Monitoring the Microwave Synthesis of d⁰-Free Disordered Rocksalt Cathodes Using In Situ Infrared Pyrometry

A detailed understanding of solid-state reaction pathways is essential for connecting predictive frameworks, such as density functional theory and machine learning, with experimental synthesis. Microwave synthesis has emerged as a powerful route for preparing inorganic materials, yet the mechanisms governing microwave-driven processes remain poorly understood, particularly for metastable compounds whose formation is highly sensitive to synthesis conditions. Disordered rocksalt oxides (DRX) are high-temperature metastable phases of interest as next-generation Li-ion cathodes. Here, we investigate the microwave reaction pathway of ${\mathrm{Li}}_{1.1}{\mathrm{Mn}}_{0.9}{O}_{1.9}{F}_{0.1}$ . Combining ex situ phase identification using x-ray diffraction and solid-state NMR with in situ infrared thermography, we show that the reaction proceeds through a reentrant order-disorder-order transformation. Layered Li-Mn-O intermediates disorder above 945 ${}^{\circ }C$ to form the DRX phase, while continued heating drives reordering back to layered structures. Infrared profiles reveal a distinct feature marking completion of the disordering transition, enabling precise reaction termination to maximize DRX phase purity. We further examine the impact of phase purity on the " $\delta$ -phase" transition during electrochemical cycling and find that residual layered phases minimally affect performance. These findings indicate that ${\mathrm{Li}}_{1.1}{\mathrm{Mn}}_{0.9}{O}_{1.9}{F}_{0.1}$ is only stable near 945 ${}^{\circ }C$ , yet its electrochemical performance tolerates synthesis-induced impurities.