Fluoride superionic conduction in TlF with the new anti-α-CuBr structure containing intrinsic F vacancies

Abstract

A new TlF phase with a cubic structure containing intrinsic F vacancies is synthesized with one-step chemical fluorination, not a conventional solid-state reaction. The crystal structure is the electronically inverted anti-α-CuBr structure of the Cu⁺ superionic conductor α-CuBr, with F occupying the Cu site and Tl occupying the Br site. The F⁻ conductivity at room temperature is increased by more than four orders of magnitude compared with the conventional orthorhombic phase, and F⁻ superionic conductivity (≥1 mS cm⁻¹) is observed at around 60 °C. Neural-network potential molecular dynamics demonstrates the F⁻ diffusion via intrinsic F vacancies. The calculated F⁻ conductivity (6.8 mS cm⁻¹ at 400 K) and activation energy (0.4 eV) are comparable to the experimental values (4.3 mS cm⁻¹ at 398 K, 0.3 eV). We propose a new design guideline that uses intrinsic F vacancies for outstanding F⁻ conductors instead of conventional material design based on the introduction of extrinsic F vacancies by aliovalent doping.