Surface Ferroelectricity in Au-Ice Capacitors*

At atmospheric pressure and below 272 K water crystallizes into an hexagonal (wurtzite) proton-disordered phase called ice Ih. At around 72 K, bulk ice Ih undergoes a phase transition to a ferroelectric (proton-ordered) phase known as ice XI. Using density functional theory we show that when hexagonal ice is placed in contact with a metallic interface, the relative surface free energy of the XI versus the Ih phase decreases, inducing a surface ferroelectric phase transition with a skin layer depth-dependent transition temperature higher than that of the bulk. We explain the origin of this enhanced stability and show that our results explain the recent experimental observations by X. Wen et al., who have measured this phase transition to occur at temperatures as high as 163 K for different metal capacitors.