Anion-mediated unusual enhancement of negative thermal expansion in the oxyfluoride of PbTiO3.

Materials featuring negative thermal expansion (NTE) properties are crucial for controlling overall thermal expansion. However, only a limited number of NTE materials are suitable as high-performance thermal expansion compensators owing to their small NTE magnitude and narrow NTE temperature range. PbTiO₃ is a typical perovskite-type (ABO₃) ferroelectric that also exhibits a unique NTE. While an enhanced NTE compared to pristine PbTiO₃ can be achieved in PbTiO₃-based ferroelectrics by improving their tetragonality via chemical substitutions at the A/B sites, the effect of anion substitution on the thermal expansion properties of PbTiO₃ has rarely been studied. Here, we report an oxyfluoride of PbTiO₃, as an O^2-/F^- solid solution with fluorine concentrations up to 10%, where PbTiO_3-xF_x demonstrated a reduced tetragonality as a function of fluorine. Notably, for the first time, we observed a reduced tetragonality accompanied by an unusually enhanced NTE in the present PbTiO_3-xF_x of PbTiO₃-based ferroelectrics, contrary to the previously reported enhanced NTE in PbTiO₃-based ferroelectrics. This unconventional phenomenon can be sufficiently understood considering the chemical bonds by the maximum entropy methods and first-principles calculations. This study not only extends the scope of NTE in PbTiO₃-based ferroelectrics but also expands the mixed-anion chemistry of the solid state.