Improper Ferroelectricity caused by bonding rather than size
19 Aug 2019
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- Rosalind Davies

 

 

A new material has been discovered that becomes an improper ferroelectric due to a break in symmetry caused solely by an electronically driven transition, rather than a geometric (size-driven) transformation.

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Representation of the CsNbW2O9 structure (left) showing the 'two-up, one-down' displacements of the Nb and W cations with the oxygen octahedra and which are responsible for symmetry-breaking at TC. This generates the meandering domain structure and six-fold 'cloverleaf' domain vertices (right), as shown by transmission electron microscopy (TEM). A second, lower temperature, tilt transition arises due to “all-in, all-out" octahedral trimers as indicated by the arrows (left).​​

Figure from article, An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites, Adv. Mater. 2019, 1903620, DOI 10.1002/adma.201903620

The material CsNbW2O9 has been characterised for the first time and shown to undergo a special type of electronic transition that causes it to become an improper ferroelectric: leading to the potential for the design of many new materials for application in nanoelectronics.

Ferroelectric materials have a spontaneous polarisation, even when there is no applied field and, crucially, this polarisation can be reversed by an electric field applied in the opposite direction. The polarisation is therefore dependent on not only the applied electric field but also its history, leading to opportunities for applications in electronic data storage. More recently, the realisation that domain walls in ferroelectrics exhibit different and exciting properties to the bulk crystal has produced a new field of “domain wall nanoelectronics".

Ferroelectric domain walls are layers at the atomic level that separate regions of uniform polarisation. The properties of these walls can be very different to the bulk materials in which they are formed, for example in the way they conduct electricity. Domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures, but relies on creating domain walls that can be reconfigured by, for example, an applied electric field.

Typically, materials demonstrate ferroelectricity only below a certain phase transition temperature, called the Curie temperature, TC. In 'proper' ferroelectrics, the distortion of the crystal that breaks symmetry at TC also gives rise to the polarisation whereas, in 'improper' ferroelectrics, the distortion that generates the polarisation is a secondary effect; the net result is that improper ferroelectrics display more complex domain structures, which often display a richer diversity of behaviour.

This study, led by Dr Finlay Morrison at the University of St Andrews, has discovered and characterised a new improper ferroelectric, CsNbW2O9. The group studied the structure of this material using the HRPD beamline both above and below the transition temperature where it becomes ferroelectric.

“HRPD was ideal for this study as its extremely high resolution allows very subtle details of crystal structure and symmetry to be detected and tracked as a function of tuning parameters, such as temperature in this case," explains beamline scientist Alexandra Gibbs.

They discovered that, although exhibiting very similar domain microstructure to a series of compounds called hexagonal manganites, the atomic scale structure of the crystals is very different, and so is the nature of the transition that occurs within the material to produce its ferroelectricity. Manganites become ferroelectric due a geometrical tilt that displaces the charged atoms in the structure. However, this new material CsNbW2O9, which adopts the 'hexagonal tungsten bronze' type structure, undergoes a symmetry-breaking transition that is entirely electronically driven i.e., by the nature of the chemical bonding.

“Not only did HRPD provide the necessary resolution to solve the crystal structure, the use of neutron scattering and its sensitivity to oxygen allowed us to unambiguously understand the chemistry of the symmetry breaking", explains Finlay Morrison. “To date much of the work in this developing area has focussed on materials from just two structure types: the proper perovskite ferroelectrics or the improper rare earth manganites, so adding another entirely different structure expands our arsenal of materials in which we can look for useful phenomena to exploit."

This discovery therefore forms the basis of a potential new class of functional materials of importance for the new field of domain wall nanoelectronics, and further understanding of this mechanism may provide a novel route to designing new ferroelectric materials with domain wall functionality by tuning their crystal symmetry through, for example, chemical composition.

Further Information

The full paper can be found at DOI: 10.1002/adma.201903620 ​

Other science highlights from HRPD​. 

Contact: Davies, Rosalind (STFC,RAL,ISIS)