While all of the polymorphs of NH$_4$I and NH$_4$Br are non-polar, a reversible electric polarization is established in the ordered $\gamma$ phases of (NH$_4$)$_{ 0.73}$(ND$_4$)$_{ 0.27}$I and (NH$_4$)$_{ 0.84}$(ND$_4$)$_{ 0.16}$Br (where D is $^2$H) 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.
Categories: cond-mat.mtrl-sci