Exploring Sodium Ion-Transport in a Rutile-Related Framework: Linking Metal Cation Electronic Configuration, Crystal Structure, Crystallinity, and Ion Mobility in NaMCl6

In recent years, a large effort has been devoted to the development of halide-based sodium solid electrolytes, yet the landscape of structural frameworks is restricted and the role of the metal cation Mⁿ⁺ for Na⁺ ion transport is poorly understood. If advances are to be made in the field, then Na⁺ transport should be examined across a broad spectrum of frameworks, particularly focusing on the electronic configuration of the metal stabilizing the crystal structure, to gain a deeper understanding of Na⁺ dynamics and improve the transport properties of these systems. Here, we introduce a promising framework for Na⁺ transport, a rutile-derived structure with a nominal composition NaSbCl₆. This work suggests that the d¹⁰ electronic configuration of Sb⁵⁺ is related to the formation of this unexplored rutile-related framework in terms of Na⁺ transport. Although recent computational predictions reveal a low energy barrier for Na⁺ transport in this framework, our experimental studies show a very limited ionic conductivity, which might be associated with a poorly interconnected ionic pathway network. Partial substitution of Ta⁵⁺ for Sb⁵⁺ notably enhances the ionic transport due to an interplay between a structural rearrangement improving the ionic pathway connectivity and a decrease in the crystallinity. Our working hypothesis is based on the possible influence of the electronic configuration of the incorporated Ta⁵⁺, a highly charged d⁰ cation with a second-order Jahn–Teller Effect. The incorporation of this cation underlines the anisotropic contraction of MCl₆ (M = Sb⁵⁺, Ta⁵⁺) octahedra and a modification of the contiguous 8-fold NaCl₈ polyhedra. Additionally, the presence of Ta⁵⁺ might destabilize the formation of the rutile-type structure, reducing the crystallite size of the material, which is positively correlated with an enhancement in the conductivity as seen in other metal halides. Through this work, we seek to provide a novel framework for Na⁺ conduction and to provide a new perspective on the role of the Mⁿ⁺ metal cation as a potential driving force for the formation of the different frameworks. Thus, it appears possible to establish better structure-transport property relationships that contribute to the understanding of sodium halide solid electrolytes.