title: 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 = 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.
authors.affiliation: Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK. paul.saines@chem.ox.ac.uk.
authors.forename: James M M
authors.initials: JM
authors.lastname: Lawler
authors.forename: Pascal
authors.initials: P
authors.lastname: Manuel
authors.forename: Amber L
authors.initials: AL
authors.lastname: Thompson
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Saines