Spontaneously polarised crystalline water ice

We describe a novel aspect of the structure of the hexagonal phase of crystalline water ice, using vacuum-ultraviolet absorption spectroscopy as a diagnostic tool. The key result is that hexagonal ice (I_h), prepared from condensation of pure water vapour, displays net molecular orientation. This reflects order in the disposition of the protons in ice I_h and our results require revision of the near century old conventional structure in which water molecular dipoles point in random directions in hexagonal ice. Orientation gives rise to spontaneous polarization and internal fields of ∼4 × 10⁶ V m^-1 in films of ice I_h tens of nanometres thick, condensed on an insulating MgF₂ substrate. Data are in the form of Stark shifts in absorption spectra, with no free charges or external electric fields introduced in the experiment.