Non-collinear ferroelectric H2O ice VIII

Abstract

We present a non-collinear ferroelectric phase of ice VIII (space group Iba2). This non-collinear ferroelectric phase should be an important base together with the anti-ferroelectric (I41/amd) and the ferroelectric (P42nm) ices to describe the structure of disordered dense H2O ice (ice VII). The enthalpy of Iba2 is slightly larger than that of I41/amd and slightly smaller than that of P42nm (by < 1 mRy/molecule). The other ices could exist at finite temperatures as the energy difference is very small. The Iba2 ice VIII coexisting with the I41/amd one can explain some anomalies observed in previous experiments.