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RESEARCH PRODUCT

First-principles study of the electronic structure of cubicGaS: Metallic versus insulating polymorphs

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The electronic structure of different polymorphs of gallium sulphide $(\mathrm{GaS})$ with cubic structure is investigated by means of first-principles band structure calculations in connection with experimental reports on a metastable semiconducting cubic form of this material. The expected metallic character of simple cubic phases containing one $\mathrm{GaS}$ group per unit cell (rocksalt or zinc-blende) is confirmed by the calculations. A cubane-based zinc-blende structure is found to exhibit a band gap which is compatible with experimental results but the unit cell parameter is much larger than the reported ones. We have also studied cubic phases containing hydrogen. It is found that two insulating polymorphs can be stable: (i) a simple cubic structure based on $[\mathrm{HGaS}{]}_{4}$ cubane units and (ii) an insulating zinc-blende structure in which hydrogen atoms occupy the empty sites of the cubic unit cell. The last structure seems to be the more likely structural alternative for the phase so far described as a semiconducting cubic phase of $\mathrm{GaS}$. The presence of hydrogen could explain the reported instability of cubic $\mathrm{GaS}$ under thermal annealing.