Origin of structural distortion and metal-insulator transition in rare-earth Nickelates

Rare-earth nickelates RNiO3 (R=rare-earth element) belongs to the ABO3 perovskite class of systems and exhibit a rich phase diagram that hosts metal-insulator transition (MIT), structural transitions, magnetism, and also superconductivity under certain perturbations.  For all systems above the MIT critical temperature T_MIT, the systems are paramagnetic metals in the orthorhombic Pbnm phase --- a weak pseudo-cubic distortion of the perfect cubic crystal.  T_MIT decreases in the excursion from right to left of the R series, the highest being LuNiO3.  This can be partially understood as a simple size effect, Lu being the smallest atom; but size cannot alone explain the MIT.  The MIT coincides with a lowering of structural symmetry that takes the system into a monoclinic P21/n phase, where Ni sites belonging to adjacent octahedra become inequivalent.  We show that this is closely related to the magnetic state of the Ni.  The MIT occurs as a consequence of the structural distortion working in tandem with disproportionation of the Ni moment, where one Ni becomes magnetic and the other nonmagnetic.  In the Pbnm phase Ni is in a d7 (3+) configuration, with a high degree of residual orbital moment.  Disproportionation enables Ni to partially quench the orbital moment and lower the total energy.