Anti-ferroelectric and chiral ordering of confined water molecules in Beryl crystal

In almost all real materials, short-range interactions predominate over dipolar interactions. An example is water, whose molecules have a large electric dipole moment, but also a strong covalent hydrogen interaction that suppresses anti-ferroelectric ordering. When the water molecules are confined in the structural channels of the beryl single crystal so that the water molecules are far enough apart to prevent hydrogen bonding, then dipolar interactions play a major role. In this case, we can observe an indication of the anti-ferroelectric arrangement of water molecules at low temperatures [1].

The beryl lattice forms a matrix for the dipoles of the water molecules. The lattice is hexagonal in the ab plane and stacked in the c direction [2]. From the simplified theoretical model, it follows that the final ground state is formed by dipoles arranged ferroelectrically in the ab plane, but in the c direction, forming a chiral structure at low temperatures and an anti-ferroelectric arrangement as the temperature increases. The dipoles can rotate along the hexagonal c-axis in the double six-well potential, so there are twelve possible equilibrium positions. In experiments, we can see signs of ordering, but not a transition to an ordered phase at non-zero temperatures [2].

There are two factors that can suppress chiral or anti-ferroelectric ordering. The first is quantum tunneling through the local double six-well potential barrier, and the second is that water molecules are not present in all sites (channels) of the beryl crystal, so there are cavities that disrupt the long-range order.

In this work, we study the phase transition of the given system under the above-mentioned conditions using the mean-field approximation and Monte-Carlo simulations. Both approaches confirm the prediction of ordering at low temperatures and its suppression due to both of the above circumstances. We quantitatively show the proportion of both types of suppression.

[1] Belyanchikov M. A. et al.Nat. Commun. 11, 3927 (2020).

[2] Gorshunov, B. P. et al. Nat. Commun.7, 12842 (2016)

[3] B. P. Gorshunov et al. J. Phys. Chem. Lett. 4, 2015 (2013).