Polar and magneto-electric properties of anti-ferrodistortive ordered Jahn-Teller distortions in a multiferroic metal-organic framework

There is great interest in hybrid organic-inorganic materials such as metal-organic frameworks (MOFs). Here, we focus on [C(NH2)3]Cu(HCOO)3, a MOF with perovskite topology which crystallizes in polar space group Pna21. In inorganic compounds, octahedral tilting and Jahn-Teller structural distortions are usually non-polar distortions. However, in this MOF cooperative interactions between the antiferro-distortive distortions of the framework and the C(NH2)3 organic cation via hydrogen bonding breaks the inversion symmetry and induces a ferroelectric polarization. Our ab-initio study supports the picture of an orbital-order-induced ferroelectricity, a rare example of dipolar ordering caused by electronic degrees of freedom. Moreover, we show that the switching of polarization direction implies the reversal of the weak ferromagnetic component. These results therefore offer an important starting point for tailoring multiferroic properties in this emerging class of materials for various technological applications.