Decoupling charge, lattice, and spin order with A-Site Cation Variance in Perovskite Nickelates*

Strain, doping, and heterostructuring are useful avenues toward exploration of phase space in oxides. These methods are well studied in the perovskite nickelates, which have close ties between structural, electronic, and magnetic orders. However, these traditional methods can be limited in their approach. Recently, theoretical predictions have suggested size variance of the A-site sublattice in strongly correlated 3d transition metal oxides may provide an extra layer of tunability to phase response. These predictions propose the presence of potential unexplored regions in the nickelate phase diagram, which may diverge from expected trends when there is little variance in the average tolerance factor. In an experimental realization of extreme A-site cation disorder, we synthesize (Y_0.2La_0.2Nd_0.2Sm_0.2Gd_0.2)NiO₃, whose parent ternary oxides each have a large range of functional and  structural phase transition temperatures. Transport and resonant elastic x-ray spectroscopy results demonstrate that the metal to insulator transition, monoclinic phase transition, and magnetic ordering temperatures are strongly influenced by A-site cation variance. These results suggest cation variance, such as that accessible in high entropy oxides, can be a critical order parameter in the design of correlated oxides, and that this parameter can provide continuous tunability to charge and magnetic orderings.