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Valence state of Eu in EuPd2P2
Journal of Magnetism and Magnetic North-Holland, Amsterdam
VALENCE
Materials
STATE OF Eu IN EuPd 2P2
E.V. SAMPATHKUMARAN Institut
407
47&48 (1985) 407-409
ftir Atom
und
Festkijrperphysik
*, B. PERSCHEID, Freie
Uniuersitiit
Berlin,
W. KRONE D
1000
Berlin
and G. KAINDL 33,
Germany
EuPd,P, has been classified as mixed-valent from its lattice volume anomaly, while our Mossbauer isomer-shifts and state. The Eu-L,,,-edge X-ray absorption spectrum, however, exhibits a magnetic measurements suggest a divalent-4f’ double-peaked structure typical for mixed-valent materials. Various reasons for this apparent discrepancy are discussed including the possible existence of a partly extended 4f radius.
Recently,
the isostructural
REPd,P,
series
of com-
(RE = Rare Earth; ThCr,Si 2 structure) was first synthesized by Jeitschko and Hofmann (J&H) (11. They noticed that the tetragonal lattice parameters a and c as well as the unit-cell volume V of EuPd, P_,exhibit deviations from the systematic trend observed for the trivalent members of this series. The deviation AV of the unit-cell volume of EuPd,P,, however, amounts to only about 5 A3, much less than the typical AV values known for divalent Eu compounds of other isotypic series of ternary RE compounds ( AJ' > 9 A3) [l-3]. This anomalously small AV prompted J&H to suggest Eu is in a mixed-valent state in EuPd,P,. In this paper, we present the results of ‘5’Eu-MGssbauer effect as well as preliminary results of Eu-L,,,-edge X-ray Absorption (XA) measurements of this compound aiming at a better understanding of the Eu-vafence state [4]. The Mossbauer isomer shift data as well as the magnetic properties [4] suggest a divalent-4f’ groundstate for all Eu ions, contrary to the expectations based on the lattice-volume anomaly [l]. While the latter might be explained by strong chemical bonding between the Eu ions and the ligands, a further complication results from our L,,,-edge XA spectra, which display a double “white-line” structure typical for mixed-valent materials. We discuss possible reasons for this apparent discrepancy, which could be indicative for partly extended 4f radius in this compound. The studied EuPdzP, sample came from the same batch as the one used in our previous investigation [4]. For experimental details corcerning sample preparation and Mossbauer spectroscopy we refer to ref. (41. The pounds
* On leave from Bombay-40005,
Tata India.
Institute
of Fundamental
Research,
0304-8853/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Eu-L,,,-edge XA spectra were obtained with a conventional laboratory XA spectrometer [5]. Mbssbauer spectra obtained at 300 and 4.2 K. respectively, are shown in fig. 1. The isomer shift of the resonance pattern, S = ( - 9.73 f 0.05) mm/s (relative to a ‘S’SmF,-source), is found to be independent of temperature within experimental accuracy. No trivalent signal, with S around zero Doppler velocity, is detected in the Mossbauer spectra, which clearly reflect the homogeneous valence state of the Eu ions. At liquid-He temperature, a well-resolved magnetic hyperfine pattern, resulting from a magnetic hyperfine field of (355 f 5) kOe and a vanishingly small electric-quadrupole coupling, is observed. It is presumably due to magnetic ordering of the Eu sublattice, in agreement with previously reported magnetic measurements [4]. All these
I
I
,
1
-60
-LO
-20 VELOCITY
0
20
LO
lmmk)
Fig. 1. Mossbauer spectra of EuPd,P, at 300 and 4.2 K, respectively. The solid lines represent the results of least-squares fits of Lorentzian line shapes to the data.
E. V. Sampathkumaran
408
er al. /
observations, as well as the measured effective magnetic moment in the paramagnetic state, pLerr= (8.0 + 0.2)~~ [4], suggest a divalent state of the Eu ion [6]. This situation is contrasted by the L,,,-edge XA spectrum displayed in fig. 2. Besides the main 2p54f7 final-state peak, which is reached from the divalent-4f’ initial state by a 2p + 5d excitation, a second “white-line” feature at about 7 eV higher energy is observed. This second peak is assigned, in the usual way, to a 2p54p6 final core state, which is expected at = 8 eV higher energy due to the reduced Coulomb energy between the 2p-hole and the localized 4f6 electrons, [8]. The L,,,-edge spectrum of fig. 2 was leastsquares fitted (solid line) by a superposition of two subspectra (dashed curves), each of which is the sum of a Lorentzian line and an arctan-like edge feature, both convoluted by a Gaussian profile for instrumental resolution [7]. This analysis leads to about 20% relative intensity for the Eu 3+ final-state subspectrum. After the L,,,-edge studies, we repeated the Mossbauer measurements with high statistical accuracy on the same absorber and again found practically no Eu3+ ions in the sample (with an upper limit of 2% relative intensity). Further comparative studies on different batches of samples as well as a critical evaluation of absorber thickness effects on the L,,,-edge XA spectra are underway. With the present results, we are thus confronted with a situation that EuPd,P, is depicted as divalent by Mossbauer and magnetic measurements, while its
--____
I
7.02
691 ENERGY
(keV)
Fig. 2. Eu-L ,,,-edge XA spectrum of EuPd,P,, fitted by the usual lineshapes (solid lines) [7]. The dashed curves represent subspectra from two final states.
Valence state of Eu rn EuPd,
P2
lattice-volume anomaly and particularly the L,,,-edge XA spectrum suggest a mixed-valent state. Presently, we can think of two possible explanations for this apparent discrepancy: (i) a 4f + ligand charge transfer in the final state under the influence of the 2p-core hole (shake-up), and (ii) partly extended Eu-4f orbital in this compound, which would cause a discrepancy of the kind observed in different types of spectroscopies. In the following we shall briefly discuss these two concepts. which are not totally independent, particularly since a strong shake-up effect in the L,,,-edge spectrum should be related to the strength of 4f-ligand hybridization. In case of such a shake-up, but otherwise well-localized Eu-4f’ electrons in the initial state. the lattice-volume anomaly of EuPd,P, could be understood on the basis of chemical bonding in this highly anisotropic crystal structure, as discussed in ref. [4]. A comparison of the AV-value observed for EuPd,P, (AI’ = 5 A3) with those found for other divalent Eu compounds with ThCr,Si, structure (AI’= 9 to 17 k3) indeed shows that the AV-values vary appreciably from series to series, with the deviation for EuPd,P, being exceptionally small. An interesting aspect in this context is also given by the observation that the lattice parameter a of EuPd,P, deviates in upward, while c deviates in downward direction from the normal systematic behaviour of trivalent REPd,P, compounds. Since the ThCrzSi, crystal structure contains layers of identical atoms stacked perpendicular to the c-axis (sequence: RE-P-Pd-P), this can be interpreted as an indication for a considerable increase in the strength of chemical bonding between the RE ion and the ligands as compared to trivalent compounds [4]. The interesting question remaining concerns the origin of such an increased bonding strength between the divalent Eu ions and the ligands in EuPd, P2. Even though the 4f orbitals in heavy RE ions are widely considered as strongly localized, a significant participation of 4f electrons in chemical bonding cannot be excluded in favourable cases. In this respect, we refer to the work of Kasuya et al., who proposed a strong mixing between 4f orbitals and boron-derived valenceband states in divalent EuB, [9]. Finally, we would like to point out that the apparent discrepancy in the discussed experimental observations on EuPd,P, could be resolved with the hypothesis [IO] that the Eu-4f orbital is partly extended in this compound, in a similar way as recently discussed for Ce systems [11,12]. Then, a lattice volume intermediate between divalent and trivalent compounds is expected, since the extended 4f electrons should practically not screen the nuclear charge for the 5s and 5p electrons
E. V Sampathkumaran et al. / Valence state of Eu in EuPd,P,
that determine the ionic RE radius and also these can contribute to the chemical bonding. On the other hand, magnetic measurements could still depict the total 4f occupancy of n f = 7. In this case, two different final states (2p54f7 and 2p54f6) are also readily explainable in the L,,,-edge excitations. Even though localized and extended 4f electrons would have different effects on the Mossbauer isomer shift, calibration problems prevent a quantitative evaluation of the measured value
D31.
Acknowledgements One of the authors (E.V.S.) von-Humboldt
work bereich-
This
foundation
was supported of the Deutsche
is thankful for
the
to Alexander-
financial
support.
by the SonderforschungsForschungsgemeinschaft.
References [l] W. Jeitschko and W.K. Hofmann, 95 (1983) 317.
J. Less-Common
Metals
409
[2] W. Reiger and E. Parthe, Monatsh. Chem. 100 (1969) 444. [3] D. Rossi, R. Marazza and R. Ferro, J. Less-Common Metals 66 (1979) P17. [4] E.V. Sampathkumaran, B. Perscheid and G. Kaindl, Solid State Common. 51 (1984) 701. [5] J. Feldhaus, PhD Thesis, Freie Universitat Berlin (1982). [6] See e.g. J.M. Lawrence, P.S. Riseborough and R.D. Parks, Rep. Progr. Phys. 44 (1981) 1. [7] E.V. Sampathkumaran, K.H. Frank, G. Kalkowski, G. Kaindl, M. Domke and G. Wortmann, Phys. Rev. B29 (1984) 5702. [S] J.F. Herbst and J.W. Wilkins, Phys. Rev. B26 (1982) 1689. [9] T. Kasuya, K. Takegahara, M. Kasaya, Y. Isikawa, H. Takahashi, T. Sakakihara and M. Date, in: Physics in High Magnetic Fields, eds. S. Chikazumi and N. Miura (Springer Verlag, Berlin, 1981) p. 150. [lo] A similar hypothesis was put forward recently in the case of TmSe,_,Te, system by Wachter et al., private communication. [ll] W.E. Pickett, A.J. Freeman and D.D. Koelling, Phys. Rev. B23 (1981) 1266. [12] M. Schliiter and C.M. Varma, Helvetica Phys. Acta 56 (1983) 147. (131 E.R. Bauminger, G.M. Kalvius and I. Nowik, in: Mossbauer Isomer Shifts, eds. F.E. Wagner and G. Shenoy (North-Holland, Amsterdam, 1978) p. 661.
VALENCE
Materials
STATE OF Eu IN EuPd 2P2
E.V. SAMPATHKUMARAN Institut
407
47&48 (1985) 407-409
ftir Atom
und
Festkijrperphysik
*, B. PERSCHEID, Freie
Uniuersitiit
Berlin,
W. KRONE D
1000
Berlin
and G. KAINDL 33,
Germany
EuPd,P, has been classified as mixed-valent from its lattice volume anomaly, while our Mossbauer isomer-shifts and state. The Eu-L,,,-edge X-ray absorption spectrum, however, exhibits a magnetic measurements suggest a divalent-4f’ double-peaked structure typical for mixed-valent materials. Various reasons for this apparent discrepancy are discussed including the possible existence of a partly extended 4f radius.
Recently,
the isostructural
REPd,P,
series
of com-
(RE = Rare Earth; ThCr,Si 2 structure) was first synthesized by Jeitschko and Hofmann (J&H) (11. They noticed that the tetragonal lattice parameters a and c as well as the unit-cell volume V of EuPd, P_,exhibit deviations from the systematic trend observed for the trivalent members of this series. The deviation AV of the unit-cell volume of EuPd,P,, however, amounts to only about 5 A3, much less than the typical AV values known for divalent Eu compounds of other isotypic series of ternary RE compounds ( AJ' > 9 A3) [l-3]. This anomalously small AV prompted J&H to suggest Eu is in a mixed-valent state in EuPd,P,. In this paper, we present the results of ‘5’Eu-MGssbauer effect as well as preliminary results of Eu-L,,,-edge X-ray Absorption (XA) measurements of this compound aiming at a better understanding of the Eu-vafence state [4]. The Mossbauer isomer shift data as well as the magnetic properties [4] suggest a divalent-4f’ groundstate for all Eu ions, contrary to the expectations based on the lattice-volume anomaly [l]. While the latter might be explained by strong chemical bonding between the Eu ions and the ligands, a further complication results from our L,,,-edge XA spectra, which display a double “white-line” structure typical for mixed-valent materials. We discuss possible reasons for this apparent discrepancy, which could be indicative for partly extended 4f radius in this compound. The studied EuPdzP, sample came from the same batch as the one used in our previous investigation [4]. For experimental details corcerning sample preparation and Mossbauer spectroscopy we refer to ref. (41. The pounds
* On leave from Bombay-40005,
Tata India.
Institute
of Fundamental
Research,
0304-8853/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Eu-L,,,-edge XA spectra were obtained with a conventional laboratory XA spectrometer [5]. Mbssbauer spectra obtained at 300 and 4.2 K. respectively, are shown in fig. 1. The isomer shift of the resonance pattern, S = ( - 9.73 f 0.05) mm/s (relative to a ‘S’SmF,-source), is found to be independent of temperature within experimental accuracy. No trivalent signal, with S around zero Doppler velocity, is detected in the Mossbauer spectra, which clearly reflect the homogeneous valence state of the Eu ions. At liquid-He temperature, a well-resolved magnetic hyperfine pattern, resulting from a magnetic hyperfine field of (355 f 5) kOe and a vanishingly small electric-quadrupole coupling, is observed. It is presumably due to magnetic ordering of the Eu sublattice, in agreement with previously reported magnetic measurements [4]. All these
I
I
,
1
-60
-LO
-20 VELOCITY
0
20
LO
lmmk)
Fig. 1. Mossbauer spectra of EuPd,P, at 300 and 4.2 K, respectively. The solid lines represent the results of least-squares fits of Lorentzian line shapes to the data.
E. V. Sampathkumaran
408
er al. /
observations, as well as the measured effective magnetic moment in the paramagnetic state, pLerr= (8.0 + 0.2)~~ [4], suggest a divalent state of the Eu ion [6]. This situation is contrasted by the L,,,-edge XA spectrum displayed in fig. 2. Besides the main 2p54f7 final-state peak, which is reached from the divalent-4f’ initial state by a 2p + 5d excitation, a second “white-line” feature at about 7 eV higher energy is observed. This second peak is assigned, in the usual way, to a 2p54p6 final core state, which is expected at = 8 eV higher energy due to the reduced Coulomb energy between the 2p-hole and the localized 4f6 electrons, [8]. The L,,,-edge spectrum of fig. 2 was leastsquares fitted (solid line) by a superposition of two subspectra (dashed curves), each of which is the sum of a Lorentzian line and an arctan-like edge feature, both convoluted by a Gaussian profile for instrumental resolution [7]. This analysis leads to about 20% relative intensity for the Eu 3+ final-state subspectrum. After the L,,,-edge studies, we repeated the Mossbauer measurements with high statistical accuracy on the same absorber and again found practically no Eu3+ ions in the sample (with an upper limit of 2% relative intensity). Further comparative studies on different batches of samples as well as a critical evaluation of absorber thickness effects on the L,,,-edge XA spectra are underway. With the present results, we are thus confronted with a situation that EuPd,P, is depicted as divalent by Mossbauer and magnetic measurements, while its
--____
I
7.02
691 ENERGY
(keV)
Fig. 2. Eu-L ,,,-edge XA spectrum of EuPd,P,, fitted by the usual lineshapes (solid lines) [7]. The dashed curves represent subspectra from two final states.
Valence state of Eu rn EuPd,
P2
lattice-volume anomaly and particularly the L,,,-edge XA spectrum suggest a mixed-valent state. Presently, we can think of two possible explanations for this apparent discrepancy: (i) a 4f + ligand charge transfer in the final state under the influence of the 2p-core hole (shake-up), and (ii) partly extended Eu-4f orbital in this compound, which would cause a discrepancy of the kind observed in different types of spectroscopies. In the following we shall briefly discuss these two concepts. which are not totally independent, particularly since a strong shake-up effect in the L,,,-edge spectrum should be related to the strength of 4f-ligand hybridization. In case of such a shake-up, but otherwise well-localized Eu-4f’ electrons in the initial state. the lattice-volume anomaly of EuPd,P, could be understood on the basis of chemical bonding in this highly anisotropic crystal structure, as discussed in ref. [4]. A comparison of the AV-value observed for EuPd,P, (AI’ = 5 A3) with those found for other divalent Eu compounds with ThCr,Si, structure (AI’= 9 to 17 k3) indeed shows that the AV-values vary appreciably from series to series, with the deviation for EuPd,P, being exceptionally small. An interesting aspect in this context is also given by the observation that the lattice parameter a of EuPd,P, deviates in upward, while c deviates in downward direction from the normal systematic behaviour of trivalent REPd,P, compounds. Since the ThCrzSi, crystal structure contains layers of identical atoms stacked perpendicular to the c-axis (sequence: RE-P-Pd-P), this can be interpreted as an indication for a considerable increase in the strength of chemical bonding between the RE ion and the ligands as compared to trivalent compounds [4]. The interesting question remaining concerns the origin of such an increased bonding strength between the divalent Eu ions and the ligands in EuPd, P2. Even though the 4f orbitals in heavy RE ions are widely considered as strongly localized, a significant participation of 4f electrons in chemical bonding cannot be excluded in favourable cases. In this respect, we refer to the work of Kasuya et al., who proposed a strong mixing between 4f orbitals and boron-derived valenceband states in divalent EuB, [9]. Finally, we would like to point out that the apparent discrepancy in the discussed experimental observations on EuPd,P, could be resolved with the hypothesis [IO] that the Eu-4f orbital is partly extended in this compound, in a similar way as recently discussed for Ce systems [11,12]. Then, a lattice volume intermediate between divalent and trivalent compounds is expected, since the extended 4f electrons should practically not screen the nuclear charge for the 5s and 5p electrons
E. V Sampathkumaran et al. / Valence state of Eu in EuPd,P,
that determine the ionic RE radius and also these can contribute to the chemical bonding. On the other hand, magnetic measurements could still depict the total 4f occupancy of n f = 7. In this case, two different final states (2p54f7 and 2p54f6) are also readily explainable in the L,,,-edge excitations. Even though localized and extended 4f electrons would have different effects on the Mossbauer isomer shift, calibration problems prevent a quantitative evaluation of the measured value
D31.
Acknowledgements One of the authors (E.V.S.) von-Humboldt
work bereich-
This
foundation
was supported of the Deutsche
is thankful for
the
to Alexander-
financial
support.
by the SonderforschungsForschungsgemeinschaft.
References [l] W. Jeitschko and W.K. Hofmann, 95 (1983) 317.
J. Less-Common
Metals
409
[2] W. Reiger and E. Parthe, Monatsh. Chem. 100 (1969) 444. [3] D. Rossi, R. Marazza and R. Ferro, J. Less-Common Metals 66 (1979) P17. [4] E.V. Sampathkumaran, B. Perscheid and G. Kaindl, Solid State Common. 51 (1984) 701. [5] J. Feldhaus, PhD Thesis, Freie Universitat Berlin (1982). [6] See e.g. J.M. Lawrence, P.S. Riseborough and R.D. Parks, Rep. Progr. Phys. 44 (1981) 1. [7] E.V. Sampathkumaran, K.H. Frank, G. Kalkowski, G. Kaindl, M. Domke and G. Wortmann, Phys. Rev. B29 (1984) 5702. [S] J.F. Herbst and J.W. Wilkins, Phys. Rev. B26 (1982) 1689. [9] T. Kasuya, K. Takegahara, M. Kasaya, Y. Isikawa, H. Takahashi, T. Sakakihara and M. Date, in: Physics in High Magnetic Fields, eds. S. Chikazumi and N. Miura (Springer Verlag, Berlin, 1981) p. 150. [lo] A similar hypothesis was put forward recently in the case of TmSe,_,Te, system by Wachter et al., private communication. [ll] W.E. Pickett, A.J. Freeman and D.D. Koelling, Phys. Rev. B23 (1981) 1266. [12] M. Schliiter and C.M. Varma, Helvetica Phys. Acta 56 (1983) 147. (131 E.R. Bauminger, G.M. Kalvius and I. Nowik, in: Mossbauer Isomer Shifts, eds. F.E. Wagner and G. Shenoy (North-Holland, Amsterdam, 1978) p. 661.