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 NH4M(HCO2)3 (M = Mn(2+), Fe(2+), Co(2+) and Ni(2+)) 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 Fe(2+) and Ni(2+) 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 Ni(2+) framework does not undergo a transition to the polar P63 phase due to the smaller size of the Ni(2+) cation. Finally trends in their anisotropic negative thermal expansion, which potentially enhances their ferroic behaviour, are quantified.