Origins of emergent high- Tc ferroelectric ordering in heteroepitaxial ice films: Sum-frequency generation vibrational spectroscopy of H2O and D2O ice films on Pt(111)

Using sum-frequency generation (SFG) spectroscopy, we have investigated ferroelectric orientational ordering of ${\mathrm{H}}₂\mathrm{O}$ and ${\mathrm{D}}₂\mathrm{O}$ crystalline-ice films grown on Pt(111) at 140 K. The $\mathrm{Im}{\ensuremath{\chi}}⁽²⁾$ SFG spectrum of the hydrogen-bonded OH (OD) stretching band of ${\mathrm{H}}₂\mathrm{O}$ (${\mathrm{D}}₂\mathrm{O}$) ice film shows a couple of peaks whose signs are all negative, indicating net-H-down (net-D-down) ferroelectric orientational ordering of ${\mathrm{H}}₂\mathrm{O}$ (${\mathrm{D}}₂\mathrm{O}$) molecules with protons (deuterons) preferentially pointing toward the Pt substrate. The spectral features are analyzed in terms of the orientational ordering and OH (OD) vibrational excitons derived from intermolecular couplings. The quantitative analysis of the SFG peak intensity demonstrates the layer-dependent strong ferroelectric orientational ordering. Temperature dependence of the SFG intensity indicates that the ferroelectric orientational ordering decays via second-order type ferroelectric-paraelectric transition at critical temperatures of ${T}_c=163$ and 167 K for ${\mathrm{H}}₂\mathrm{O}$ (50 bilayers thickness) and ${\mathrm{D}}₂\mathrm{O}$ (64 bilayers thickness) ice films, respectively, which are more than twice as large as that of ferroelectric bulk ice XI (${T}_c\ensuremath{\sim}72\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{H}}₂\mathrm{O}, {T}_c\ensuremath{\sim}76\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{D}}₂\mathrm{O}$). The isotope shift of ${T}_c$ for the high-${T}_c$ ferroelectric ice films on Pt(111) is only about 4 K, which is in stark contrast to a large isotope shift of ${T}_c$, approximately several tens of Kelvin, of typical high-${T}_c$ hydrogen-bonded ferroelectrics. The interadsorbate and adsorbate-substrate interactions at the ice/Pt interface and nuclear quantum effects in the hydrogen-bond network of ice film are responsible for the observed thermal stability of ferroelectric orientational ordering and its isotope effect.