The Role of Spontaneous Polarization in the Negative Thermal Expansion of Tetragonal PbTiO3-Based Compounds

Abstract

PbTiO₃-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO₃–0.3BiFeO₃ and 0.7PbTiO₃–0.3Bi(Zn_1/2Ti_1/2)O₃, whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz_Pb/Bi) is weakened in 0.7PbTiO₃–0.3BiFeO₃ but well-maintained in 0.7PbTiO₃–0.3Bi(Zn_1/2Ti_1/2)O₃. There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi–O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.