Stability and Band-Gap Tuning of the Chalcogenide Perovskite BaZrS3 in Raman and Optical Investigations at High Pressures

We report experiments and calculations investigating the pressure and temperature dependences of the optical phonons in BaZrS3, and the pressure dependence of its absorption edge. BaZrS3 is a chalcogenide perovskite in a novel class of materials being considered for photovoltaics. It is studied by Raman spectroscopy as functions of temperature (at 1 atm) and pressure (at 120 and 295 K), and by pressure-transmission spectroscopy at 295 K. Density-functional-theory calculations predict the allowed Raman lines, their intensities, their pressure shifts, and the band-gap pressure shift. Cooling shifts all but one of the phonon peaks to higher frequencies; the temperature coefficients are typical of semiconductors. A strong low-temperature peak at 392.3 cm-1 is attributed to resonant forbidden LO scattering; its shift with temperature has the opposite sign. The pressure coefficients of the phonon frequencies for all observed Raman peaks are positive, indicating no mode softening. The rates of pressure shift also are typical, and show the customary scaling with phonon frequency. Experiment and theory show good agreement on the pressure-induced frequency shifts. The BaZrS3 absorption edge moves to lower energy with pressure, reflecting a reduction of the band gap. The measured shift is ∼-0.015 eV/GPa, slightly less than the density-functional-theory result of -0.025 eV/GPa. We find no evidence that the perovskite structure of BaZrS3 undergoes any phase changes under hydrostatic pressure to at least 8.9 GPa. Our results indicate the robust structural stability of BaZrS3, and suggest cation alloying as a viable approach for band-gap engineering for photovoltaic and other applications.