Angew.Chem.Int.Ed.http://dx.doi.org/10.1002/anie.201301836(2013)

Credit: © 2013 WILEY

Some crystalline materials generate an external electric current in response to a temperature change — a 'pyroelectric' effect. This can be exploited in temperature sensors, such as for alarm devices, because body heat from a distance creates a sufficient voltage. The phenomenon arises from spontaneous polarization: as molecules move slightly within the lattice, changes in their dipole moments lead to the creation of two oppositely charged faces and a voltage across the material. This effect, determined by a crystal's symmetry, occurs for the ten crystal classes that are non-centrosymmetric polar (that is, those having a dipole in their unit cell).

Meir Lahav, Igor Lubomirsky and co-workers at the Weizmann Institute of Science have now observed pyroelectricity in nonpolar, centrosymmetric crystals of amino acids. The researchers grew crystals of α-glycine on a glass substrate, in an aqueous solution and in the presence of l-alanine, whose occlusion prevents crystal twinning that could lead to artefacts. Crystals of different morphologies, with different faces exposed at the interface with the aqueous solution, were prepared. All of them exhibited a pyroelectric effect at their {010} faces, despite the crystals' centrosymmetric lattice.

The characteristics of the current — direction and time dependence — showed that the pyroelectric effect did not arise from the bulk crystal or from the glass substrate, but from the presence of a thin polar layer near the surface. Characterization at different temperatures provides evidence that the pyroelectricity arises from thin polar films of hydrated α-glycine. Although water molecules are also adsorbed at other faces, they can only adopt a suitable polar configuration near the {010} surfaces. This is in good agreement with the fact that the surface pyroelectricity disappeared on heating the crystals above 80 °C, which coincides with the removal of the adsorbed water molecules. Similar surface pyroelectricity was observed at specific faces of other nonpolar crystals of amino acids, l- and d-alanine, showing that the limitations arising from a crystal's symmetry can be circumvented.