Strong hybridization driving unusual enhanced negative thermal expansion in PbTiO3-based ferroelectrics

Abstract

New conceptsThe discovery of unusual negative thermal expansion (NTE) is of fundamental significance, as it enables the controllable regulation of thermal expansion properties.As a key member of functional materials, PbTiO₃-based perovskite ferroelectrics exhibit NTE over a broad temperature range below the Curie temperature (T_C ). Notably, favorable enhancement of NTE can be achieved by tailoring the tetragonality of PbTiO₃ through chemical substitutions. However, the microscopic mechanisms underlying this intriguing phenomenon remain elusive. This work reported two PbTiO₃-based compounds of 0.5PbTiO₃-0.5BiFeO₃ and 0.6PbTiO₃-0.4BiCoO₃ -that possess comparable tetragonality yet contrasting NTE behaviors. Our results reveal that the distinct covalent characteristics of Fe-O and Co-O bonds give rise to divergent temperature-dependent unit cell volume responses: 0.5PbTiO₃-0.5BiFeO₃ exhibits strong nonlinear NTE, while 0.6PbTiO₃-0.4BiCoO₃ displays colossal volume contraction. These findings clarify the microscopic origin of NTE in PbTiO₃-based ferroelectrics and lay the foundation for the design of advanced materials with enhanced NTE performance.