Photon Factory Activity Report 2004 #22 Part B (2005)
Electronic Structure of Condensed Matter

2C/2003G149

Valence changes associated with the metal-insulator transition in Bi1-xLaxNiO3
Hiroki WADATI*1, Masaru TAKIZAWA1, Thanh Trung TRAN1, Kiyohisa TANAKA1,Takashi MIZOKAWA1, Atsushi FUJIMORI1, Akira CHIKAMATSU2, Hiroshi KUMIGASHIRA2,
Masaharu OSHIMA2, Shintaro ISHIWATA3, Masaki AZUMA3, Mikio TAKANO3
1
Department of Physics and Department of Complexity Science and Engineering,
University of Tokyo, Kashiwa 277-8561, Japan2
Department of Applied Chemistry, University of Tokyo, Tokyo 113-8656, Japan3
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

Experimental
Preparation and characterization of polycrystalline Bi1-x
LaxNiO3 (x = 0, 0.05, 0.1, 0.2) are described elsewhere [1,2]. For x = 0 (BiNiO3), the system is insulating. Electricalresistivity decreases with La substitution. For x = 0.05
and 0.1, a broad MI transition occurs as a function oftemperature. For x = 0.2, the system is metallic.
Experiment was performed at BL-2C of Photon Factory.
The photoemission and x-ray absorption measurements-10were performed under an ultrahigh vacuum of 10 Torr atroom temperature. The photoemission spectra weremeasured using a Scienta SES-100 electron-energy
analyzer. The total energy resolution was about 200-500
meV depending on the photon energy. The Fermi level(EF) position was determined by measuring gold spectra.
The XAS spectra were measured by the total-electronyield method. All the measurements were performed atroom temperature. Clean surfaces of the samples wereobtained by repeated in-situ scraping with a diamond file.
Results and Discussion
Figure 1 shows the Ni 2p XAS spectra of Bi1-xLaxNiO3.
The Ni 2p3/2 peak overlaps the La 3d3/2 → 4f absorption
peak. As reported previously [3], the Ni 2p XAS spectra
3+2+of RNiO3 (Ni ) are very different from that of NiO (Ni ):The 2p3/2 spectrum consists of two peaks labeled as A andB, as shown in Fig. 1. The lower energy peak A isstronger than the higher energy one B for NiO, whereas
the two peaks have similar intensities in RNiO3. The2+
spectrum of x = 0 is similar to that of Ni , which isconsistent with the report that the valence of Ni is 2+ inthis material [1]. With increasing x, the relative intensityof the higher energy peak increases, indicating that the2+3+valence of Ni gradually increases from Ni toward Ni .
In order to evaluate the Ni valence from Ni 2p XAS morequantitatively, we have subtracted the contribution of La
3d3/2 XAS and compared the spectra with those of BiNiO32+3+(Ni ) and PrNiO3 (Ni ). From the relative intensities ofthe two peaks, one can estimate the Ni valence as afunction of x. Here, we have assumed that the spectrum of2+x = 0 represents pure Ni , and adopted that of PrNiO3 as a3+
reference of Ni . The valence of Ni is approximately(2+x)+ rather than 3+, the value expected when theaverage valence of Bi becomes 3+ due to the Lasubstitution. The result indicates that La substitution does
not change the average valence of Bi appreciably, while
suppressing the charge disproportionation at the Bi site,
and that the valence of Ni is ~ 2+x. This result isconsistent with the core-level photoemission study.
Bi1-xLaxNiO3AT = R. T.
Intensity (arb. units)

Introduction
Perovskite-type RNiO3 exhibits a metal-insulator (MI)transition as a function of the ionic radius of the rareearth R ion. BiNiO3 was expected to be metallic because
LaNiO3 remains metallic down to 1.5 K and the ionic3+3+radius of Bi is larger than that of La . Contrary to thisexpectation, BiNiO3 is an insulating antiferromagnet
3+
because the Bi ions are charge-disproportionated into Bi
5+
and Bi and thus the oxidation state of the Ni ion
becomes 2+ rather than 3+ [1]. It was also reported thatthe substitution of La for Bi suppresses the charge
disproportionation and makes the system conducting [2].
Therefore, an interesting question is how the electronicstructure changes from the charge-disproportionatedinsulating BiNiO3 to the metallic LaNiO3. To settle thisquestion, we investigated the electronic structure of Bi1-x
LaxNiO3 by photoemission and x-ray absorption
spectroscopy (XAS).

B
Ni 2p3/2

Ni 2p1/2

x=0
La 3 d5/2

La 3d3/2

x = 0.05
x = 0.1x = 0.2820830840

850860870880

Photon Energy (eV)FIG. 1: Ni 2p XAS spectra of Bi1-xLaxNiO3. The dashedlines represent the La 3d3/2 absorption peaks.
References[1] S. Ishiwata et al., J. Mater. Chem. 12, 3733 (2002).
[2] S. Ishiwata et al., cond-mat/0502066.
[3] M. Medarde et al., Phys. Rev. B 46, 14975 (1992).
* wadati@wyvern.phys.s.u-tokyo.ac.jp

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