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X-ray absorption spectroscopic study of a mixed valence system, EuPd2Si2
9 February 1981
PHYSICS LETTERS
Volume 81 A, number 7
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
Company
397
Volume 8 1A, number 7
PHYSICS LETTERS
/
6960
70
/
\cEuPd&
90
80 ENERGY
7oc
(eV)
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 398
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.
PHYSICS LETTERS
Volume 81 A, number 7
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
Company
397
Volume 8 1A, number 7
PHYSICS LETTERS
/
6960
70
/
\cEuPd&
90
80 ENERGY
7oc
(eV)
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 398
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.