Bismuth selenide, a benchmark topological insulator, grows in a trigonal structure at ambient conditions and exhibits a number of enticing properties related to the formation of Dirac surface states. Besides this polytype, a metastable orthorhombic modification with the Pnma space group, o-${\mathrm{Bi}}_2{\mathrm{Se}}_3$, has been produced by electrodeposition and high-pressure high-temperature synthesis. It displays significant thermoelectric properties in the midtemperature range, particularly upon Sb doping. However, very little experimental information is available on the fundamental properties of this orthorhombic polytype, such as, e.g., the electronic band gap and the lattice dynamics. Here we report the temperature dependence of the Raman spectra of o-${\mathrm{Bi}}_2{\mathrm{Se}}_3$ between 10 K and 300 K, which displays an anharmonic behavior of the optical phonons that can be modeled with a two-phonon decay channel. To analyze the data, we performed ab initio calculations of the electronic band structure, the phonon frequencies at the center of the Brillouin zone, and the phonon dispersion relations along the main symmetry directions, examining the effect of spin-orbit coupling in both phonon and electronic energies. Lastly, we report cathodoluminescence experiments at 83 K that set a lower limit to the electronic band gap at 0.835 eV, pointing to an indirect nature, in agreement with our calculations. These results shed light on the essential properties of orthorhombic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ for further understanding of the potential of this semiconductor for thermoelectrics and other relevant applications.

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• First-principles calculations
• Optical phonons
• Phonons
• Thermoelectrics
• Topological insulators
• Raman spectroscopy