Journal of Magnetism and Magnetic Materials 104-107 (1992) 661-662 North-Holland Correlation between mixed valence behaviour of cerium and the magnetic and superconducting phenomena of CeFe 2 and CeRu 2 Jesfis Chaboy a, Joaquln Garcia a and Augusto Marcelli b a ICMA, CSIC-Universidad de Zaragoza, Zaragoza E-50009, Spain b Laboratori Nazionali di Frascati, INFN, 00044 Frascati, Italy This contribution concerns an X-ray absorption investigation of the cubic Laves phase rare earth compounds CeFe2, CeRu 2 and their related hydrides. These compounds have been investigated by Extended X-ray Absorption Fine Structure (EXAFS) both at cerium and iron sites and by X-ray Absorption Near Edge Spectroscopy (XANES) at different edges of both ruthenium and iron species. The hydrogen absorption process is correlated with the suppression of the cerium fluctuating valence state in the case of CeFe2, while the intermediate valence character is detected in CeRu 2 also upon the absorption process. These results and the change of 3d-5d hybridization determined by structural disorder effects in the hydridate phase of these systems can explain the anomalous magnetic behaviour of the hydridate phases when compared with those of the whole series REFe 2 and RERu 2. It is well established that cubic Laves phase com- pounds like CeFe z and CeRu 2 are able to absorb large amounts of hydrogen in a reversible way [1]. Moreover, in CeFe 2 it has been observed a marked change in the magnetic properties upon H 2 absorption, i.e., the satu- ration magnetization of the hydride corresponds to 2.1 /ZB/Fe, it is about 70% higher than in the original intermetallic compound. The Curie temperature of the hydride (358 K) is also about 60% higher than in CeFe 2 (230 K) [2]. These results appear not correlated with the values observed in other REFe 2 systems where Tc is considerably reduced in the hydridated deriva- tives. In addition the increase of Fe magnetic moment for the YFezH 4 compound was found to be only about 25% higher than in YFe 2 addressing the possibility of a substantial change in the valence state of Ce ions upon H 2 uptake. The case of CeRu 2 is interesting too, due to the presence of a superconducting state below the critical temperature 6.2 K which disapears upon hydriding. This result is still a matter of controversy about the origin of superconductivity in this compound. In fact it was originally assigned to the 4d ruthenium chains because in this system rare earth ions (Tb, Gd, La, Ho) were introduced substitutionally without destroying the superconducting state [3]. However, recent works as- cribe the presence of superconducting state to 4f elec- tron of cerium [4] in agreement with XPS results which measure an f-electron count near one. This agreement is in contrast with the usual assignment of tetravalent state to cerium in this compound which correlates the suppression of the superconducting state after hydrida- tion to the change of Ce 4+ to Ce 3+ [5]. To investigate the effect of hydrogen on the valence state of cerium ions in both systems, we performed X-ray absorption experiments on the L 3 edge of the rare earth in the compounds CeFezH x with x= (0, 2.8, 3.75) and CeRu2H x with x = (0, 2.9, 4). The measurements were carried out at the PULS station at the Frascati Synchrotron Facility in transmission mode using a monochromator equipped with a Si (1 1 1) crystal. The cerium valence was extracted from the normalized spectra via deconvolution process using arctangent functions to describe the transitions into the continuum states and Lorentzian functions for the 5d-states associated to the 4f n+l and 4f" configura- tions [6]. From this analysis we deduce a cerium valence change from v = 3.29 in the case of CeFe z to v = 3 after hydridation, substantially independent of hydro- gen concentration. On the contrary, in the case of CeRu2H x the cerium ions still show mixed valence behaviour and the change of the cerium valence from v = 3.3 in the intermetallic alloy to l' = 3.1 in the case of the hydrides is again independent of hydrogen con- centration. Unambiguous determination of the presence of trivalent state of cerium in CeFezH x makes possible the application of a molecular field model to describe the variation of the Curie temperature along the REFe 2 series. This simple model, neglects the RE-RE inter- action and Tc can be written as: 3kTc=aFeFe + [a2eFe + 4aFeREaREFe]l/2, (1) where axy terms represent the magnetic interaction energy between the x and y spins. These energies can be expressed in terms of the coupling constants Jveve and JREFe by means of the relations aFeFe = ZFeFeJFeFeSFe(SFe + 1), (2) aREFeaFeRE = ZIZeSFe( SFe + 1)JR2EF,(gj -- 1) 2 xJ(J + 1), (3) 0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved