We report remarkable multiferroic effects in polycrystalline Bi2Fe4O9. High-resolution x-ray diffraction shows that this compound has orthorhombic structure. Magnetic measurements confirm an antiferromagnetic transition around 260K. A pronounced inverse S-shape anomaly in the loss tangent of dielectric measurement is observed near the Néel temperature. This feature shifts with the application of magnetic field. These anomalies are indicative of substantial coupling between the electric and magnetic orders in this compound.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of ${\mathrm{Ba}}_3{\mathrm{ZnIr}}_2{\mathrm{O}}_9$ is a realization of a novel spin-orbital liquid state. Our results reveal that ${\mathrm{Ba}}_3{\mathrm{ZnIr}}_2{\mathrm{O}}_9$ with ${\mathrm{Ir}}^{5+}$ ($5d^4$) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive $J=0$ state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK.