Solids get stretched

Negative pressures, though much investigated in liquids (see Physics Today, January 2011, page 14 ), have been thought to be impossible to sustain in solids. Colloquially, materials under negative pressure are said to be under tension. But more specifically, that tension must be isotropic—thin films stretched in just one or two dimensions don’t qualify. Now Nava Setter (Swiss Federal Institute of Technology, Lausanne) and her colleagues have developed a method to create freestanding crystalline nanomaterials stretched in all three dimensions. The researchers synthesized nanowires of lead titanate in the material’s so-called PX phase—a low-density, metastable crystal structure—and induced them to transform into the denser, stable perovskite phase. That reaction begins at the nanowire’s surface, and as it works its way inward, the outer layer forms a rigid shell that encloses a volume 13% larger than the rest of the perovskite nanowire would normally fill. Some of the excess volume is taken up by spherical pores, as shown in the cross-sectional images in the figure; the rest is accounted for by tensile stress of a few gigapascals. In accordance with a 12-year-old prediction about ferroelectric materials under negative pressure, the transformed PbTiO₃ had its ferroelectric transition temperature spike by more than 100 °C, and its spontaneous polarization doubled. Those enhancements persisted for more than two years. The researchers anticipate that their technique can be applied to many other materials with low-density metastable phases and could enhance other useful material properties in ways theorists haven’t yet explored. (J. Wang et al., Nat. Mater., in press, doi:10.1038/nmat4365 .)