Fluorine as a Key Element in Solid-State Chemistry of Mixed Anions 3d Transition Metal-Based Materials for Electronic Properties and Energy.

Mixed anion compounds containing fluorine and based on 3d transition elements represent a class of materials with significant interest in solid-state chemistry. Indeed, their highly varied chemical composition, structural diversity, and the resulting electronic properties provide a rich playground for imagining new applications in the field of energy. The anions and the chemical bonds they form with the 3d transition elements are at the heart of this review. Key parameters such as electronegativity, hardness, and polarizability are introduced and discussed to better understand the charge capacity of the anion and the bonds formed in the solid. Oxyfluorides represent the most studied family due to the size similarity of the two anions, and part of the review is dedicated to the specific synthesis of these materials by systematically adjusting the fluorine content within various structures and analyzing the electronic and electrochemical properties of these compositions. The final sections focus on materials with structures often exhibiting a two-dimensional character, where ionic blocks coexist with covalent layers, such as fluorochalcogenides, fluoropnictides, and fluorotetrelides. The compositions and structures are systematically correlated with the electronic properties.