Zircon Structure as a Prototype Host for Fast Monovalent and Divalent Ionic Conduction

“Beyond Li-ion” energy storage solutions based on ions such as Na, Mg, Ca, and Zn have attracted increasing attention due to growing concerns about the cost, resource availability, and safety of the currently dominant Li-ion batteries. One of the greatest challenges for beyond-Li systems, especially multivalent ones, is the lack of materials with high ionic mobility. In this study, we find that zircon-type YPO<sub>4</sub> presents a unique structural environment that enables superior conduction of multiple species including Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, and Zn<sup>2+</sup>, even in the dilute carrier concentration regime. This highly unusual capability originates from one-dimensional (1D) percolating channels of adjacent, distorted octahedral sites, which results in a smoothly varying coordination environment and correspondingly low activation barriers. Low decomposition energy of multiple compositions of doped YPO<sub>4</sub>, where the carrier ions are introduced into the system along with subvalent doping into P sites, confirmed good stability and synthesizability. Among these compositions, we found Na<sub>0.0625</sub>YSi<sub>0.0625</sub>P<sub>0.9375</sub>O<sub>4</sub> exhibiting good Na<sup>+</sup> conductivity of 0.99 mS/cm at 300 K with an activation energy of 220 meV. Zircon-structured Na<sub>0.05</sub>YSi<sub>0.05</sub>P<sub>0.95</sub>O<sub>4</sub> was successfully synthesized; however, the highest density achieved (78%) was insufficient to conclusively establish its conductivity. Finally, we identify dopant–carrier association in doped YPO<sub>4</sub> as a key challenge for long-range diffusion in this structure family.