Ferroelectric Ordering in Nanosized PbTiO₃

The insight into the three-dimensional configuration of ferroelectric ordering in ferroelectric nanomaterials is motivated by the application of the development of functional nanodevices and the structural designing. However, the atomic deciphering of the spatial distribution of ordered structure remains challenging for the limitation of dimension and probing techniques. In this paper, a neutron pair distribution function (nPDF) was utilized to analyze the spontaneous polarization distribution of zero-dimensional PbTiO3 nanoparticles in three dimensions, via the application of reverse Monte Carlo (RMC) modeling. The comprehensive identification with transmission electron microscopy verified the linear characteristics of polarization along the c-axis in the main body, while electric polarization distribution on the surface was enhanced abnormally. In addition, the correlation of dipole vectors extending to three unit cells below the surface is retained. This work shows an application of the micro/macroscale information to effectively decode the polarization structure of nanoferroelectrics, providing new views of designing nanoferroelectric devices

Structure and Phase Transformation in the Giant Magnetostriction Laves-Phase SmFe₂

As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for its abundant magnetic properties. Samarium–iron alloy system SmFe₂ is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe₂ has been identified as a cubic <i>Fd</i>3̅<i>m</i> structure at room temperature; however, the cubic symmetry, in principle, does not match the spontaneous magnetization along the [111]_cubic direction. Here we studied the crystal structure of SmFe₂ by high-resolution synchrotron X-ray powder diffraction, X-ray total scattering, and selected-area electron diffraction methods. SmFe₂ is found to adopt a centrosymmetric trigonal <i>R</i>3̅<i>m</i> structure at room temperature, which transforms to an orthorhombic <i>Imma</i> structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111]_cubic to [110]_cubic direction and is further evidenced by the inflection of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the field-cooling–zero field-cooling curve, and the anomaly in the specific heat capacity measurement. The revised structure and phase transformation of SmFe₂ could be useful to understand the magnetostriction and related physical properties of other RM₂-type pseudocubic Laves-phase intermetallic compounds