LaPdo3: The first PdIII oxide with the perovskite structure [14]

Full text of DOI:10.1021/ja016522b

J. Am. Chem. Soc. 2001, 123, 10413-10414
10413
LaPdO3: The First PdIII Oxide with the Perovskite
Structure
Seung-Joo Kim,† Sylvain Lemaux,† Ge´rard Demazeau,*,†
Jong-Young Kim,‡ and Jin-Ho Choy‡
Institut de Chimie de la Matie`re
Condense´e de Bordeaux, UPR.CNRS 9048
87 aVenue A. Schweitzer, 33608 Pessac Cedex, France
National Nanohybrid Materials Laboratory
School of Chemistry and Molecular Engineering
Seoul National UniVersity, Seoul, 151-747, Korea
ReceiVed June 29, 2001
Figure 1. X-ray powder diffraction patterns, Rietweld refinement (upper
and lower tick marks represent the Bragg positions for LaPdO3 and KCl,
respectively), and schematic presentation of crystal structure for LaPdO3.
Perovskite-type oxides containing transition-metal ions with
degenerated electron configurations have attracted a considerable
interest due to their specific physical properties1 including colossal
negative magnetoresistance [high-spin Mn3+ (3d4)], charge dis-
proportionation [high-spin FeIV (3d4)], metal-insulator transition
[low-spin NiIII (3d2)], and high-Tc superconductity [Cu2+ (3d9)].
Although PdIII (d7) may also represent promising candidates in
these fields, the main investigations have been carried out within
fluorides2 and halogen-bridged salts3 systems, owing to a con-
figurational instability of PdIII (d7) and a strong tendency to
disproportionate into PdII and PdIV. In early works, Pd2F6 with
the LiSbF6-type structure have been characterized by a cationic
ordering with the formulation PdIIPdIVF6.4 Under high-pressure
Pd2F6 has shown a pronounced decrease of the resistivity, which
suggests the charge redistribution (PdII + PdIV f 2PdIII).5 The
stabilization of PdIII as single valent form has been achieved in
NaPdF4 and K2NaPdF6, and EPR measurements confirm the
electronic configuration (t62gd12)2. The oxides containing PdIII are
quite rare. Recently the seriezs of APd2O4 (A ) rare earth) have
been prepared under high pressure where the Pd with mixed
valence of +2.5 is coordinated by four oxygen atoms forming
approximately square planar PdO4 group.6 The aim of this work
is to stabilize PdIII in an oxygen lattice through the high-pressure
techniques for which we select perovskite structure, very stable
under the pressure conditions.
heated at 1100-1150 °C for 10 min. After being quenched to
room temperature, the sample was washed with distilled water
to remove KCl and subsequently dried at 120 °C for 2 h. The
composition of final black powder was determined as LaPd-
O2.98(0.02 with chemical titration and microprobe analysis.7 The
compound was stable to 500 °C at which it started to decompose
to La2Pd2O5 + 0.5O2.
X-ray diffraction profile (Figure 1) for the final black powder
could be indexed using a primitive orthorhombic unit cell
implying that LaPdO3 would adopt the GdFeO3-type structure
(space group Pbnm) with unit cell parameters a ) 5.5898(3) Å,
b ) 5.8502(3) Å, c ) 7.8666(4) Å. Significant Jahn-Teller effect
was not observed although c/x2a ratio(0.995) is smaller than 1.8
Considering the difference (∆r ) 0.07 Å) between the radii of
oxide and fluoride ions, the average PdIII-O bond distance (2.08
Å) is compatible with the average PdIII-F bond distance (2.00
Å) estimated from recent EXAFS study on K2NaPdF6 with the
elpasolite-type structure where the (PdF6) octahedra exhibit a
Jahn-Teller distortion.9
To confirm the oxidation state of Pd, an X-ray absorption
spectroscopic study was performed at the Pd L3 edge (3173 eV).10
Two Pd oxides were used as references (PdIIO and Zn2PdIVO411).
The Pd L3-edge XANES spectra (Figure 2) for all compounds
are characterized by one main peak which primarily corresponds
to the electric dipole-allowed transition from 2p states to 4d ones.
Since no multiplets effect was detected in the Pd L3 XANES
spectra of fluorides,9 it can be assessed that multiplet transitions
are also absent from the spectra of the oxides, due to a higher
covalency of the Pd-O bond compared to that of the Pd-F bond.
Therefore the Pd L3 edge spectra of these oxides is directly
relevant of the empty electronic states of the 4d orbitals.12 A shift
of the peak to higher energies is clearly observed while moving
from PdIIO to Zn2PdIVO4. The peak in the Pd L3 XANES of
Stoichiometric amounts of La2O3 and PdO in the ratio 1:2 were
mixed together and then fired at 1040-1070 °C for 2 weeks.
The product was composed of mainly La2Pd2O5 and small amount
of La4PdO7 and Pd metal. To help the oxidation of Pd metal, the
previous product was treated under oxygen pressure (100 MPa)
at 800 °C for 1 day. The resultant product was mixed with KClO3
as oxygen source and treated under vacuum at 120 °C to eliminate
water contamination. The mixture was encapsulated in a platinum
tube, pressurized under 5 GPa using a Belt-type apparatus, and
† Institut de Chimie de la Matie`re Condense´e de Bordeaux.
‡ Seoul National University.
(1) For example, see: (a) von Holmolt, R.; Wecker, J.; Holzapfel, B.;
Samwer, K. Phys. ReV. Lett. 1993, 71, 2331. (b) Takano, M.; Nakanishi, N.;
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(2) (a) Tressaud. A.; Khairoun, S.; Dance, J. M.; Grannec, J.; Demazeau,
G.; Hagenmuller, P. C. R. Acad. Sci. Paris, Ser. II 1982, t.295, 183. (b)
Khairoun, S.; Dance, J. M.; Demazeau, G.; Tressaud, A. ReV. Chim. Miner.
1983, 20, 871.
(3) For example, see: (a) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem.,
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Tressaud, A.; Wintenberger, M.; Bartlett, N.; Hagenmuller, P. C. R. Acad.
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(7) After dissolving the sample in HCl solution containing Br-, Br2 liberated
from the re-dox reaction of PdIII and Br- was titrated with hydrazine sulfate
standard solution. The end point was determined by bi-amperometry.
Microprobe analysis revealed that K, Cl, and Pt were not contained in the
final sample (<0.5%).
(8) c/x2a ratio does not work in some trivalent nickelates (d7), PrNiO3
(0.994) and NdNiO3 (0.998) where no static Jahn-Teller effect has been
observed.
(9) De Nada¨ı, C.; Demourgues, A.; Grannec, J. Nucl. Instrum. Methods
Phys. Res. B 1997, 133, 1.
(10) The XANES spectra of Pd L3-edge were measured at room temperature
in transmission mode using a Si(111) double-crystal monochromator on station
D44 in LURE (Orsay-France). The spectra were calibrated in energy with
the presence of argon traces in the ionization chamber (Ar K-edge: 3202 eV).
The spectra were normalized at 3185 eV.
(11) Demazeau, G. Omeran, I.; Pouchard, M.; Hagenmuller, P. Mater. Res.
Bull. 1976, 11, 1449.
(12) Choy, J. H.; Kim, D. K.; Demazeau, G.; Jung, D. Y. J. Phys. Chem.
1994, 98, 6258.
(5) Langlais, F.; Demazeau, G.; Portier, J.; Tressaud. A.; Hagenmuller, P.
Solid State Commun. 1979, 29, 473.
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Chen, B.-H.; Walker, D.; Scott, B. Chem. Mater. 1997, 9, 1700.
10.1021/ja016522b CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/02/2001

10414 J. Am. Chem. Soc., Vol. 123, No. 42, 2001
Communications to the Editor
Figure 3. (a) Fourier transform of k3-weighted Pd K-egde for LaPdO3
(b) k3-weighted Fourier-filtered EXAFS functions of Pd K-edge for
LaPdO3.
Figure 2. Normalized Pd L3-edge XAS spectra of PdIIO, LaPdIIIO3, and
Zn2PdIVO3 at room temperature. The dotted line represents the average
spectrum of PdIIO and Zn2PdIVO3.
LaPdO3 (3175.7 eV) is exactly located at the middle position of
two peaks for PdII (3174.6 eV) and PdIV (3176.8 eV) references.
The shift induced by the difference in oxidation state is less
predominant in such oxides compared to the fluorides (only 2.2
eV between the PdII and PdIV oxides compared to 4 eV in the
fluorides9), owing to the lower ionicity of Pd-O bond of than
Pd-F one. The peak intensity for LaPdO3 is also between them,
which is correlated with d-band vacancies. Additionally, the
spectrum of LaPdO3 can clearly be distinguished from the average
of the Pd L3 spectra of PdII and PdIV oxides. Therefore, the formal
oxidation state of Pd in LaPdO3 is close to III rather than II +
IV. However, the peak for LaPdO3 is broader than those for PdIIO
and Zn2PdIVO4. Such broadening may be attributed from strong
covalency due to the high degree of hybridization of Pd 4d and
O 2p orbitals.13 A similar effect has been observed in comparing
the Cu L3-edge of metallic cuprate with the insulating one.14
The local structure of PdIII was estimated by the EXAFS
analysis on Pd K-edge.15 The first shell (1.1 Å < R < 2.1 Å) in
R space (phase shift: uncorrected), which seems to be nearly
isotropic, corresponds to the nearest neighbors of Pd ion, that is,
(Pd-O) shells (Figure 3a). For this shell the radial distance (R),
the edge position (E0) and the Debye-Waller factor (σ2) were
refined. Due to the strong correlation between σ2 and the
coordination number (CN), the CN was fixed to 6. A good fit
accuracy is obtained for this shell by considering standard
harmonic pair distribution, which suggests the absence of a large
distortion of (PdO6) octahedra in LaPdO3 (Figure 3b). The average
Pd-O bond distance is estimated to be 2.06 Å, which is well
consistent with XRD results mentioned above.
Figure 4. Temperature dependence of the molar magnetic susceptibility
of LaPdO3 after correction of the core diamagnetic term.
ø ) ø(0) - aT2 + C/T
with ø(0) ) 1.0 × 10-4 emu/mol, a ) 1.1 × 10-10 emu/K2 mol,
and C ) 1.5 × 10-3 emu K/mol (Figure 4). The first and second
terms are Pauli paramagnetic susceptibility and its small temper-
ature dependency, respectively. The last term represents a small
Curie component. We cannot rule out a trace of impurities as
sources of the Curie term. Such a magnetic behavior is consistent
with the metallic electron configuration (t62gσ*1) for PdIII which
is deduced from the XANES/EXAFS analyses. The absence of
large enhancement of the Pauli susceptibility observed in LaNiO3
(5.1 × 10-4 emu/mol) is probably due to weak electron-correlation
effect in the 4d compared to the 3d system.17
From these physicochemical characterizations LaPdO3 appears
the first oxygen-perovskite containing PdIII.
Acknowledgment. We thank S. Belin and.V. Briois in LURE (French
Synchrotron in Orsay) for their help in Pd L3 XAS measurements and
the French Ministry of Foreign Affairs for a Pasteur Scholarship
(S.J. KIM).
The molar magnetic susceptibility taken at H ) 10 kOe for 5
K e T e 300 K can be fitted to the formula,16
(13) de Groot, F. M. F.; Hu, Z. W.; Lopez, M. F.; Kaindl, G.; Guillot, F.;
Tronc, M. J. Phys. Chem. 1994, 101, 6570.
Supporting Information Available: The results of Rietveld refinemet
on powder XRD patterns and the fitted structural parameters in EXAFS
analysis (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(14) Sondericker, D.; Fu, Z.; Johnston, D. C.; Eberhardt, W. Phys. ReV. B
1987, 36, 3983.
(15) The XAS measurements on the Pd K-edges were carried out with
synchrotron radiation at the beam line 10B of the Photon Factory (KEK-PF)
in Tsukuba, Japan. A Si(311) monochromator channel-cut monochromator
was used. The data analysis was performed according to the previously
published procedures. For example, see: Choy, J. H.; Kim, D. K.; Hwang, S.
H.; Demazeau, G.; Jung, D. Y. J. Am. Chem. Soc. 1995, 117, 8557.
(16) Sreedhar, K.; Honig, J. M.; Darwin, M.; McElfresh, M.; Shand, P.
M.; Xu, J.; Crooker, B. C.; Spalek, J. Phys. ReV. B 1992, 46, 6382.
JA016522B
(17) (a) Goodenough, J. B.; Mott, N. F.; Pouchard, M.; Demazeau, G.
Mater. Res. Bull. 1973, 8, 647. (b) Zhou, J.-S.; Goodenough, J. B.; Dabrowski,
B.; Klamut, P. W.; Bukowski, Z. Phys. ReV. Lett. 2000, 84, 526.