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 polar1,2. Consequently, ice is not piezoelectric and cannot generate electricity under pressure3. On the other hand, the coupling between polarization and strain gradient (flexoelectricity) is universal4, so ice may in theory generate electricity under bending. Here we report the experimental demonstration that ice is flexoelectric, finding a coefficient of 1.14+-0.13 nC/m, comparable to that of ceramics such as SrTiO3, TiO2, or PbZrO3. Additionally, and unexpectedly, the sensitivity of flexoelectric measurements to surface boundary conditions has also revealed a ferroelectric phase transition around ~160K 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. Importantly, there are also consequences for natural phenomena. In particular, we have calculated the flexoelectric charge density generated in ice-graupel collisions, and found it to be comparable to the experimental charge transferred in such events, suggesting a possible participation of ice flexoelectricity in the charging up of thunderstorms.