Strain control of electronic phase in rare-earth nickelates

We use density functional plus U methods to study the effects of a tensile or compressive substrate strain on the charge-ordered insulating phase of LuNiO₃. The numerical results are analyzed in terms of a Landau energy function, with octahedral rotational distortions of the perovskite structure included as a perturbation. Less than $4%$ tensile or compressive strain leads to a first-order transition from an insulating structure with large-amplitude breathing-mode distortions of the NiO₆ octahedra to a metallic state in which breathing-mode distortions are absent but Jahn-Teller distortions in which two Ni-O bonds become long and the other four become short are present. Compressive strain produces uniform Jahn-Teller order with the long axis aligned perpendicular to the substrate plane, while tensile strain produces a staggered Jahn-Teller order in which the long bond lies in the plane and alternates between two nearly orthogonal in-plane directions, forming a checkerboard pattern. In the absence of the breathing-mode distortions and octahedral rotations, the tensile-strain-induced transition to the staggered Jahn-Teller state would be of second order.