Influence of Aliovalent Substitution on Structure and Dynamics in Sodium Halide Na3-2xY1-xNbxCl6Solid Electrolytes

Abstract

Sodium halide solid electrolytes are garnering increased interest because of their synthetic flexibility to incorporate a variety of cations, thereby altering their structure and properties. Aliovalent substitution is said to increase ionic conductivity by promoting polyanion rotation. Herein, we synthesise and assess a series of Na_3-2xY_1-xNb_xCl₆, probing their complex structures using complementary powder X-ray diffraction and variable-temperature 1D and 2D solid-state nuclear magnetic resonance spectroscopy. The bond-valence energy landscapes of the end members are visualised to reveal potential Na-ion transport pathways. A structural threshold is reached for Na₂Y_0.5Nb_0.5Cl_6, revealing a limit for Nb polyhedral distortion while the unit cell volume is retained up to 50% Nb substitution. Na₂Y_0.5Nb_0.5Cl₆ shows the greatest RT ionic conductivity enhancement in the series, from 10^–11 S cm^–1 to 10^–5 S cm^–1.