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VALENCE DETERMINATION IN EuM2X2
COMPOUNDS : LIII-EDGE VERSUS MÖSSBAUER ISOMER SHIFT
G. Wortmann, B. Perscheid, W. Krone
To cite this version:
G. Wortmann, B. Perscheid, W. Krone. VALENCE DETERMINATION IN EuM2X2 COMPOUNDS : LIII-EDGE VERSUS MÖSSBAUER ISOMER SHIFT. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-979-C8-982. �10.1051/jphyscol:19868188�. �jpa-00226094�
JOURNAL D E PHYSIQUE
Colloque C8, supplbment au n o 12, Tome 47, dbcembre 1986
VALENCE DETERMINATION IN EuM2X2 COMPOUNDS : Lm-EDGE VERSUS MOSSBAUER ISOMER SHIFT
G. WORTMANN, B. PERSCHEID and W. KRONE
Fachbereich Physik, Freie Universitst Berlin, 0-1000 ~ e r l i n 33, F.R.G.
ABSTRACT. - A comparative study by Eu-LIII X-ray absorption and 1 5 1 ~ u - ~ 6 s s b a u e r spectroscopy is presented for the E U P ~ ~ - ~ A U , S ~ ~ series. Possible ways to distinguish between mixed-valent behaviour and final-state effects i n the LIII-edge spectra are discussed.
I. INTRODUCTION
LIII-edge X-ray absorption (XA) spectroscopy is now widely used for valence studies in rare-earth (RE) systems with valence instabilities / l / , although this method is a deep-core or high-energy spectroscopy, which leaves the RE ions i n an excited atomic state. In applying the LIII-XA method, it is generally assumed. that the double peaked LIII-edge spectra contain the undisturbed information on the 4f-electron occupancy i n the ground state. I n this way, the term v(LIII) = 2 + 1(4fn)/(1(4fn) + 1(4fn+')) is defined as the valence v(LIII) measured by LIII-XA, where I(4fn) and 1(4fn+') are the intensities of the final states with 2p54fn5d* and 2p54fn+15d*
configurations, respectively. This implies (i) that both final states have equal transition probabi- lities and (ii) that no change in the 4f configuration occurs in the absorption process despite of the creation of a hole in the 2p shell. Such final-state effects, however, are well known in other deep-core spectroscopies, for instance 3d-XPS /2/, and have been shown recently to occur also in LIII-XA spectroscopy /3,4/. In Ce systems hybridization effects of the 4f electrons were found to be relatively pronounced /3/. But also in systems with heavier R E metals such as EuPd2P2, where Eu is divalent as proved by a variety of methods, the Eu-LIII edge spectra exhibit an additional trivalent component. This was attributed to the covalent character of the RE-ligand bonding giving rise to hybridization-induced final-state effects /4/. In order to prove the reliability of the LIII-XA method we performed LIII-XA and Mossbauer isomer shift (IS) studies of ternary systems with the tetragonal ThCr2Si2 structure /5,6/. In this contribution, we report on such a comparative study on the quasi-ternary system EuPd2-,Au,Si2. This series of compounds is well known because of the temperature-induced valence transition in EuPd2Si2 and the loss of of mixed-valent behaviour with increasing X, accompanied by a spectacular occurence of magnetism above X = 0.18 /5,7,8,9/.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868188
CS-980 JOURNAL D E P H Y S I Q U E
11. EXPERIMENTAL
The samples with X = 0 , 0.5, 0.63, 1.0, 1.5, and 2.0 as well as EuAu2Ge2 were prepared by argon-arc melting. Their phase purity was checked by X-ray diffraction. Small parts o f the samples were finely ground in purified argon atmosphere and mixed with outgassed paraffin in order to obtain homogeneous absorbers. Identical absorbers were used for Mossbauer and X A measurements. The 1 5 1 ~ u - ~ b s s b a u e r studies, employing a 1 5 1 ~ m ~ 3 source, were performed at various temperatures, mostly at 300K, 77K and 4.2K. The L I I I - X A measurements were done at the EXAFS-I1 beam line at HASYLAB/DESY. The Si(ll1) double-crystal monochromator provided a resolution o f ~ 1 . 8 eV at 7 keV. In addition to the standard X A measurements performed on powdered samples in transmission geometry, all samples were investigated in bulk form (as ingots) by taking X A spectra with the fluorescence technique by using a Ge diode for the detection o f the Eu-L X-rays. This latter method is particularly useful i n the present case with basically Eu2+ systems, where the use o f finely powdered samples is problematic because o f possible oxide formation.
111. RESULTS AND DISCUSSION
E u - L T I I X A fluorescence spectra o f the series and o f EuAu2Ge2 are shown in Fig. 1 . These spectra are governed by dominant white lines at energies typical for E U ~ + ions.
Additional structures typical for E U ~ + ions are observed in the EuPd2-,Au,Si2 series with their relative amounts decreasing with increasing X . The spectra were least-squares fitted by a superposition o f two L I I I - X A profiles separated by an energy AE o f about 8 eV. Each profile contains the sum o f an arctan function (representing the edge) plus a Lorentzian (representing the white line), both convoluted by a Gaussian for instrumental resolution. This fitting routine assumes equal ratios o f white-line intensity to edge height for the E U ~ + and E U ~ + subspectra. The results for v ( L I I I ) and AE derived from the L I I I - X A spectra are presented in Fig. 2 and Fig. 3.
All 1 5 ' ~ u Mossbauer spectra o f the powdered samples (not shown here) exhibit a prominent line with isomer shifts S (at room temperature) between -7.9mm/s (EuPd2Si2) and -11.4mm/s (EuAu2Ge2). In addition, small E U ~ + components were detected ranging from 0.5 to 4 % o f the spectral intensity. These impurities may arise from Eu3+ oxide contaminations or from Eu sites in the regular compound with distorted n.n. shell coordinations /5-10/. Details o f the spectral analysis and the correction o f the v(LII1) values for the E U ~ + impurity components in the powdered samples may be found in R e f . 6. It should be mentioned that the present IS data agree well with those published in a detailed Mossbauer study o f the E U P ~ ~ - ~ A U ~ S ~ ~ series /g/.
Fig. 2 shows in the inset for the EuPd2-,AuXSi2 series the dependence o f v ( L I I I ) and S on the Au concentration X . The correlation between v ( L I I I ) and S is given together with data from a previous investigation o f the temperature dependence o f these parameters in EuPd2Si2 / 5 / . In all correlations shown i n Fig. 2, the slope changes at about X = 0.5. It is important to note that the relative variation o f v(LIII) with respect to S is steeper in the concentration range 0 S X d 0.5 and smaller in the range 0.5 < X S 2.0. This can also be seen in the v ( L I I I ) versus S correlation. The non-linear behaviour indicates that the underlying mechanisms act differently on v ( L I I I ) and S. Following the line o f recent arguments /4,8,9/, this behaviour can be explained by the. mixed-valent character o f the alloy series ending smoothly around around X = 0.5 and
by hybridization effects for higher Au concentrations. (Hybridization effects are also
10 reflected by the pronounced variation of the
- - - magnetic ordering temperatures as well as by the transferred magnetic hyperfine fields /8,9/). Further support to this argumentation comes from the present LIII-XA data, for which in Fig. 3 the energy separation AE is plotted as function of the derived v(LIII) values. The linear variation of AE versus v(LIII), which is observed in the temperature-induced valence transition in EuPd2Si2 / 5 / and other mixed-valent Eu systems / l / , changes dramatically above X =
0.5 for the EuPd2-,AuXSi2 series. This is in line with AE = 8.0(1) eV observed in the LIII-XA spectra of EuPd2P2 /4,6/. The underlying shielding mechanism causing the change in AE will be discussed elsewhere. The absence of a trivalent component in the LIII-spectrum of EuAu2Ge2 further corroborates these arguments. Ge neighbours are less covalent than Si neighbours and favour the divalent state
05 of Eu in EuM2X2 samples /6,10/.
00- . - - - - . - - - - - - The present LIII-XA data of the 69L 696 698 700 EuPd2-,AuXSi2 series are in gross disagree-
ENERGY IkeVl
ment with LIII-XA measurements reported in Fig. Eu LIII-edge fluorescence Ref. 8, where v(LIII) values around 2.3 were spectra of EuM2X2 compounds. found for the whole series. On the other hand, the value v(LIII) = 2.18 reported in Ref. 7 for X = 0.5 is in good agreement with our data. It should be stressed in this context that the determination of valencies near to the integral numbers 2+ or 3+ needs careful measurements with the LIII-XA method, since all perturbing effects, arising for instance from imperfect absorbers (thickness, holes, impurities) or from insufficient spectral resolution lead always to a reduction in the height of the dominant white-line, which can be easily misinterpreted as a mixed-valent behaviour.
From Fig. 2 one can conclude that for the case of X = 0.5 both mixed-valent behaviour and hybridization effects contribute to the observed E U ~ + component in the LIII-XA spectrum, with their relative amounts depending on the local environment of the Eu ions (number of Pd neighbours). Therefore, one can state that final-state effects vary between v(LIII) = 0.12 and 0.09 for X = 0.5 and 2.0, respectively, in the EuPd2-,AuxSi2 series. This finding gives an uncertainty of =0.1 in the determination of absolute valencies in these compounds, when the LIII-XA method
C8-982 JOURNAL DE PHYSIQUE
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m and correspond to measurements performed in transmission and absorption, respectively. o isomer shifts are from Ref. 8.
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ACKNOWLEDGEMENTS
This work was supported by the Bundesminister fiir Forschung und Technologie, Contracts 05 256 KA and 05 313 AXB, and by the Deutsche Forschungsgemeinschaft, SfB-161, TP D3. The authors thank Prof. G. Kaindl for stimulating discussions and support.
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