Large Negative Thermal Expansion Induced by Synergistic Effects of Ferroelectrostriction and Spin Crossover in PbTiO3 -Based Perovskites

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but muchdesired property of thermal expansion, which is valuable not only inscientific interests but also in practical applications. However, most of theavailable NTE materials are limited to a narrow temperature range, andthe NTE effect is generally weakened by various modifications. Here, wereport an enhanced NTE effect that occurs over a wide temperaturerange (α̅V=−5.24×10−5°C−1,25−575°C), and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering inthe present perovskite-type ferroelectric of (1−x)PbTiO3−xBiCoO3. Specifically, for the composition of 0.5PbTiO3−0.5BiCoO3, an extensivevolumetric contraction of ∼4.8 % has been observed near the Curie temperature of 700°C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the large NTE originates from a synergistic effect ofthe ferroelectrostriction and spin crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, orferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms, and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.