Electrostatic Interactions versus Second Order Jahn–Teller Distortion as the Source of Structural Diversity in Li₃MO₄ Compounds (M = Ru, Nb, Sb and Ta)

With the advent of layered rocksalt oxides showing anionic redox activity toward Li, there has been an increased focus on designing new rocksalt structures and, more particularly, compounds pertaining to the Li 3 MO 4 family. The structural richness of this family is nested in its ability to host many different cations, leading to the formation of superstructure patterns whose predictability is still limited. Thus, there is a need to understand the formation of such superstructures, as cationic arrangements have a crucial effect on their physical properties. Herein we propose a combined experimental and theoretical approach to understand the interactions governing cation ordering in binary systems of general composition given by Li 3 M y M′ 1– y O 4 (M and M′ being Ru, Nb, Sb, and Ta). Through complementary X-ray diffraction and X-ray absorption spectroscopy techniques, we reveal a solid-solution behavior for the Li 3 Ru y Sb 1– y O 4 system, as opposed to Li 3 Sb y Nb 1– y O 4 that enlists four rocksalt structures with different cation orderings. We use DFT calculations to rationalize such a structural diversity and find that it is controlled by a delicate balance between electrostatic interactions and charge transfer due to a second order Jahn–Teller distortion. This insight provides a new viewpoint for understanding cationic arrangements in rocksalt structures and guidelines to design novel phases for applications such as Li-ion batteries or ionic conductors.