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

Abstract

This study probes the magnetic and ferroelectric ordering of the NH₄M(HCO₂)₃ (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 P6₃ 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.