Magnetic ordering in the Jeff=0 nickelate NiRh2O4 prepared via a solid-state metathesis

In spinel-type nickelate NiRh2O4, magnetic ordering is observed upon the sample synthesized via kinetically controlled low-temperature solid-state metathesis, as opposed to previously reported samples obtained through conventional solid-state reaction. Our findings are based on a combination of bulk susceptibility and specific heat measurements that disclose a Neel transition temperature of T-N = 45 K in this material, which might feature spin-orbit entanglement in the tetragonally coordinated d(8) Mott insulators. The emergence of magnetic ordering upon alteration of the synthesis route indicates that the suppression of magnetic ordering in the previous sample was rooted in the cation mixing assisted by the entropy gain that results from high-temperature reactions. Furthermore, the J(eff) = 0 physics, instead of solely the spin-only S = 1, describes the observed enhancement of effective magnetic moment well. Overseeing all observations and speculations, we propose that the possible mechanism responsible for the emergent magnetic orderings in NiRh2O4 is the condensation of the J(eff) = 1 exciton, driven by the interplay of the tetragonal crystal field and superexchange interactions.