Magnetic ordering in the $J_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 0 Nickelate NiRh${}_2$O${}_4$ prepared via a solid-state metathesis

In spinel-type nickelate NiRh${}_2$O${}_4$, 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 N$\acute{e}$el 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 $J_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 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 NiRh${}_2$O${}_4$ is the condensation of $J_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 0 exciton, driven by the interplay of the tetragonal crystal field and superexchange interactions.