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

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(Zn1/2Ti1/2)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(Zn1/2Ti1/2)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.