Local ferroelectric polarization in antiferroelectric chalcogenide perovskite BaZr S 3 thin films

Bulk chalcogenide perovskite $\mathrm{BaZr}{\mathrm{S}}_{3}$ (BZS), with a direct band gap in the visible region, is an important photovoltaic material, albeit with limited applicability owing to its antiferroelectric (AF) nature. Presently, ferroelectric (FE) perovskite-based photovoltaics are attracting enormous attention for environmental stability and better energy conversion efficiency through enhanced charge separation. We report on AF-FE phases of BZS thin film using temperature-dependent Raman investigations and first-principles calculations. Origin of localized FE phase is established from an anomalous behavior of ${A}_{g}^{7}\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and ${B}_{1g}^{5}\ensuremath{\sim}420\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ modes, which involve the vibration of atoms at the apical site of $\mathrm{Zr}{\mathrm{S}}_{6}$ octahedra. Additionally, ${B}_{1g}^{1}$ and ${B}_{2g}^{2}$ ($\ensuremath{\sim}85\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) modes appear while antipolar ${B}_{2g}^{1}$ ($\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) disappears below 60 K. Our first-principles calculations confirm that FE appears as a result of the loss of center of inversion symmetry in $\mathrm{Zr}{\mathrm{S}}_{6}$ due to the existence of oxygen impurities placed locally at apical sites of sulfur.