Inducing ferroelectricity in NH_4I and NH_4Br via partial replacement of protons by deuterons

While all of the polymorphs of NH_4I and NH_4Br are non-polar, a reversible electric polarization is established in the ordered γ phases of (NH_4)_0.73(ND_4)_0.27I and (NH_4)_0.84(ND_4)_0.16Br (where D is ^2H) via dc electric fields. The presence of two groups of orbital magnetic moments appears to be responsible for the asymmetric lattice distortions. Our findings provide an alternative pathway for hydrogen-based materials to potentially add a ferroelectric functionality.