X-RAY ABSORPTION SPECTROSCOPIC STUDY OF A MIXED VALENCE SYSTEM, EuPd#i,
R NAGARAJAN, E.V. SAMPATHKUMARAN,
Tata Institute of Fundamental
L.C. GUPTA and R. VIJAYARAGHAVAN
Research, Bombay 400005,
India
and
BHAKTDARSHAN
Department
and B.D. PADALIA
of Physics, Indian Institute of Technology,
Powai, Bombay 400076,
India
Received 27 November 1980
La X-ray absorption spectroscopic study of Eu firmly establishes that the compound, EuPdzSiz, is a mixed valence system. It is found that the relative intensities of the absorption peaks corresponding to divalent and trivalent europium ions in
EuPdz Siz are a strong function of temperature.
The intermetallic compound EuPd,Si, crystallizes
in the ThCr2Si2-type tetragonal structure. Recently it
has been reported [l] that EuPd,Si, exhibits a lattice
volume anomaly. Moreover, this system has been
found to have some unusual properties which have not
been observed so far in any of the europium systems
[2] . These are: (i) The isomer shift (IS) of 151 Eu varies
from z-1 mm/s (at 77 K) to m-7.5 mm/s (at 300 K).
Not only is the overall variation of the IS with temperature largest in this material in comparison with that of
any other europium compound, the rate of variation of
the IS with temperature is also the highest in this system.
(ii) There is a distinct maximum of the magnetic susceptibility at about 200 K. These results have been interpreted [2] in terms of the valence fluctuation (or
mixed valence) phenomenon
[3]. This implies that Eu
ions in EuPd,Si, undergo fast fluctuations (=1013/s)
between di- and tri-valence states. EuPd2Si2 has, therefore, been termed as a unique mixed valence system of
europium [2].
X-ray absorption spectroscopy (XAS) is becoming
an important tool in the understanding of valence fluctuation phenomenon
[4-61. This technique has a
probing time of the same order as that of X-ray photoelectron spectroscopy (= lo-l6 s). The results obtained from XAS are free from the surface condition of the
sample and interpretation
of the results is simple and
0 03 l-9 163/8 1/OOOO-0000/$02.50
0 North-Holland
straightforward. In this technique, one measures the
position of the L3 absorption peak of the rare earth
(corresponding to an electronic transition from the 2p
core level to the first unoccupied level at the top of the
conduction band) and this position normally differs by
x7 eV for the two different valence states of a rare
earth ion [6] . This method has been employed to
very few mixed valence Eu systems like EuCu,Si,
[4].
Jn this letter, we report such measurements on
EuPd,Si, at 300 K and at 120 K. Mijssbauer and magnetic susceptibility measurements on EuPd2Ge2 and
EuRu, Si,, which also have the same crystal structure
as EuPd,Si,, have revealed that Eu ions are in the diand tri-valence states, respectively, in these systems [2] .
EuPd2Ge2 and EuRu2Si2 have, therefore, been chosen
as references for the comparison of absorption profiles.
The samples were prepared by arc melting followed
by vacuum annealing at 800°C for 1 week. These compounds, thus prepared, have been characterized by
X-ray diffraction. A curved mica crystal spectrograph
and a MO target X-ray tube were used in the present
investigation. Low temperature study was made by
mounting the sample on a copper ring attached to the
copper cold finger of a liquid nitrogen cryostat. The
temperature was measured at the sample using a copper resistance thermometer. The details regarding the
experimental set up and procedure of recording the specPublishing
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Fii. 1. Ls absorption spectra of Eu in EuPds Sis (300 K and
120 K), EuPdsGes (300 K) and EuRuaSis (300 K). The intensities have been normalised with respect to the peak positions
of the curves.
tra are discussed elsewhere [4]. The error estimated
in the measurement of energies was found not to exceed 0.5 eV.
Fig. 1 shows the L, absorption curves for Eu in
EuPd2Si2, taken at room temperature (300 K) and
120 K. The curves for Eu2+ in EuPd2Ge2 and Eu3+
in EuRu2Si2, chosen as references, are included
in the same figure for the purpose of comparison. It is
apparent from the figure that a single absorption peak
appears for EuPd2Ge2 as well as for EuRu,Si
and the
peak positions corresponding to Eu2+ and Eu 5+ differ
by -7 eV. This difference in energy can be explained
on the basis of the fact that the energy for the transfer
of an electron from the 2p core level to the first unoccupied state at the Fermi level depends on the intervening 4f electrons [7]. In EuPd,Si,, we observe two
peaks at 300 K as well as at 120 K, which suggest that
9 February 1981
Eu ions are present in both valence states in this system. The observed spectra of EuPd,Si, were deconvoluted by assuming the line shapes and positions of
the 2+ and 3+states of Eu ions in EuPd,Si, to be the
same as those of Eu2+ in EuPd2Ge2 and Eu3+ in
EuRu,Si,, respectively. A similar procedure has been
adopted in analysing other mixed valence systems
like EuCu2Si2 and TmSe [4,7]. This analysis suggests
that the relative population, Eu2+/Eu3+, of the two
valence states is a strong function of temperature and
the values of Eu2+/Eu3+ at 300 K and 120 K are
-2.33 and eO.43, respectively. The observed trend in
the case of EuPd,Si, is similar to that noted for EuCu,Si,
[4]. Stated otherwise, the population of Eu3+ ions in
EuPd, Si, increases at the expense of Eu2+ ions as one
decreases the temperature. The values of the average
valency of Eu ions in EuPd2Si2 corresponding to the
above relative populations are -2.3 at 300 K and
x2.7 at 120 K. These values are in close agreement
with those obtained from Mossbauer and susceptibility
data [2] .
Finally, it may be stated that the present study establishes the presence of mixed valence in EuPd2Si2
and confirms that the average valency of Eu ions is
strongly dependent on temperature in this system. Further this study emphasizes the importance of the X-ray
absorption spectroscopic technique in understanding
the phenomenon of valence fluctuation in rare earth
systems.
Two of the authors (Bhaktdarshan and B.D. Padalia)
wish to express their thanks to the Council of Scientific
and Industrial Research, New Delhi, for financial assistance in the form of a project.
References
[I] D. Rossi, R. Marazza and R. Ferro, J. Less. Common Met.
66 (1979) 17.
[2] E.V. Sampathkumaran, L.C. Gupta, R. Vijayaraghavan,
K.V. Gopalakrishnan, R.G. Pi&y and H.G. Devare, to be
published.
[3] J.M. Robinson, Phys. Rep. 51 (1979) 1, and references
therein.
[4] T.K. Hatwar et al., Solid State Commun. 34 (1980) 617.
[S] C.N.R. Rao et al., Chem. Phys. Lett. 76 (1980) 413, and
references therein.
[6] E.E. Vainshtein, S.M. Blokhin and Yu.B. Padevno, Sov. Phys.
Solid State 6 (1965) 2318.
[7] H. Launois, M. Rawiso, E. Holland-Horitz, R. Pott and
D. Wohlleben, Phys. Rev. Lett. 44 (1980) 1271.