Flexoelectricity and surface ferroelectricity of water ice

Frozen water at ambient pressure—common ice, also known as hexagonal Ih ice—is a non-polar material, even though individual water molecules are polar. Consequently, ice is not piezoelectric and cannot generate electricity under pressure. However, it may in principle generate electricity under bending, because the coupling between polarization and strain gradient (flexoelectricity) is always allowed by symmetry. Here we measure the flexoelectricity of ice and find it to be comparable to that of benchmark electroceramics such as TiO₂ and SrTiO₃. Moreover, the sensitivity of flexoelectric measurements to surface boundary conditions has revealed a ferroelectric phase transition around 160 K confined within the near-surface region of the ice slabs. Beyond potential applications in low-cost transducers made in situ in cold locations, these findings have profound consequences for our understanding of natural phenomena involving ice: our calculations of the flexoelectric charge density generated in ice–graupel collisions inside thunderstorm clouds compare favourably to the experimental charge transferred in such events, suggesting a possible participation of ice flexoelectricity in the generation of lightning.