Double perovskite: new ferroelectricity
Ferroelectric materials are composed of crystals possessing a spontaneous electric polarization which can be reversed in direction by the application of a strong external electric field. Perovskite oxides — for example, lead zirconate titanate (PbZrxTi1–xO3) and bismuth ferrite (BiFeO3) — are the most important ferroelectric materials, particularly for technological applications. Now, a research team headed by Profs Jianshi Zhou and John B. Goodenough from the University of Texas at Austin show that double perovskite Ca2–xMnxTi2O6 — produced in large yields using a facile method — undergoes an unusual ferroelectric transition.
In the absence of lone pair cations such as Pb2+ and Bi3+, perovskites with a smaller geometric tolerance factor (t) than 1 are usually free from ferroelectricity. The first material breaking this rule is synthetic perovskite CaMnTi2O6, which exhibits a ferroelectric–paraelectric phase transition at 630 K. Aided by neutron diffraction and first-principles calculations, the researchers reveal that in sharp contrast to proper ferroelectricity, in which dipoles arise from the displacement, the order–disorder transition here at the square-planar coordinated Mn2+ site gives rise to a polar structure with dipoles created at octahedral Ti4+ sites, and tetrahedral and coplanar Mn2+ sites. Moreover, unlike solid state reactions under high pressure, the spark plasma sintering route used in this work allows for gram-scale synthesis, paving the way for the mass production of this material.