Flexoelectricity and surface ferroelectricity of water ice

The phase diagram of ice is complex and contains many phases, but the most common (frozen water at ambient pressure, also known as Ih ice) is a non-polar material despite individual water molecules being polar. Consequently, ice is not piezoelectric and cannot generate electricity under pressure. On the other hand, the coupling between polarization and strain gradient (flexoelectricity) is universal, so ice may in theory generate electricity under bending. Here we report the experimental demonstration that ice is flexoelectric, finding a coefficient comparable to that of ceramics such as SrTiO3 or TiO2. Additionally, and unexpectedly, the sensitivity of flexoelectric measurements to surface boundary conditions has also revealed a ferroelectric phase transition around 163 confined in the near-surface region of the ice slabs. The electromechanical properties of ice may find applications for low-cost transducers made in-situ in cold and remote locations, but perhaps more important are the consequences for natural phenomena involving ice electrification. In particular, we have calculated the flexoelectric polarization generated in collisions between ice and graupel particles, which reproduces the experimentally reported results for contact electrification in such events, known to cause electrification in storm clouds.