Earth abundant transition metal oxides are attracting broad interest for thermochemical production of renewable fuels and other gaseous commodities. Despite progress, a major challenge remains achieving fast and reversible redox kinetics as well as large oxygen exchange capacities. Here, we present insights on the optimal doping of manganese oxide nanocrystals for their efficient and stable utilization as a redox material for thermochemical water splitting. The detailed investigation of the evolution of the material properties over a broad range of possible Ce-Mn compounds reveals a single key structural parameter affecting the thermochemical performance. We observe that the expansion of the MnO lattice is essential for activating its reduction from oxide to carbide, and thus for H2O splitting during its subsequent re-oxidation. This is optimally achieved for a very narrow window of dopant concentration peaking at 3% Ce content, which provides the largest distortion of the manganese oxide crystal lattice. In contrast, smaller or higher Ce amounts of 1% and 5%, respectively, result in significantly smaller lattice expansions either due to an insufficient dopant amount or to the segregation of Ce in large CeO2 domains. We use these findings to propose a mechanism for the enhancement of the redox kinetics of this metal oxide, which may provide guidance for the design of a family of future materials for thermochemistry.