MAGNETIC AND HYDROGEN ABSORPTION PROPERTIES OF (La1-x Yx)Co5-HYDRIDES

The 5 9 ~ o NMR and magnetization measurement in (Lal-,Y,) Co5Hy have been performed at 77 K. The magnetizations of these compounds decrease with increasing hydrogen content (y) and are indepedent of Y-concentration (2). This result can be interpreted by an electron transfer effect in the direction H atom -+ Co atom. The pseudobinary intermetallic compound (Lal-,Y,) C05 have CaCu5 type structure and absorb large quantities of hydrogen gas [I-31. In this paper, the pressure-composition characteristics and the effect of hydrogen on the magnetic properties of (Lal-,Y,) Co5 were investigated in order to obtain more detailed information about the behaviour of hydrogen atom. The samples were prepared by melting the constituent metals in an induction furnace under a purified argon atmosphere followed by annealing at about 900 "C for one week. The X-ray analysis showed that all sample had CaCu5 type structures. The hydrogen absorption was performed as follows. The samples were first heated at 300 OC while continuously evacuating. Then at 100 OC they were exposed to high purity hydrogen gas of 4 atm. The amount of hydrogen absorbed was calculated from a pressure change in an apparatus of known volume. Magnetization measurements were made using a vibrating sample magnetomater at 77 K in field up to 8 kOe. The NMR signals of 5 9 ~ o nuclei were observed in zero external field at 77 K by a spin echo method. The a-axis in (Lal-,Y,) C05 decrease and while the oaxis slightly increase with increasing Y concentration. By hydrogen absorption, the crystal structure changes from hexagonal type structure to orthorhombic structure which slightly distorts from a hexagonal one. The lattice expansion which induced by hydrogen absorption are about 4 5.3 % for the a-axis and 0.9 1.2 % for the c-axis. This result shows that the effect of hydrogen absorption is larger for the aaxis than the c-axis. Figure 1 shows the absorption pressure-composition isotherms for (Lao.gY0.1) CO~H,,. The plateau pressure in (Lal-,Y,) c05Hy increases with increasing Y concentration. This increase of the plateau pressure seems to be related with the reduction of the a-axis which is induced by the substitution of Y for La. The thermodynamic parameters calculated from the relationship between the plateau pressure and the reciprocal temperature in absorption isotherms are summarized in table I. The enthalpy changes (AH) Fig. 1. Pressur~composition isotherms for hydrogen absorption in (Lao.gYo.1) Co6Hy. Table I. The thermodynamic parameters in (Lal-,Y,)Co5H,. Concentration AH AS AG (at 300 K) (r) (kcal/mol H2) (cal/K.mol H2) (kcal/mol Hz) and the entolopy changes (AS) decrease with increasing Y concentration. This shows that the stability of hydride decreases with substituting Y. Figure 2 shows a variation of the magnetic moment per Co atom as a function of hydrogen content (y) in (Lal-,Y,) CosH, (x = 0, 0.2). The magnetization in (Lal-,Y,) Co5 is independent of Y concentration. While the magnetic moment per Co atom in (Lal-,Y,) Co5Hy decreases with increasing hydrogen content (y) . This reduction in Co moment can be interpreted by an electron transfer in the direction H atom -+ Co atom. The two 5 9 ~ o Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888235