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

Abstract

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 nanodevice 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.

Corrigendum to: “Phase transitions in the hexagonal tungsten bronze RbNbW2O9” [J. Solid State Chem. 286 (2020) 121275]

“…The orthorhombic Cmcm structure and both hexagonal structures feature octahedral tilting, as observed in Cmc21. Our previous work showed that the octahedrally tilted Cmc21 structures in both RbNbW2O9 [10] and CsNbW2O9 [9] are described by inclusion of A6 + and A3 + distortion modes, consistent with previous work. [ [14][15][16] In contrast to CsNbW2O9, which has a more complex supercell at ambient temperature due to an additional K3 distortion mode, the structural evolution of RbNbW2O9 is unlikely to proceed via P63cm as this requires an A4 + mode, which has previously been shown by Whittle et al [14] to result in disconnection of the BO6 octahedral framework.…”

“…Above 973 K, a decrease in the c-axis parameter is observed like that previously observed in CsNbW2O9. [9] While this is suggestive of a structural transition to the paraelectric (P6/mmm) phase, all refinements above 973 K are still a slightly better fit in the polar P6mm model (Table S1 and Figure S5) suggesting that the transition to P6/mmm (TC) occurs above the available data range. Evolution of the lattice parameters versus temperature based on the Cmc21 (288-623 K), P63mc (673 K), P6mm (723-973 K) and P6/mmm (>973 K) phases is shown in Fig.…”

“…with the loss of tilting in the related CsNbW2O9 system. [9] Rietveld refinement of diffraction data between 723 and 973 K indicated a good fit using the P6mm model. The paraelectric P6/mmm model was also tested in this temperature range but the fits were of an inferior quality (Table S1).…”

“…The most recent examples of these materials have focused on the structures of the ANbW2O9 series with A = K + , Rb + and Cs + . [8][9][10] Investigations into the structure-property relationships of CsNbW2O9 have shown this material to be an electronically-driven improper ferroelectric with an interesting domain microstructure that may potentially be useful in domain wall nanoelectronics. [9] Although neither RbNbW2O9 nor KNbW2O9 appear to be improper ferroelectrics, recent work involving detailed crystallographic characterisation of the ambient temperature structures utilising high-resolution powder neutron diffraction (PND) has shown that both compositions feature octahedral tilting at low temperature, as also observed in CsNbW2O9.…”

Multiferroic heterostructures for spintronics

“…The family of improper ferroelectric, i.e. geometric ferroelectrics h-RMnO3 [15] or electronically-driven improper ferroelectric CsNbW2O9 [212] are prominent candidates for the design of functional and stable charged domain walls.…”