A plastic crystal that can switch polarisation

Researchers from Japan’s Hokkaido University have developed a ferroelectric plastic crystal with switching properties that make it suitable for memory-related applications.

Like other ferroelectric materials, the crystal’s atoms have the ability to ‘switch’ their electric polarisation from one direction to another, making one side of the material positive and the other negative — a particularly useful property for storing binary bits of data in memory devices. Unlike other materials, the crystal is non-toxic, cost-efficient and flexible.

The researchers combined quinuclidine, an organic amine, with perrhenic acid to form the crystalline salt quinuclidinium perrhenate. Quinuclidinium perrhenate consists of globular ionic molecules that give the crystal unique characteristics, described by the researchers in the journal Nature Chemistry.

The crystal is ferroelectric above room temperature, then turns into a plastic, more pliable phase at higher temperatures. At the higher temperatures, the molecules in the crystal have randomly different polarity axes, but they can be aligned in one direction by applying an electric field as the crystal cools, bringing it back to a ferroelectric state. Being able to control the polarity addresses a major challenge faced by researchers working with organic compound-based ferroelectric crystals, which are less symmetric than inorganic crystals and can thus be polarised only in one direction, leading to a very weak overall polarisation of randomly oriented crystals.

Furthermore, the crystal has the distinct advantage of transitioning to a plastic state when heat is applied. This plasticity — as opposed to fracturing that occurs in regular organic and inorganic crystals when a mechanical stress is applied — makes it suitable for use as a thin ferroelectric film in devices. One such application could be in non-volatile ferroelectric random-access memory devices, which maintain memory when the power is turned off.

Exploring crystals composed of molecules similar to quinuclidine could lead to the discovery of more ferroelectric crystals, according to the researchers, who added that chemical modifications of the molecules’ constituent ions could also improve their performance.

Image caption: The newly synthesised crystal is ferroelectric above room temperature (a–b, e–f) and turns into ‘plastic phase’, meaning highly deformable, at higher temperature (a–c). The electric polarity of each molecule can be aligned in one direction by applying electric field as it cools (c–e). Harada J et al, 11 July 2016, Nature Chemistry, DOI: 10.1038/NCHEM.2567.