Enhanced ferroelectricity in perovskite oxysulfides

A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and for exploring unusual physical phenomena. However, owing to its volatility, sulfur substitution to oxide materials is challenging, and thus the sulfurization effects on the associated properties have been limitedly studied. Here, a facile method for sulfurization to a perovskite oxide $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ is developed and demonstrated. A thiourea $(\mathrm{C}{\mathrm{H}}_{4}{\mathrm{N}}_{2}\mathrm{S})$ solution is used as a precursor for the sulfurization and its doping-level control. By manipulating the sulfur concentration ($x$), we systematically examine the physical properties of sulfur-doped $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3\ensuremath{-}x}{\mathrm{S}}_{x}$ films. An enhancement in the tetragonality and ferroelectricity by sulfurization is observed with the band-gap reduction, which is consistent with our theoretical predictions. In the sulfurized films, the ferroelectric phonon modes become softened progressively, probably due to the substitution of apical oxygens with sulfur atoms. Our work is of practical interest for designing ferroelectric photovoltaic devices with high performances.