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RNiO _3 perovskites have been described to present thermally driven metal-insulator transitions (at T _MI ) as a function of the rare-earth ion size (R = Pr to Lu). Aiming to extend the stability range of RNiO _3 for smaller R ^3+ ions, we prepared Lu _1−x Sc _x NiO _3 (x = 0, 0.1, 0.2) perovskites, being Sc ^3+ ions substantially smaller than Lu ^3+ , by using a multi-anvil high-pressure synthesis device at 10 GPa. We have studied the structural evolution of Lu _0.9 Sc _0.1 NiO _3 by synchrotron x-ray diffraction (SXRD) from room temperature to 350 °C. The symmetry of the lattice evolves from monoclinic ( P2 _1 /n ) to orthorhombic ( Pbnm ) upon heating across T _MI (≈320 °C), with the existence of two chemically and crystallographically distinct nickel sites in the insulating, monoclinic regime, whereas the metallic phase has a single NiO _6 environment. A simultaneous structural and electronic transition implies an abrupt evolution of the lattice parameters and size of the NiO _6 octahedra upon entering the metallic regime, leading to the merging of the disproportionated Ni-O bond lengths. The magnetic properties correspond to the establishment of antiferromagnetic correlations at the Ni sublattice; a decrease of the T _N ordering temperature from 122 K (x = 0) to 113 K (x = 0.2) is observed as the Sc content increases, which is concomitant with a more distorted perovskite structure.