Neutron and synchrotron diffraction data provide the first observation of changes in the crystal symmetry at the metal-insulator transition in $R{\mathrm{NiO}}_3$ perovskites. At high temperatures, ${\mathrm{YNiO}}_3$ is orthorhombic and metallic but below $T_{\mathrm{MI}}=582\mathrm{}\mathrm{K}$ it changes to a monoclinic insulator due to a charge disproportionation $(2{\mathrm{Ni}}^{3+}\to {\mathrm{Ni}}^{3+\delta }+{\mathrm{Ni}}^{3-\delta })$ that develops at the opening of the gap. We report the presence of two alternating ${\mathrm{NiO}}_6$ octahedra with expanded (Ni1) and contracted (Ni2) Ni-O bonds and a magnetic structure $[\mathbf{k}=(1/2,0,1/2\left)\right]$ with unequal moments at Ni1 and Ni2 octahedra.

We show that the Mott transition in orbitally degenerate systems can, and often does, proceed not in the standard “Mott insulator—weakly correlated metal” sequence, but via a novel intermediate phase with a charge (rather than orbital) ordering. Lifting an orbital degeneracy this way can be viewed as an alternative to a Jahn-Teller distortion. This may occur in a crossover between localized and itinerant regimes, if Hund’s rule coupling overcomes the on site Coulomb repulsion. We show both by calculations and by experiment that this scenario is realized in rare-earth nickelates, and argue that the same phenomenon takes place in many other systems.