  Perovskite metal-organic frameworks (MOFs) have recently emerged as potential
candidates for multiferroicity. However, the compounds synthesized so far
possess only weak ferromagnetism and low polarization. Additionally, the very
low magnetic transition temperatures ($T_c$) also pose a challenge to the
application of the materials. We have computationally designed a mixed metal
perovskite MOF -[C(NH2)3][(Cu0.5Mn0.5)(HCOO)3]- that is predicted to have
magnetization two orders of magnitude larger than its parent
([C(NH2)3][Cu(HCOO)3]), a significantly larger polarization (9.9 {\mu}C/cm2),
and an enhanced $T_c$ of up to 56 K, unprecedented in perovskite MOFs. A
detailed study of the magnetic interactions revealed a novel mechanism leading
to the large moments as well as the increase in the $T_c$. Mixing a
non-Jahn-Teller ion (Mn$^{2+}$) into a Jahn-Teller host (Cu$^{2+}$) leads to
competing lattice distortions which are possibly responsible for the enhanced
polarization. The MOF is thermodynamically stable as evidenced by the computed
enthalpy of formation, and can likely be synthesized. Our work represents a
first step towards rational design of multiferroic perovskite MOFs through the
largely unexlpored mixed metal approach.
