Exploring $\rm Mg^{2+}$ and $\rm Ca^{2+}$ Conductors Via Solid-State Metathesis Reactions

The scarcity of viable electrode and electrolyte materials vastly hinders the advancement of magnesium and calcium batteries. This study utilises solid-state metathetical reactions involving chalcogen- and pnictogen-based honeycomb layered oxides with alkaline-earth halides/nitrates to synthesise $\rm Mg^{2+}$- and $\rm Ca^{2+}$-based materials previously achievable only under high-temperature/high-pressure conditions, as well as new metastable materials with unique crystal versatility. Particularly, we employ metathetical reactions involving $\rm Li_4MgTeO_6$, $\rm Na_2Mg_2TeO_6$, and $\rm Na_4MgTeO_6$ with $\rm MgCl_2$/$\rm MgSO_4$/$\rm Mg(NO_3)_2$.$\rm 6H_2O$ or $\rm Ca(NO_3)_2$.$\rm 4H_2O$ / $\rm CaCl_2$.$\rm 2H_2O$ at temperatures not exceeding 500 $^\circ$C to produce $\rm Mg_3TeO_6$ polymorphs, ilmenite-type $\rm CaMg_2TeO_6$/$\rm Mg_2CaTeO_6$, and double perovskite-type $\rm Ca_2MgTeO_6$. Thus, we demonstrate that these materials, conventionally requiring gigascale pressures or high temperatures (>1000$^\circ$C) for their proper synthesis, are now readily accessible at ambient pressure and considerably lower temperatures. Meanwhile, despite sub-optimal pellet densities, the synthesised ilmenite-type \magenta {$\rm Mg_3TeO_6$ (high-pressure polymorph)} and double perovskite-type ${\rm Ca}_2M{\rm TeO_6}$ ($M = \rm Mg, Ca, Zn$) materials exhibit remarkable bulk ionic conductivity at room temperature, marking them as promising compositional spaces for exploring novel $\rm Mg^{2+}$ and $\rm Ca^{2+}$ conductors. Furthermore, this study extends the applicability of metathetical reactions to attain Mg- or Ca-based antimonates, ruthenates, titanates, phosphates, and silicates, thus opening avenues to novel high-entropy multifunctional nanomaterial platforms with utility in energy storage and beyond.