Hydrogen absorption by the ternary borides RCo 4B (R ≡ La, Pr, Sm) and RNi 4B (R ≡ Ca, La, Pr)

Abstract

The hydrogen absorption properties of the ordered hexagonal ternary borides RCo 4B (R ≡ La, Pr, Sm) and RNi 4B (R ≡ Ca, La, Pr) were investigated in order to obtain information about the parameters leading to hydride formation in CaCu 5-type intermetallics. RCo 4B compounds crystallize with the CeCo 4B structure and absorb about 4.5 hydrogen atoms per formula unit. Multiplateau behavior indicates that at least three distinct hydride phases exist around room temperature in the pressure range 0–100 atm. In contrast with the RCo 5H z phases, which are orthorhombic for z > 1, RCo 4BH z phases exhibit an orthorhombic structure only in the α phase region ( z ⩽ 1), with all higher hydrides reverting to the hexagonal structure. CaNi 4B, which also crystallizes with the CeCo 4B structure, absorbs 3.2 hydrogen atoms per formula unit at 100 atm and 324 K. The other RNi 4B compounds produce X-ray powder patterns which are similar to CeCo 4B-type patterns but which exhibit several anomalies. In the case of LaNi 4B, singlecrystal studies confirm that a long-period superstructure extends along the basal plane directions. The RNi 4 (R ≡ La, Pr) ternaries show a maximum hydrogen uptake of 1.5 hydrogen atoms at room temperature in the pressure range 0–100 atm. This appears to indicate that only built1 2 R in the formula RNi 4B is active in absorbing hydrogen (according to the subunit built1 2 RH 3 ). It is assumed that the R which is active in bonding with hydrogen in these latter materials is situated in the R-T plane, while the R in the R-B plane is inactive in this respect. Models for hydrogen site occupancy in the RCo 4B hydrides are proposed on the basis of crystallographic considerations.