GeS Phases from First-Principles: Structure Prediction, Optical Properties, and Phase Transitions upon Compression

Germanium (Ge) based binary chalcogenides have emerged as promising materials for various electronic and optoelectronic applications due to their unique properties. Binary chalcogenides are compounds made up of two elements from the chalcogen family (sulfur, selenium, and tellurium) and can be combined with germanium to form a wide range of materials with distinct properties. Ge-based binary chalcogenides such as GeTe, GeSe, and GeS have been shown to have promising properties for photovoltaic cells. These materials have a high absorption coefficient, which makes them efficient absorbers of light. Additionally, they have a direct bandgap, which means that they can be used to make thin-film solar cells with high efficiency. Moreover, Ge-based binary chalcogenides have also been investigated for their thermoelectric properties. These materials have a high Seebeck coefficient, which means that they can generate a voltage when there is a temperature difference across them. They also have a low thermal conductivity, which means that they can maintain a temperature difference. This makes them ideal for use in thermoelectric generators that can convert waste heat into electricity. Furthermore, GeS has been shown to have promising properties for use in thin-film transistors, including a high electron mobility and a low threshold voltage. Indeed, GeS, have drawn significant research attention for potential use in optoelectronic devices. This can be attributed to its attractive features like less toxicity, high stability and high carrier mobility.

One of the most important structures of GeS is that it contains a layered orthorhombic α–Pnma structure formed via the intralayer covalent bonding. While α–Pnma remains the most studied and obvious structural attribute of GeS, the recent discovery of rich phase diagrams beyond α–Pnma in related compounds like GeSe, SnS, and SnSe has increased curiosity for a similar discovery in GeS through pressure and nanoscale modifications. Meanwhile, the two most well-recognized phase diagrams of GeS are phase II and phase III, reported at relatively high pressures. In GeS, pressure-induced phase transitions exhibit a structural rearrangement sequence similar to that of SnS at lower pressures. However, several phase diagrams reported in related compounds are yet to be obtained experimentally in GeS.

To this note, Dr. Long Truong Nguyen and Professor Guy Makov from Ben-Gurion University of the Negev investigated the possibility of other GeS polymorphs and phase diagrams beyond the well-known orthorhombic α–Pnma phase that is yet to be discovered or explored extensively. In their approach, a combination of evolutionary algorithm and density functional theory calculations of the phonon and energetics spectra was employed to systematically investigate the GeS compounds in both high-pressure and ambient conditions. Their research work is currently published in the peer-reviewed journal, Crystal Growth and Design.

The authors reported six new and dynamically stable GeS structures at ambient conditions, enabled by the study and accurate calculation of their optical and electronic properties of the unconventional phases. These new structures consisted of three layered and three nonlayered phases and exhibited formation energies close to the ground state orthorhombic phase and could be stabilized through high-pressure induced structural transitions. Three of the new layered phases exhibited different tetrahedral arrangements, while the other three nonlayered phases were akin to rock salt formation. Due to the effects of these phases, the useful GeS bandgap range was expanded across a wider range from 0.39 – 1.59 eV.

A lone pair analysis was used to interpret the pressure-induced phase transition mechanisms of GeS. The lone pairs-induced distortion played a critical role in the formation of the structures of the phases. For instance, the lone pair localization weakening on the Ge atoms was the main cause of the structural transition mechanism due to compression. The resulting new Pnma and Cmcm GeS structures was thermodynamically preferred over α–Pnma at some pressure conditions, especially 15 and over 35 GPa. Furthermore, the resulting optical and electronic properties showed greater potential for higher absorption, optical conductivity, wide effective working range and anisotropy, making them promising materials for optoelectronic applications.

In summary, the study predicted six new unconventional GeS phases different from the α–Pnma GeS. The unconventional layered phases displayed efficiency comparable to or even higher than that of conventional phases. In contrast, the nonlayered phases were more isotropic, even at similar optical and electronic properties. The findings related to the newly predicted GeS phases closed the existing literature gap regarding the understanding of group IV monochalcogenide class of materials. In a statement to Advances in Engineering, Professor Guy Makov stated that their findings will help leverage the structure-property relationship for extended applications of GeS compounds.