Proton Ordering Induces a Polar Structure in the Antiferromagnetic Solid Proton Conductor FeH₆(PO₄)₃

Materials exhibiting coexisting exploitable properties often result in especially attractive behavior from both fundamental and applied perspectives. In particular, magnetoelectric materials combining ferroelectric and magnetic properties are increasingly becoming paramount nowadays. Here, we show that FeH₆(PO₄)₃ exhibits proton conductivity and the coexistence of magnetic and polar structural features, suggesting that such frameworks may be of broader interest beyond the field of proton conductors. By a combination of neutron diffraction and second harmonic generation experiments, we have demonstrated that FeH₆(PO₄)₃ crystallizes in the polar R3c space group. Inversion symmetry breaking is triggered by a polar proton ordering within the structure. In FeH₆(PO₄)₃, this particular cation ordering in combination with the polar displacement of the adjacent structural units results in a polar crystal structure with a calculated net polarization of approximately 10 μC cm^-2 between 10 and 300 K. Together with an antiferromagnetic state below 28 K, determined from a combination of neutron diffraction and magnetic measurements and associated with the particular Fe³⁺ octahedral arrangement, the result is the coexistence of both properties. By a detailed study of this system with a full description of the crystal structure as well as the ionic and magnetic properties, we aim to spark further investigations in magnetoelectric materials existing in the solid ionic conductor phase space.