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RESEARCH PRODUCT

High-pressure, high-temperature phase diagram of InSe: A comprehensive study of the electronic and structural properties of the monoclinic phase of InSe under high pressure

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We report on an investigation of the high-pressure, high-temperature phase diagram of InSe. We optically observed the phase transition from the rhombohedral polytype (InSe-I) to the monoclinic phase (InSe-II) and determined the phase boundary up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. High-pressure resistivity measurements were performed to complement the optical measurements. Monoclinic and cubic InSe (InSe-III) were observed to be metastable around $14.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and evidence suggesting the existence of an as yet unidentified new high-pressure and high-temperature phase was found. By means of optical absorption measurements on InSe-II samples, we observed that InSe-II is a semiconducting phase. It has an absorption coefficient about ten times larger than InSe-I at the absorption edge, which shifts upon compression to low energies. These results were analyzed under the light of first-principles band-structure calculations. We also determined the refractive index of InSe-II and its pressure evolution. Finally, Raman and x-ray diffraction measurements were performed in recovered InSe-II samples, being its crystalline structure refined and its phonon modes determined.