Negative-pressure-induced enhancement in a freestanding ferroelectric - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Nature Materials Année : 2015

Negative-pressure-induced enhancement in a freestanding ferroelectric

Résumé

Ferroelectrics are widespread in technology 1 , being used in electronics and communications 2 , medical diagnostics and industrial automation. However, extension of their operational temperature range and useful properties is desired 3–5. Recent developments have exploited ultrathin epitaxial films on lattice-mismatched substrates, imposing tensile or compres-sive biaxial strain, to enhance ferroelectric properties 6,7. Much larger hydrostatic compression can be achieved by diamond anvil cells 8,9 , but hydrostatic tensile stress is regarded as unachievable. Theory and ab initio treatments 10 predict enhanced properties for perovskite ferroelectrics under hydro-static tensile stress. Here we report negative-pressure-driven enhancement of the tetragonality, Curie temperature and spontaneous polarization in freestanding PbTiO 3 nanowires, driven by stress that develops during transformation of the material from a lower-density crystal structure to the perovskite phase. This study suggests a simple route to obtain negative pressure in other materials, potentially extending their exploitable properties beyond their present levels. Pressure influences the properties of ferroelectrics as a simple consequence of elasto-electric coupling. Biaxial stress has been achieved in films on lattice-mismatched substrates, resulting in strongly enhanced properties, whereas negative pressure is considered practically unrealizable. However, when a material undergoes a phase transformation from a low-density structure to a denser one, the conditions of the transformation can dictate stretching of the dense structure, resulting in a built-in negative pressure and its corresponding property tuning. Here we implement this concept for the prototypical ferroelectric PbTiO 3 (PT). We produced freestand-ing PbTiO 3 particles under the appropriate conditions, studied the structural fingerprints of the negative pressure, and found that these particles had enhanced tetragonality, spontaneous polarization and Curie temperature. The experimental work was complemented by theory, ab initio modelling and numerical simulations. PbTiO 3 exists in two different crystalline structures, the common ferroelectric perovskite and a body-centred tetragonal structure, named the 'PX' phase 11,12. The unit-cell volume of the PX phase is approximately nine times that of perovskite, whereas the number of atoms in the unit cell is eight times that in perovskite. The remainder of the volume is taken up by a longitudinal central pore along the c-axis. Thus, the density of perovskite is 13% higher than that of PX. In perovskites, oxygen octahedra are interconnected by their corners (Fig. 1a), whereas in the PX phase, oxygen octahedra are connected by both their corners and their edges (Fig. 1b). Therefore, although their cation to oxygen ratio is identical, the conversion from PX to perovskite requires the presence of extra oxygen, which is absorbed at the beginning of the conversion and released again on its completion 13. These two features, the difference in density and the oxygen required for the phase conversion, provide the condition for negative-pressure creation. When a body of PX phase is heated in air, the perovskite phase first forms at the surface where the oxygen is abundant; the released oxygen is gradually supplied to the inner parts of the body and the conversion proceeds inwards. However, whereas the outer part accommodates the shrinkage, the inner part, when transforming, is bound by the rigid outer part, resulting in a strong tensile stress in the core. PbTiO 3 nanowires were initially prepared in the PX phase (see Methods and Supplementary Information). The perovskite nanowires, with thicknesses ranging from 20 to 500 nm (Fig. 1c) were obtained by annealing the PX nanowires in air at ∼540 • C. Nearly spherical pores with a typical diameter of around 10 nm did not break their monocrystallinity (Supplementary Fig. 1a). Annealed nanowires showed a tetragonal perovskite structure, confirmed by X-ray diffraction (XRD) and Raman spectroscopy (Supplementary Fig. 1b,c). The spontaneous polarization direction, the c axis of the tetragonal perovskite cell, did not adopt any specific orientation with respect to the nanowire geometry. Bending occurred in some of the perovskite nanowires and 90 • domains were frequently found across the bent regions (Supplementary Fig. 1d). The c/a ratio of individual nanowires was investigated by high-resolution transmission electron microscopy (HR-TEM) for nanowires thinner than 120 nm and selected area electron diffraction (SAED) for thicker wires. Remarkably, in a small range of diameters near 110 nm, there was an enormously enhanced tetragonality (c/a − 1) up to ∼0.13 (Fig. 1d), almost doubling the tetragonality from bulk PbTiO 3 (ref. 14). Both thinner and thicker wires exhibited c/a ratios similar to the bulk PbTiO 3 value. Polarization switching under an applied field confirmed ferroelectricity of the wires. Supplementary Fig. 2 demonstrates polarization switching with a scanning probe in a wire of diameter ∼105 nm. Off-centring of Ti with respect to the four nearest Pb columns in a wire of 116 nm diameter was about twice the bulk value as shown by the high-resolution high-angle annular dark-field (HAADF) measurement, and a spontaneous polarization
Fichier non déposé

Dates et versions

hal-01260079 , version 1 (22-01-2016)

Identifiants

Citer

Jin Wang, Ben Wylie-van Eerd, Tomas Sluka, Cosmin Sandu, Marco Cantoni, et al.. Negative-pressure-induced enhancement in a freestanding ferroelectric. Nature Materials, 2015, 14 (10), pp.985. ⟨10.1038/NMAT4365⟩. ⟨hal-01260079⟩
168 Consultations
1 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More