Microscopic Mechanisms of Superionic Na-ion Conductivity in Crystalline and Amorphous NaMOCl₄ (M = Nb, Ta) Solid...

Sodium-ion solid electrolytes offer a sustainable route toward next-generation batteries, but few match the performance of their lithium counterparts. Halide-based NaMOCl₄ (M = Nb, Ta) has recently emerged as a promising analogue to LiMOCl₄, yet its structure-transport relationships remain unclear due to poor crystallinity in experiments. Here, we combine density functional theory and machine-learned molecular dynamics to reveal that crystalline NaMOCl₄ exhibits negligible room-temperature conductivity with high activation barriers arising from vacancy-mediated diffusion below an order-disorder transition. Above this transition, rotational and translational motion of the [MO_2/2Cl₄ ^-]_∞ chains create new Na sites and enhances transport. In contrast, the amorphous phase inherently supports facile, three-dimensional Na diffusion through dynamic framework flexibility. These results show that ordered crystalline phases hinder ionic transport, while disorder - either thermally induced or structural - facilitates it, revising prior assumptions from the Li system and providing design principles for high-conductivity Na halide electrolytes.