Fluoride-ion conduction in Ba4M3F17 (M = Bi3+, Y3+) with conductive M6F13 cage units

Abstract

Nanoporous cage structures serve as ideal units for ionic conduction due to the weak bonds formed between the central ions and the surrounding framework, which facilitates high ionic mobility. However, in the realm of inorganic compounds, research into ionic conduction within cage structures containing mobile central ions has been limited primarily to the oxide-ion conductor 12CaO·7Al₂O₃, and the superiority of the cage structure over other conducting units remains unclear. This study investigates Ba₄M₃F₁₇ (M = Bi³⁺, Y³⁺), which incorporates M₆F₁₃ cage units and a fluorite-type framework commonly associated with F⁻ conduction. Bond valence energy landscape calculations suggest that F⁻ conduction pathways in Ba₄M₃F₁₇ are primarily facilitated through the M₆F₁₃ cage units rather than the fluorite-type framework. The calculated barriers for F⁻ migration within the cage units are extremely low, although the rigid cation-ordered fluorite-type framework may hinder fast ionic conduction in Ba₄M₃F₁₇. The conductivity is improved by modifying the cage units and quantity of F vacancies, reaching 5.8 × 10⁻⁶ S cm⁻¹ at 200 °C for Ba₄Bi_3−xPb_xF_17−x (x = 0.3). This research provides valuable insights into the role of the cage structure in ionic conductors, contributing to the development of high-performance ionic conductors based on cage structures.