Organic materials are increasingly being considered as a viable alternative to inorganics for many applications. They are far cheaper, and their structure can be easily modified through chemistry, which makes them highly versatile. Organic semiconductors and superconductors have been known for a long time, and the attention is now turning to ferroelectric materials, in which spontaneous electrical polarization can be induced by an external electric field. Several organic ferroelectric materials have been discovered so far.

Fig. 1: A typical crystal of diglycine picrate (inset) and its high-resolution X-ray diffraction pattern.

Now, Mohd Shakir, B. K. Singh, Binay Kumar and G. Bhagavannarayana from the National Physical Laboratory and University of Delhi in India have observed ferroelectricity in crystals of glycine picrate.1 This result is particularly surprising because glycine picrate has a centrosymmetric structure, that is, its crystal lattice includes a center of symmetry, around which an inversion of the elements leads to the same structure. As the scientists explain, in such compounds, “the net dipole moment or polarization becomes zero and hence in such crystals one cannot observe certain effects, for instance ferroelectricity or second harmonic generation.” The team was intrigued by the recent finding of second harmonic generation in this material, and suspected that it might also exhibit ferroelectricity.

The measurements fully confirmed their hypothesis. They observed both ferroelectricity and piezoelectricity over a wide temperature range. The data showed that both the remanent electrical polarization (remaining after removal of the external electric field) and the coercive electric field (the opposite external electric field at which the net polarization becomes zero) decrease with increasing temperature until both disappear at 105 ºC. Surprisingly, this is not the ferroelectric Curie temperature, as heating the samples further led to a sharp increase in both quantities. Instead, the result implies that at around 105 ºC, the material undergoes a second-order phase transition between different ferroelectric phases.

The exact origin of the observed ferroelectricity is still unclear, but several structural tests confirmed the centrosymmetric character of the structure, leading the team to believe that under a strong enough electric field, a small local distortion of the geometry could occur. This could in turn result in a local non-centrosymmetric structure inside a centrosymmetric one, giving rise to ferroelectricity.

Aside from the interest in understanding the fundamental mechanism behind this observation, the results could also have practical implications. “This type of unusual and astonishing experimental finding of ferroelectricity may lead researchers to look for other such materials, which could enhance the range of organic ferroelectrics and help fulfill the emerging design and development requirements of all-organic electronics,” say the research team.