Enthalpies of formation of RNi 5 compounds

Abstract

Abstract The RNi5 compounds (with R = La, Ce, Pr, Sm, Y) crystallize in the hexagonal CaCu5 type structure. This close packed stacking is characterized by a cell parameter directly related to the R and Ni atomic radii, , and by the c parameter which depends almost entirely on the Ni atomic radius, c = The a lattice parameter variation inside the series RNi5 displays an anomaly for CeNi5 compound [1]. This anomaly has been studied previously by means of magnetic properties and X-ray absorption [2]; the X-ray absorption experiments show that in CeNi5, Ce has the same valence state close to 4+ as in the insulator CeO2. In the present study the enthalpies of formation of the RNi5 compounds are measured by a calorimetric method. The variation of enthalpies of formation inside the series RNi5 shows an anomaly for CeNi5 compound; this study allows us to estimate the energy associated to this anomaly. The enthalpy of formation of the RNi5 compound corresponds to the reaction: To + 5 To → To Its value is deduced from the dissolution enthalpies of the compound and its components in liquid aluminum by using the relation: ΔHTof(RNi5) = Q∞R + 5Q∞Ni − Q∞RNi5 where Q∞x (x = R, Ni, RNi5) is the heat of dissolution at infinite dilution of the compound or the elements. The enthalpies of dissolution were measured using an isoperibolic calorimeter which has been described elsewhere [3,4].7] t001 . Experimental and Calculated Values of the Enthalpies of RNi5 Compounds. Compound ΔH298f experimental kJ mol−1 rR/r4f ΔH298f calculated kJ mol−1 LaNi5 −158.9 3 −150.6 CeNi5 −199 PrNi5 −160.6 3.31 −165.2 SmNi5 −182 3.635 −182 YNi5 −204.6 Experimental enthalpies of formation are shown in Table I. Our value of the enthalpy of formation of LaNi5 compound is an agreement with the more recent experimental values obtained by O'Hare [5] and S. Watanabe [6] which are − 164 and − 157.7 kJ mol−1 respectively. The a lattice parameter and the variations of enthalpy of formation inside the series RNi5 are presented in Fig. 1. The evolution of these two quantities is appreciably identical, the abnormal behaviour of Ce in CeNi5 compound being exhibited in the two cases. The variation of the enthalpy of formation can be explained by physical properties of the rare earth metals. Geschneidner [7] has explained the crystal structure sequence of the lanthanide metal when compressed ar alloyed by the participation of the 4f electrons in the bonding for the light lanthanide metals (La to Ho). The 4f contribution to the bonding is essentially zero for the heavy lanthanide metals (Er to Lu). Geschneidner [8] found that the metallic radius of the rare earth rR divided by the 4f orbital radius r4f, calculated by Freeman [9], gives a number which is at least a qualitative estimate of the relative participation of the 4f electrons in the bonding in these metals. The assumption of this radius ration can be applied to the intermetallic compound and in our case allows us to calculate empirically the enthalpies of formation of RNi5 compounds where R is heavy lanthanide metal must be equal to the value of the enthalpy of formation of the YNi5 compound (−204.6 kJ mol−1 since Yttrium without 4f electrons has a behaviour similar to the heavy lanthanide metals. For the RNi5 compounds, where R is a light lanthanide metal, the enthalpies of formation are deduced from the enthalpy of ErNi5 by the study of the rR/r4f ratio. The calculated values are compared with the experimental ones in Table I. There is a good agreement between calculated and experimental results. This approach allows us to give an explanation about the peculiar value of the enthalpy of formation of CeNi5 compound. If Ce has a behaviour similar to La and Pr in the RNi5 compound the enthalpy of formation of the CeNi5 compound should be obtained by linear interpolation from the values of the enthalpies of formation of LaNi5 and PrNi5 compounds; the obtained value should be equal to −160 kJ mol−1, value plainly different of the experimental value − 199 kJ mol−1. Moreover the experimental values of the enthalpies of formation of CeNi5 and YNi5 compounds are very close and let us think that the 4f electrons of Ce do not participate in the bonding in the CeNi5 compound. Our energetic study does not allow us to conclude in favour of a valence state close to 4+ of Ce in CeNi5 compound, nevertheless our results are not in disagreement with this assumption and show that Cerium has effectively an abnormal behaviour in the CeNi5 compound.