Polarization -tilting-induced near-zero thermal expansion in PbTiO3 nanosheets.

Ferroelectrics exhibiting volumetric thermal stability are essential for thermal-shock-resistance device applications. However, due to the crucial contribution of spontaneous volume ferroelectrostriction (SVFS) to lattice expansion, the coexistence of strong ferroelectricity and zero thermal expansion (ZTE) is difficult. Herein, wide-range near-ZTE behavior (α V = 3.1 × 10-6 °C-1, 25-475 °C) and considerable ferroelectric structure (c/a ratio: 1.044) were both observed in 3.5 nm PbTiO3 nanosheets, resulting from the ab plane strain induced by polar-axis-reduction. The robust ferroelectricity and ab symmetry breaking were confirmed by piezoelectric force microscopy (PFM) and Raman spectroscopy. The neutron pair distribution function (nPDF) combined with Reverse Monte Carlo (RMC) simulations suggests that the polarization is characterized by a fixed magnitude and a tilting direction during heating, while the unit cell volume remains constant. It comes from the deviation of Ti atoms from the O6 center, along with the rotation and the deformation of the O6 octahedron. Phase-field simulations demonstrated thermally induced domain tilting towards the ab direction and supported polarization magnitude locking. The thermally stable polarization magnitude, electric-field-controllable polarization direction, and near-ZTE behavior in ultrathin two-dimensional materials hold great promise for ferroelectric applications that are immune to thermal failure. This work presents a feasible structure-guided strategy to develop high-performance nanoferroelectrics.