High Resolution Image Download MS PowerPoint Slide Mg 2 TMH n complex hydrides, where TM represents various combinations of transition metals, were synthesized by reactive ball milling of Mg and TM powders under H 2 pressure. TM was an equimolar mixture of three (Fe, Co, and Ni), four (Mn, Fe, Co, and Ni), or five (Cr, Mn, Fe, Co, and Ni) elements. The Mg/TM ratio was either 2:1 or 3:1. For 2:1 samples, a single fcc hydride phase Mg 2 TMH n with a K 2 PtCl 6 -type structure was detected by X-ray diffraction along with a residual, unreacted metal phase. By contrast, in samples where the Mg/TM ratio was 3:1, the tetragonal MgH 2 hydride was also observed. The formation of Mg 3 TMH n complex hydrides, previously reported for TM = Cr and Mn under high-pressure conditions, was not detected. The maximum hydrogen content in the as-milled state was about 5 wt% for samples with a 3:1 Mg/TM ratio as determined by temperature-programmed desorption. The as-milled hydrides exhibited similar onset temperatures for desorption independently of the TM composition, suggesting no destabilization induced by elements like Mn and Cr that are known to form only unstable, high-pressure hydrides. The reversible hydrogen storage, investigated by pressure–composition isotherms in a Sieverts-type apparatus, arises from both the Mg-MgH 2 and the Mg 2 TM-Mg 2 TMH n transformations. Within the 0.1–20 bar and 285–320 °C window, the samples with a 3:1 Mg/TM ratio exhibit a reversible gravimetric capacity in the 3.7–4.2 wt% range depending on TM composition, while those with a 2:1 ratio are in the 3.0–3.2 wt% range. The decreased reversible capacity compared to the initial hydrogen content was associated with the phase segregation of the transition metals, particularly Cr and Mn, which was highlighted by X-ray diffraction and transmission electron microscopy with nanoscale microanalysis.