X-ray absorption spectroscopic study of specific valence in CeO2, PrO2 and TbO2

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Journal of Magnensm and Magnenc Materials 63 & 64 (1987) 518-5211 North-Holland, Amsterdam

X - R A Y A B S O R P T I O N S P E C T R O S C O P I C S T U D Y O F S P E C I F I C V A L E N C E IN C e O 2 , PrO2 A N D TbO2 R C K A R N A T A K 1'3, J M E S T E V A 1 ~, H D E X P E R T 12, M G A S G N I E R 2, P E C A R O 2 and L A L B E R T 2 1LURE, Bdtt 209d, Umversu~ Pans-Sud, 91405 Orsay, France 2Laboratotre des Elements de Trans~non dans les Sohdes Equtpe de Recherche 210, CNRS, I Place A Briand, 9219~ Meudon cedex, France ~Laboratolre de Spectroscople Atomutue et lonulue Un~t~ Assoct~e 040775 Bat 350 Unwers~t~ Pans-Sud, 9140~ Orsav France

The Lil ] and M]v-v spectra of fluorite related CeO2, PrO2 and TbO2 were studied In these spectra the presence of spectral contributions from localuzed and extended type of f states is revealed Th~s study yields a reasonable descnpUon of the valence state m these oracles

1. Introduction T h e study of the fluorite related ROx (R = Ce, Pr and T b , 1 5 ~< x <~ 2) systems [1] were found to be interesting from the point of vtew of valence anomalies In the rare earths (RE) These systems, with a choice of three elements and the same crystal structure offer statable examples for a c o m p a r a t i v e study of 4f level hybridization in Ce based and the fluctuating valence Sin, Eu and T m materials W e recently studied the ROx systems In detail by M~v-v and L m absorption spectroscopy (XAS) [2-4] T h e absorption spectra of the stable intermediate oxides show features which are clearly Identified as contrlbutions from m v a l e n t and tetravalent sites As the oxygen concentration x reaches 2, as in CeO2, PrO2 and TbO2, the spectral features show a specific behaviour which is quite different from that of pure trwalent c o m p o u n d s In the present communlcaUon we will briefly describe the X - r a y absorpuon study of these oxides and c o m p a r e them with those of other R E materials presenting valence problems

charaeterlzed by two types of distinct features T h e intense peaks m fig 1 show broad multIplet structures T h e M~v-v multlplets of CeO2, PrO2 and TbO2 are found to be slmdar to those observed m corresponding La, Ce and Gd metals [5] and trivalent compounds, thus indtcatmg a reduction m the local 4f count So the

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2. Results and discussion T h e selected examples of the M y edge features obtained from CeO2, CeNI2, PrO2 and TbO2 are presented m fig 1 These spectra are

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R C Kamatak etal / Specific valence m CeO2, PrOe and

main lines in the RO2 spectra reveal the presence of the transmons from locahzed ground state T h e signatures of these multlplets in ROE correspond to t °, fi and f7 ground configuration respectively in CeO2, PrO2 and TbO2 In CeNI2 such identification yields mainly fl ground state In addition to the main multlplet lines, one additional weaker feature Y (fig 1) is systematically observed at about 5 eV a b o v e each Mtv and M v p e a k in RO2, CeNi2 and other a C e materials [6,7] T h e vertical arrows in fig 1 indicate the hmlts upto which the distinguishable features of the 3 d - 4 f multiplets are observed In CeOz, CeN12 and PrO2 it is found to be well separated from the main lines whereas in TbO2 it lies within the wide 3 d - 4 f multlplet and this separation is reduced to about 3 eV T h e Y type structures are observed to have similar forms in CeO2, CeNl2 and PrO2 and these structures are not Identified with any known R E multlplets Their separation ( = 5 eV) from the main lines is too high as c o m p a r e d to the observed separation (1 7 e V ) between the tetravalent and trivalent 3d94f n+l final atomic multlplets These observations suggest that the Y structures are due to transitions to f symmetry extended type of f states T h e most interesting fact about the ROE is the observation of unusually broad multiplets These multlplets are found to be 4 0 - 5 0 % wider than the similar multiplets observed in the corresponding Z - 1 elements [8] and can not be explained by invoking the multiplet expansion due to increase in the 3d spin-orbit separation for higher Z It is worthy to mention here that the CeN12 multlplet width (2 2 eV) is found to be similar to those observed in the Ce intermetalhcs and other trivalent Ce c o m p o u n d s However, some minor variations in the widths in the Ce intermetalhcs due to change in relative intensity of the 3d94f 2 multiplet lines due to population of the spin-orbit f7/2 level with 4f level hybridization [9] is observed These widths (2 2 eV) are greater than that observed (1 7 eV) for the single Mv main line in CeO2 T h e important informations which we obtain from the 3 d - 4 f transition in RO2 and the Ce materials are (1) Co-existence of the localized

TbO 2

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and the extended character of the last 4f electron in these materials T h e 3d X A S are able to distinguish them by using the multiplet identification (2) T h e main distinction between the Ce lntermetalhcs, the RO2 and the fluctuating valence Sm, Eu and T m materials lies in the identity of the main line multlplets and their different widths (1) T h e 3 d - 4 f multiplet in a C e lntermetalhcs clearly demonstrates the presence of nearly one f electron in localized level T h e 4f level hybridization with the valence band leads to a fractional delocahzation of this electron T h e vacant states thus created are found to be of extended nature as evidenced by the observation of the transitions giving rise to the Y type of structures Due to their specific nature these states are also responsible for certain spectral features in other core level and BIS spectra of these materials In the context of Ce materials these states should not be confused with the to type of the localized states and the 4f o c c u p a n c y (nf) in each type of states can be precisely distinguished (n) T h e delocahzation mechanism of the last f electron in ROa IS quite different [10] from that of Ce intermetalhcs In this case the vacant states thus created show localized f states corresponding to those for trivalent compounds of Z - 1 elements and the extended states similar to those in Ce intermetalhcs T h e unusual broadening of the final multlplets is thought to be due to the interaction of the excited f electron with the finite continuum of the f symmetry states [10] (ni) Atomic 3 d - 4 f multiplets due to two distinct valence states are observed in fluctuating valence Sm, Eu and T m materials T h e description of the ground state of RO2 which we obtain from the M~wv spectra is further supported by the LIII edge measurements [11] T h e single white line at the L m edge of the trivalent R E c o m p o u n d s is due to a resonance m the 2p-Ed cross section modified by the local density of the unoccupied states In CeO2 and PrO2 two absorption peaks separated by 7 - 8 eV with more or less c o m p a r a b l e intensity are observed In TbO2 the low energy p e a k appears

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R C Karnatak et al / Spect]ic valence m CeO~_, PrO2 and TbO~

as a prominent shoulder Besides these mare peaks, a weaker low energy shoulder is systematically observed in RO2 This is found to be coincident with the position of the main peak of the corresponding trlvalent c o m p o u n d Below this shoulder an additional preabsorption type of weak feature ~s also observed in RO2 T h e two main peaks in CeO2 and PrO2 and the high energy peak and the p r o m m e n t shoulder in TbO2 are interpreted as due to mainly ground state configuratton T h e highest energy peak m these oxides is assigned to fn (n = 0, 1 and 7 respectwely for Ce, Pr and Tb) localized ground state T h e lower energy peak and the prominent shoulder tn T b ts considered to be due to the screening of the 2p hole by the 4f admixture in the valence band T h e weaker shoulder is interpreted as shake down feature m these oxides T h e weak preabsorption type of structures are of some interest due to their large separation (18 eV) from the f" mare lines Their interpretation needs further considerations related to the properties of the extended type of f symmetry states An estimate of the 4f occupancy (n0 related to the population of the 4f electrons m the extended states can be obtained from the L m spectra of RO2 In fact, the area under each peak ts proportional to the 2p--ed cross sections relevant to the occupted and unoccupied f states Thus in a crude estimation, the area under the low energy mare peak and the total area under the LIH structures yields the n4 values of about 0 5, 0 6 and 0 3, respectively for CeO2, PrO2 and TbO2 T h e s e values for CeO2 and PrO2 are m a g r e e m e n t with the values of 4f occupancy obtained by band structure calculations [12]

3. Conclusion T h e X - r a y absorption spectra are able to provide useful reformation on the valence state

of the last 4f electron i n R O 2 and gwe further insight mto the valence problem m Ce materials In the M w - v spectra the localized and the extended f symmetry states can be dtstmgmshed by 3d-4f multiplet ~dentification These spectra clearly demonstrate an appreciable 4 f - 2 p oxygen hybridization in a heavy R E oxide hke TbO2 T h e 4f occupancy m the valence band can also be obtained from these spectra However, the present study m&cates that the estimated values of n~ should not be taken as a measure of fractional valence in these oxides

References [1] L Eyrmg, m Non-Stolchlometnc oxides, ed o q Sorensen (Academic Press, New York, 1981) p 337 [2] R C Karnatak, M Gasgmer, H Dexpert, J M Esteva, P E Caro and L Albert, J Less-Common Metals 110 (1985) 377 [3] M Gasgmer, L Eyrmg, R C Karnatak, H Dexpert, J M Esteva, P E Caro and L Albert, 17th Rare Earth Research Conference, Hamilton, Canada (June 1986) [4] J M Esteva, R C Karnatak, H Dexpert, M Gasgmer, P E Caro and L Albert, IV Intern Conf on EXAFS and Near Edge Structures, Fontevraud, France (July 1986) [5] B T Thole, G van der Laan, J C Fuggle G A Sawatzky, R C Karnatak and J -M Esteva, Phys Rev B32 (1985) 5107 [6] J C Fuggle, F U Hlllebrecht, J - M Esteva, R C Karnatak, O Gunnarsson and K Schonhammer, Phys Rev B 27 (1983) 4637 [7] G Kamdl, G Kalkowskl, W D Brewer, B Perscheld and F Holtzberg, J Appl Phys 55 (1984)1910 G Kamdl, G Kalkowskl, W D Brewer, E V Sampathkumaran, F Holtzberg and A Schavch v Wmenau, J Magn Magn Mat 47&48 (1985)181 [8] R C Karnatak, J - M Esteva and J P Connerade, J Phys B 14 (1981) 4747 [9] G van der Laan, B T Thole, G A Sawatzky, J C Fuggle, R C Karnatak, J -M Esteva and B Lengeler, J Phys C 19 (1986) 817 [10] R C Karnatak et al (unpubhshed) [11] H Dexpert et al (unpubhshed) [12] D D Koelhng, A M Boring and J H Wood, Sohd State Commun 47 (1983) 227