Polar–Nonpolar Transition-Type Negative Thermal Expansion with 11.1% Volume Shrinkage by Design

Chemical substitution for the tuning of the working temperature of phase-transition-type negative thermal expansion (NTE) materials generally reduces the volume shrinkage during the transition. We have investigated the effects of electron doping and reduction of 6s2 lone-pair activity in PbVO3 with a large polar distortion (c/a = 1.23) and found that the combination of Bi and Sr substitutions for Pb enables a temperature-induced polar to non-polar transition with 11% volume shrinkage, even larger than the pressure-induced volume collapse of PbVO3 (∼10.6%), and is the largest value among the NTE materials reported so far. The domain structure of the coexisting cubic and tetragonal phases with such a huge volume difference was successfully observed by high-angle annular dark-field scanning transmission electron microscopy and the spatial distribution of domains by Bragg coherent X-ray diffraction imaging. The temperature hysteresis is reduced by repeated heating/cooling cycles, suggesting that the changes in the domain structure dominate the NTE properties.