Boron Selenide and Boron Sulfide Chalcogenides

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A gray crystalline compound of bismuth and selenium, bismuth(III) selenide is a semiconductor and a thermoelectric material. It is a critical component for research, development and production of advanced technologies which require optimum properties, performance and quality.

boron selenide has been observed to exhibit strong nonlinear optical responses in both a single crystal and thin GaSe films doped with boron. The characterization of the electronic-absorption spectra and the kinetics of relaxation of photocurrent are in good agreement with theoretical predictions. The observed nonlinear response is attributed to the presence of a defect mode which appears to arise from the substitution of selenium atoms for oxygen ions in the lattice.

In the present study two boron-rich chalcogenides, namely boron sulfide o-B6S and boron selenide o-B6Se, were synthesized under high pressure – high temperature conditions (BL04B1 beamline, SPring-8 Japan; PSICHE wiggler source, SOLEIL France). The elemental composition of the new compounds was determined from energy-dispersive X-ray spectroscopy data. The crystal structures were characterized by using ab initio evolutionary crystal structure prediction and Rietveld refinement of experimental powder X-ray diffraction patterns. Their stoichiometry is B6X (X = S, Se). Theoretical calculations with the USPEX code showed that the o-B6S structure has Pmna symmetry and is stable in the 0-20 GPa pressure range. The thermodynamically lowest-enthalpy structures with a Pmna-like structure (convex hulls shown in Fig. 1a,b) are metastable at 20 GPa.

The Raman spectra of o-B6S and o-B6Se are compared with those obtained from a-B2Se3, an isostructural crystalline form of boron selenide. The o-B6S and o-B6Se Raman bands are associated with various tilting oscillations of the B12-icosahedral units around different crystallographic [100], [010] and [001] directions (rocking and wagging of the equatorial and polar boron atoms of one icosahedral unit) and with the partial occupation of 4 h sites by Se atoms in o-B6S. The results suggest that the structural changes caused by high pressure may affect also the vibration modes of the phonon system.

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