Phase evolution during reactive flash sintering of Li6.25Al0.25La3Zr2O12 starting from a chemically prepared powder

•Li6.25Al0.25La3Zr2O12 ceramics were produced by reactive flash sintering (RFS), starting from a chemically prepared precursor powder.•Single-phase dense samples were obtained in just 30 s with an electric field of 50 V cm−1 and a current limit of 150 mA mm−2.•The ceramics displayed the cubic garnet structure and a bulk ionic conductivity of 0.18 mScm−1 at room temperature.•Phase evolution during RFS is studied by in-situ synchrotron XRD and compared with conventional heating processing.•Reaction pathway during RFS is different, which can be used for stabilizing phases that are not thermodynamically favored.