that can be exploited for applications in non-linear optics, photovoltaic (multi-junction solar cells) and visible response photocatalytic materials