Insights into the composition exploration of novel hydrogen storage alloys: evaluation of the Mg-Ni-Nd-H phase diagram

In order to provide an efficient tool to explore alloy composition and processing conditions for Mg-based alloys with good hydriding/dehydriding (H/D) properties, investigation of the Mg-Ni-Nd-H quaternary system was carried out by experimental measurements and CALPHAD thermodynamic analysis combined with first-principles calculations. A new stable compound Nd16Mg96Ni12 with the space group of Cmc2(1) was identified in the Mg-Ni-Nd system by synchrotron powder X-ray diffraction (SR-PXRD). The phase equilibria and phase transformation related to Nd4Mg80Ni8, Nd16Mg96Ni12, NdMg5Ni and NdMg2Ni were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The formation enthalpies of these ternary compounds were calculated by the density functional theory. The existence of the quaternary hydrides NdMgNi4H4 and NdMg2Ni9H12 was taken into consideration. Based on the obtained thermodynamic description of the Mg-Ni-Nd-H system, the hydrogen storage capacities and pressure-composition-temperature (P-C-T) curves were predicted and two new ternary compounds Nd4Mg80Ni8 and Nd16Mg96Ni12 were designed as hydrogen storage alloys with excellent properties. Meanwhile, the H/D mechanism of the Mg-Ni-Nd alloy was revealed.