Influence of aliovalent substitution on structure and dynamics in sodium halide Na3−2xY1−xNbxCl6 solid 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 sodium-ion transport pathways. A structural threshold is reached for Na₂Y_0.5Nb_0.5Cl_6, revealing a limit for niobium 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.