Correlation between mixed valence behaviour of cerium and the magnetic and superconducting phenomena of CeFe2 and CeRu2

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 C e F e 2 and C e R u 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 p h a s e comp o u n d s like C e F e z a n d C e R u 2 are able to absorb large a m o u n t s of hydrogen in a reversible way [1]. Moreover, in C e F e 2 it has b e e n observed a m a r k e d c h a n g e in the m a g n e t i c p r o p e r t i e s u p o n H 2 absorption, i.e., the saturation m a g n e t i z a t i o n of the hydride c o r r e s p o n d s to 2.1 /ZB/Fe, it is a b o u t 70% higher than in the original intermetallic c o m p o u n d . T h e Curie t e m p e r a t u r e of the hydride (358 K) is also a b o u t 60% higher t h a n in C e F e 2 (230 K) [2]. T h e s e results a p p e a r not correlated with the values observed in o t h e r R E F e 2 systems w h e r e Tc is considerably r e d u c e d in the h y d r i d a t e d derivatives. In addition the increase of Fe m a g n e t i c m o m e n t for the Y F e z H 4 c o m p o u n d was f o u n d to be only a b o u t 25% higher t h a n in Y F e 2 addressing the possibility of a substantial c h a n g e in the valence state of Ce ions upon H 2 uptake. T h e case of C e R u 2 is interesting too, due to the p r e s e n c e of a s u p e r c o n d u c t i n g state below the critical t e m p e r a t u r e 6.2 K which disapears upon hydriding. This result is still a m a t t e r of controversy a b o u t the origin of superconductivity in this c o m p o u n d . In fact it was originally assigned to the 4d r u t h e n i u m chains b e c a u s e in this system rare e a r t h ions (Tb, Gd, La, Ho) were i n t r o d u c e d substitutionally without destroying the s u p e r c o n d u c t i n g state [3]. However, r e c e n t works ascribe the p r e s e n c e of s u p e r c o n d u c t i n g state to 4f electron of cerium [4] in a g r e e m e n t with X P S results which m e a s u r e an f-electron c o u n t n e a r one. This a g r e e m e n t is in c o n t r a s t with the usual assignment of t e t r a v a l e n t state to cerium in this c o m p o u n d which correlates the suppression of the s u p e r c o n d u c t i n g state after hydridation to the c h a n g e of Ce 4+ to Ce 3+ [5]. T o investigate the effect of hydrogen on the valence state of cerium ions in b o t h systems, we p e r f o r m e d X-ray a b s o r p t i o n e x p e r i m e n t s on the L 3 edge of the rare e a r t h in the c o m p o u n d s C e F e z H x with x =

(0, 2.8, 3.75) and C e R u 2 H x with x = (0, 2.9, 4). T h e m e a s u r e m e n t s were carried out at the P U L S station at the Frascati S y n c h r o t r o n Facility in transmission m o d e using a m o n o c h r o m a t o r e q u i p p e d with a Si (1 1 1) crystal. T h e cerium valence was extracted from the normalized spectra via deconvolution process using a r c t a n g e n t functions to describe the transitions into the c o n t i n u u m states a n d L o r e n t z i a n functions for the 5d-states associated to the 4f n+l and 4f" configurations [6]. F r o m this analysis we d e d u c e a cerium valence c h a n g e from v = 3.29 in the case of C e F e z to v = 3 after hydridation, substantially i n d e p e n d e n t of hydrogen c o n c e n t r a t i o n . O n the contrary, in the case of C e R u 2 H x the cerium ions still show mixed valence b e h a v i o u r a n d the c h a n g e of the cerium valence from v = 3.3 in the intermetallic alloy to l' = 3.1 in the case of the hydrides is again i n d e p e n d e n t of hydrogen concentration. U n a m b i g u o u s d e t e r m i n a t i o n of the p r e s e n c e of trivalent state of cerium in C e F e z H x m a k e s possible the application of a molecular field model to describe the variation of the Curie t e m p e r a t u r e along the R E F e 2 series. This simple model, neglects the R E - R E interaction a n d Tc can b e written as:

3kTc=aFeFe + [a2eFe + 4aFeREaREFe]l/2,

(1)

where a x y terms r e p r e s e n t the m a g n e t i c interaction energy b e t w e e n the x a n d y spins. T h e s e energies can be expressed in t e r m s of the coupling constants Jveve a n d JREFe by m e a n s of the relations aFeFe = ZFeFeJFeFeSFe(SFe + 1),

(2)

aREFeaFeRE = ZIZeSFe( SFe + 1)JR2EF,(gj -- 1) 2

0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

x J ( J + 1),

(3)

662

J. Chaboy et aL / Mixed valence of Ce in CeFe 2 and CeRu 2

Table 1 Magnetic ordering temperatures for REFe2 series and their hidridate derivatives (from ref. [2]). The Curie temperatures calculated using a molecular field model, are shown in column 4 Compound

7[. (K) REFe2H x

Tc (K) REFe 2

TL. (K)* REFe 2

YFe 2 CeFe 2 SmFe 2 GdFe 2 TbFe 2 HoFe 2 ErFe 2

308 358 333 388 303 298 265

545 230 676 785 711 612 590

545 544 627 785 718 627 592

where Zvevc is the average n u m b e r of similar neighhour atoms a r o u n d an Fe atom, a n d Z j and Z 2 are the n u m b e r of Fe n e i g h b o r s a r o u n d an R E atom a n d the n u m b e r of R E n e i g h b o r s a r o u n d an Fe atom, respectively. T h e value of the F e - F e coupling c o n s t a n t was obtained by the data of the Y F % c o m p o u n d , w h e r e the Y ion is not magnetic. T h e n , this value was used to estimate the R E - F e coupling c o n s t a n t in G d F e 2 which was used t h a n to calculate the Curie t e m p e r a t u r e for the whole R E F e 2 series. E s t i m a t e d values for Tc are r e p o r t e d in table 1 showing a satisfactory a g r e e m e n t with experimental data with the exception of the cerium based c o m p o u n d . However, the most i m p o r t a n t result concerns this system and explain the " a n o m a l y " of the e x p e r i m e n t a l Tc value. In fact C e F e 2 is not a trivalent system as the o t h e r iron b a s e d compounds. T h e existence of a hypothetical C e F e 2 system with trivalent cerium would lead to a reduction of Tc as in the rest of the R F e 2 hydrides indicating that the a n o m a l o u s behaviour can be associated in the intermetallic comp o u n d of C e F e 2 to the cerium mixed valence state, and that t h e r e is no anomaly in the hydride b e h a v i o u r w h e r e cerium is in a trivalent configuration. O n the o t h e r h a n d we u n d e r l i n e the a g r e e m e n t o b t a i n e d by X A S analysis a n d m a g n e t i c b e h a v i o u r in the framework of a simple m e a n field model. This is certainly a nice example of the capability of the X A S

t e c h n i q u e to extract information not only about 4t" occupation n u m b e r s but also in the dynamic of the electron states. Finally we discuss the case of C e R u 2. T h e p r e s e n c e of the cerium mixed valence b e h a v i o u r also upon H absorption help us to address the 4d r u t h e n i u m chains as the origin of the superconductivity. In fact there is no reason to keep on assigning the loss of the superconductivity to the change of the cerium valence after the H 2 u p t a k e because it is certainly a negligible p h e n o m e n o n . Careful analysis of the local o r d e r in this system shows that R u - R u hybridization is certainly more involved in the s u p e r c o n d u c t i n g phase. In fact, analysis of E X A F S spectra on the L 3 edge of the rare e a r t h in the case of L a R u z H x and C e R u 2 H ~ [7] clearly show an increase of the disorder of the two systems after hydridation with a strong reduction of the n u m b e r of n e i g h b o r s in the first coordination s p h e r e and the increase of the R u - R u i n t e r a t o m i c distance. As a consequence, it seems more reasonable to correlate the loss of superconductivity in these two hydridate systems to loss of c o h e r e n c e b e t w e e n Ru chains. This i n t e r p r e t a t i o n is s u p p o r t e d also by the decrease of the critical t e m p e r a t u r e from 6.2 K in the case of C e R u 2 to 4.4 K in L a R u 2 w h e r e the R u - R u interatomic distance are respectively 3.12 and 3.19 A. This work was s u p p o r t e d by the a g r e e m e n t I N F N C I C Y T and by spanish C I C Y T projects MAT88-03027 and MAT90-0734. W e would like to t h a n k D. F r u c h a r t and S. Miraglia who provided us with the samples. References [1] K.H.J. Buschow and A.M. van Diepen, Solid State Commun. 19 (1976) 79. [2] K.H.J. Buschow, Physica B 86-88 11977) 79. [3] M. Wilhelm and B. Hillebrand, J. Phys. Chem. Solids 31 11970) 559. [4] A. Yanase, J. Phys. F 16 11986) 15/)1. [5] G.X. Tessema, J. Peyrad, A. Nemoz, J.P. Senateur, A. Roualt and R. Fruchart, J. de Phys. Lett. 40 (1979) L-105. [6] J. R6hler, J. Magn. Magn. Mater. 47 & 48 (1985) 175. [7] J. Chaboy, Thesis, Universidad de Zaragoza 11991).