Probing ferroic transitions in a multiferroic framework family: a neutron diffraction study of the ammonium transition metal formates

This study probes the magnetic and ferroelectric ordering of the NH<inf>4</inf>M(HCO<inf>2</inf>)<inf>3</inf> (M = Mn2+, Fe2+, Co2+ and Ni2+) frameworks using neutron diffraction, improving the understanding of the origins of the properties of these fascinating multiferroics. This rare study of the magnetic structure of a family of metal-organic frameworks shows that all four compounds exhibit antiferromagnetic coupling between neighbouring cations bridged by formate ligands. The orientation of the spin, however, changes in a highly unusual way across the series with the spins aligned along the c-axis for the Fe2+ and Ni2+ frameworks but lying in the ab plane for the other members of the series. This work also sheds new light on the nature of the ferroelectric order-disorder transition in these materials; probing changes in the ammonium cation across the transition and also shows that the Ni2+ framework does not undergo a transition to the polar P6<inf>3</inf> phase due to the smaller size of the Ni2+ cation. Finally trends in their anisotropic negative thermal expansion, which potentially enhances their ferroic behaviour, are quantified.