A large hysteresis centered around room temperature represents one of the mandatory goals of research on functional switchable materials. In the thoroughly studied field of spin-crossover, such behaviour appears very rarely and essentially concerns coordination networks. A new compound showing a large spin-crossover hysteresis spanning room temperature demonstrates in a definitive manner that this goal is achievable in molecular discrete compounds without damaging the single-crystal character.

M 4 A 2O9 transition metal oxides, with M a divalent cation and A Nb or Ta, that exhibit a structure derived from corundum constitute an interesting class of materials due to their possible magnetoelectric properties. The lack of a linear magnetoelectric effect in Ni4Nb2O9, unlike Mn4Nb2O9 or Co4Nb2O9, is explained by a comprehensive investigation, combining synchrotron x-ray and neutron diffraction with magnetization, dielectric, and polarization measurements. The m′m′m magnetic point group associated with the Pb′cn′ ferrimagnetic structure induced below 76 K by the orthorhombic Pbcn structure of Ni4Nb2O9 forbids indeed such an effect. The crystal structure and magnetic ground state of Ni4Nb2O9 are discussed and compared with those of magnetoelectric P3¯c1M4A2O9 compounds whose structure derives from corundum.