An Electronically-Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogues for the Hexagonal Manganites

Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realised that domain wall engineering offers exciting new opportunities for nano-electronics and nano- device architectures. We report a novel improper ferroelectric, CsNbW2O9, with the hexagonal tungsten bronze structure. Powder neutron diffraction and symmetry mode analysis indicates that the improper transition (TC ~ 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However in contrast to the manganites the symmetry breaking in CsNbW2O9 is electronically-driven (i.e., purely displacive) via the second order Jahn-Teller effect in contrast to the geometrically-driven tilt mechanism of the manganites. Nevertheless CsNbW2O9 displays the same kinds of domain microstructure as those found in the manganites, such as characteristic six-domain ‘cloverleaf’ vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics.