A group of researchers from the Institute of Physics, together with an international team, studied the dynamics of water molecules in beryl crystals. As reported in their recent article in Nature Communications , they succeeded in proving for the first time that, at low temperatures, these localized water molecules tend to align, exhibiting so-called incipient ferroelectricity.
The water molecules are known to exhibit a high dipole moment. Trapped in the beryl crystal lattice, they are subject to a regular arrangement at sites where they can reorient. The directions of their orientations are imposed by the crystal lattice which belongs to the hexagonal system; consequently, there are six equivalent orientations for each of the molecules. Owing to their dipole moments, the molecules can interact and influence dynamically their orientations. Such correlations cannot be observed in liquid water nor ice, where the dipole-dipole interactions are dominated by the stronger short-range hydrogen bonding. Beryl represents a convenient host medium, imposing a moderate spacing among the water molecules (of the order of 1 nm) which prevents the hydrogen bonds to form. It is also possible to grow water-free beryl crystals, which enables a clear identification of the contribution of the water molecules themselves.
In the Institute of Physics, the dynamical behaviour of beryl crystals was studied by several techniques of the broad-band dielectric spectroscopy. It was shown that the collective vibrations of the crystal water exhibit a soft mode – a vibrational resonance whose frequency decreases upon cooling. In many known crystals, soft modes initiate a phase transition to an ordered (ferroelectric) state which occurs below a certain, so-called Curie temperature. The parameters of the water soft mode in beryl can be described by known equations which yield that the Curie temperature for water in beryl is below absolute zero. This implies that, together with the dipole-dipole interaction, there are other inter-atomic forces which prevent the ordered state from appearing. The published discovery opens the possibility, though, that a different host crystal would enable a ferroelectric ordering. In view of the importance of water, which appears in different confined forms in all living organisms and many inorganic objects, one can expect that the ordering of electrical dipole moments of water molecules plays an important role in some systems.