Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO₃

The rare physical property of negative thermal expansion (NTE) is intriguing because materials with a large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, the applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO₃. Intriguingly, the substitution of S for O in PbTiO₃ further increases the tetragonality of PbTiO₃. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion of _V = -2.50 × 10^-5 K^-1 was achieved over a wide temperature range (300-790 K), which is in contrast to that of pristine PbTiO₃ (_V = -1.99 × 10^-5 K^-1, RT-763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve a large NTE over a wide temperature range in PbTiO₃-based ferroelectrics.