Effect of internal pressure on incipient ferroelectricity of nano‐confined water molecules observed in hydrothermally grown beryl crystals

Quasistatic dielectric permittivity of D₂O type I molecules (electric dipole moment perpendicular to the crystallographic c‐axis) within hydrothermally grown beryl crystals characterized by different internal pressure and content of D₂O type II molecules (dipole moment parallel to the c‐axis) is measured at temperatures 4–300 K. All crystals are found to display quantum paraelectric behavior of the D₂O‐I molecular subsystem permittivity, that is, permittivity growth while cooling from room temperature followed by saturation below 15–40 K. Processing the data with the Barrett expression shows that excess internal pressure and excess content of D₂O‐II molecules lead to an increase in quantum temperature T₁ and a decrease in the Curie constant C, with the Curie temperature T_C remaining unchanged. The discovered strengthening of quantum effects (growth of T₁) within an ensemble of dipole–dipole‐coupled D₂O‐I molecules is associated with an enhanced azimuthal tunneling of these molecules within the hexagonal localizing potential. The data indicate the possibility of using crystal growth conditions to “tune” the strength of quantum effects in the network of polar water molecules, which provides a workbench for further studies of exotic phases of a lattice of coupled “point” electric dipoles.