Domain Structure Observation of Phase Transition-type Negative Thermal Expansion Materials

Abstract: 

Chemical substitution for the tuning of working temperature of phase-transition type negative thermal expansion (NTE) materials generally reduces the volume shrinkage during the transition. It was found that electron-doped PbVO₃undergoes a polar-to-nonpolar transition accompanied by a huge NTE. The parent compound, PbVO₃, has a tetragonal perovskite structure with a space group of P4mm and has a large c/a ratio of 1.23 due to the ordering of the d_xy orbital in addition to the stereo chemical activity of 6s² lone pair of Pb²⁺. We have investigated the effects of electron doping and reduction of 6s² lone-pair activity in PbVO₃ 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 in Pb_0.82Sr_0.18VO₃ was successfully observed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the spatial distribution of domains by Bragg coherent X-ray diffraction imaging (BCDI). The temperature hysteresis is reduced by repeated heating/cooling cycle, suggesting that the changes in domain structure dominates the NTE properties.