Hydrogenation properties and crystal structure of YMgT4 (T = Co, Ni, Cu) compounds

New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthesized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches 6.8 at. H/f.u. The Pressure-Composition-Temperature studies of YMgCo4-H2 and YMgNi4-H2 systems revealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases the stability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropy of the YMgCo4H6.8 hydride formation were calculated: ΔH= -27.9 ± 0.8 kj mol-1 H2 and ΔS = -93.4 ± 2.6 J mol-1 H2 K-1. The YMgCo2Ni2-H2 system shows intermediate thermodynamic properties compared to the ternary hydrides (ΔH = -28.8 ± 0.2 kJ mol-1 H2 and ΔS = ―117.6 ± 2.4J mol-1 H2 K-1). The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with a cell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does not interact with hydrogen under normal conditions.