In situ X-ray diffraction study of the hydriding combustion synthesis of Mg₂NiH₄

An in situ X-ray diffraction study of the hydriding combustion synthesis of Mg₂NiH₄ directly from the compact of magnesium and nickel mixture in a pressurized hydrogen atmosphere has been conducted. During two cycles of heating (hydriding) and cooling (dehydriding) in a temperature range from 300 to 823 K, the X-ray diffraction patterns were obtained in a pressure of 1.0 MPa at eleven points of the temperature. The obtained X-ray diffraction patterns reveal nine reactions existing in two cycles of heating and cooling. In the first cycle, (1) Mg+H₂→MgH₂, partially, (2) MgH₂→Mg+H₂, (3) 2Mg+Ni-→Mg₂Ni, in the heating period, and (4) Mg₂Ni + 2H₂→Mg₂NiH₄ (HT), partially, (5) Mg₂NiH₄ (HT)→Mg₂NiH₄ (LT), in the cooling period. In the second cycle, (6) Mg₂NiH₄ (LT)→Mg₂NiH₄ (HT), (7) Mg₂NiH₄ (HT)→Mg₂Ni+2H₂, in the heating period, and (8) Mg₂Ni+2H₂→Mg₂NiH₄ (HT), partially, (9) Mg₂NiH₄ (HT)→Mg₂NiH₄ (LT), in the cooling period. The X-ray diffraction intensity of Mg₂Ni after the second cycle of hydriding decreases to 1/6 times of that after the first cycle of hydriding. In contrast, the X-ray diffraction intensity of Mg₂NiH₄ after the second cycle of hydriding is six times of that after the first cycle of hydriding, although only one single phase of Mg₂Ni exists at 823 K before hydriding either the first cycle or the second cycle.