Revisiting Jahn-Teller transitions in correlated oxides with Monte Carlo modeling

Jahn–Teller (JT) distortions are a key driver of physical properties in many correlated oxide materials. Cooperative JT distortions, in which long-range orbital order reduces the symmetry of the average structure, are common in JT-distorted materials at low temperatures. This long-range order will often melt on heating, via a transition to a high-temperature state without long-range orbital order. The nature of this transition has been observed to vary with different materials depending on crystal structure; in LaMnO3 the transition has generally been interpreted as order-disorder, whereas in layered nickelates ANiO2 (A=Li,Na) there is a displacive transition. However, recent theoretical work has suggested that previous evidence for order-disorder may in fact be a consequence of phonon anharmonicity, rather than persistence of JT distortions. In this work, we run Monte Carlo simulations with a simple Hamiltonian that is modified to include terms dependent on the JT amplitude ρ, which is allowed to vary within the simulation. Our simulations yield distributions of JT amplitudes consistent with displacive rather than order-disorder behaviour for both perovskites and layered nickelates. We also find significant differences between the transition observed for perovskites compared with layered nickelates, which we attribute to differing extensivity of configurational entropy on the two lattices, suggesting a crucial role for lattice geometry in determining behaviour.