Physica B 329–333 (2003) 826–828

Co K-edge XANES and spin-state transitionof RCoO3 (R ¼ La; Eu)

J.Y. Chang, B.N. Lin*, Y.Y. Hsu, H.C. Ku

Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC

Abstract

Co K-edge X-ray absorption near-edge spectra at 300 K and magnetic studies for the rhombohedral LaCoO3 with

low-spin state to intermediate-spin (IS) state transition TsC105 K and orthorhombic EuCoO3 ðTsC295 KÞ are

reported. The small pre-edge feature observed is from 1s–3d dipole transition, which is weakly allowed through the

hybridization of Co 4p states with 3d states of neighboring Co atoms. For both samples in the IS state, pre-edge can be

fitted using three peaks with energy separation P2 � P1 ¼ 2:0 eV for R ¼ La and Eu, and P3 � P2 ¼ 1:5 eV for R ¼ La

and 1:3 eV for R ¼ Eu: Since the thermally excited IS state is a mixed t52ge1g and d7

%L ðt62ge

1g or t52ge

2g), with almost filled

t2gm states. From reported band structure calculation, the three peaks P1; P2; P3 are probably corresponding to the

unfilled t2gk; egm and egk states, respectively.

r 2003 Elsevier Science B.V. All rights reserved.

Keywords: Spin-state transition; XANES

Rhombohedral LaCoO3 ðR%3cÞ with trivalent

Co3þ ð3d6Þ is a nonmagnetic Mott insulator at low

temperature which undergoes a thermally induced spin-

state transition to a paramagnetic state with TsC90–

100 K and a metal–insulator transition above 500 K [1–

8]. It remains controversial that whether the spin-state

transition is from a low-spin ðLS : t62g;S ¼ 0Þ state to an

intermediate-spin ðIS : t52ge1g;S ¼ 1Þ state or to a high-

spin ðHS : t42ge2g;S ¼ 2Þ state. The LDA+U calculation

shows that IS state is energetically comparable to LS

state, but much more stabilized than HS state due to the

larger Co–O d–p hybridization [1]. The X-ray photo-

emission and absorption studies indicate that the ground

state is a CoIII LS state with heavily mixed d6 and d7

%L

(t62ge1g with ligand hole) character due to strong d–p

hybridization and the spin-state transition is a gradual

LS–IS transition [2]. The infrared studies indicate a local

lattice distortion during the LS–IS transition due to

dynamical Jahn–Teller effect [3].

For orthorhombic rare earth RCoO3 (Pbnm) com-

pounds, possible TsC220 K for PrCoO3 and C275 K

for NdCoO3 are reported [6]. Since various reported

data are confused by magnetic rare earth R3þ signal

[5,6], in order to identify the true spin-state transition

in orthorhombic phase, EuCoO3 is chosen as the

best candidate with nonmagnetic Eu3þ ground state

ðJ ¼ 0Þ [8].

The samples RCoO3þd (R ¼ La; Eu) were synthesized

by standard solid-state reaction. Sintered pellets were

annealed in O2; air or Ar to check the variation of

oxygen content. Final oxygen content parameter dC0

was crosschecked by comparing the X-ray diffraction,

magnetic susceptibility and X-ray absorption near-edge

spectra (XANES) data [8]. The structural studies

indicate that there is no interstitial oxygen site. With

fully occupied oxygen sublattice and R and Co

vacancies, the correct composition is RzCozO3 with

z ¼ 3=ð3 þ dÞ; and maximum formula unit volume is

achieved at the stoichiometric RCoO3: The maximum

*Corresponding author. Tel.: +886-3-574-2523; fax: +886-

3-572-3052.

E-mail addresses: d863351@ox.nthu.edu.tw (B.N. Lin),

hcku@phys.uthu.edu.tw (H.C. Ku).

0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-4526(02)02595-4

formula unit volume of 55:91 (A3

for rhombohedral

LaCoO3þd and 52:83 (A3

for orthorhombic EuCoO3þd

derived from X-ray diffraction studies indicate that

samples prepared are very close to the stoichiometric 113

ratio ðdC0Þ with Co3þ [8].

Molar magnetic susceptibility wmðTÞ in 3 T zero-field-

cooled (ZFC) for LaCoO3 and EuCoO3 are shown in

Fig. 1. The susceptibility shows similar behavior as

previously reported data and decreases abruptly from

TsC105 K for LaCoO3 [2,3]. Since TsC140 K for

La0:75Eu0:25CoO3 and 200 K for La0:5Eu0:5CoO3 can

be clearly observed from magnetic data [8], an extra-

polated TsC295 K is deduced for EuCoO3; which is

unfortunately masked by the magnetic signal of Eu3þ

excited state ðJ ¼ 1Þ [8].

The Co K-edge XANES spectra for RCoO3 (R ¼ La;Eu) at room temperature ðT > TsÞ are shown in Fig. 2.

The energy is calibrated by a Co metal foil. The E0 for

two standards CoO ðCo2þÞ and Co2O3 ðCo3þÞ indicates

a substantial shift of E0 with increasing Co formal

valence. Almost identical E0 for La and Eu samples

indicates that they are very close to the stoichiometric

composition with Co3þ:The K-edge XANES is sharp with a long, low-energy

tail. The main edge is attributed to 1s–4p dipole

transition to Co 4p states. Since crystal structure of

RCoO3 system is identical to LaMnO3þd system (Pbnm

for dC0 and R%3c for d > 0), the edge shape (A, B, C, D)

can be explained by 4p partial density of states,

broadened by finite lifetime of 1s core hole. The Co 4p

states, like Mn 4p state, are highly delocalized and

extend over several Co atoms. The observed small pre-

edge feature P is corresponding to 1s–3d dipole

transition, which is weakly allowed through the hybri-

dization of 4p states with 3d states of neighboring Co

atoms.

Fig. 3 shows the low intensity pre-edge P feature for

RCoO3 (R ¼ La; Eu). For both samples in the IS state,

pre-edge can be fitted using three peaks with energy

separation P2 � P1 ¼ 2:0 eV for R ¼ La and Eu, and

P3 � P2 ¼ 1:5 eV for R ¼ La and 1:3 eV for R ¼ Eu

(inset for LaCoO3). Since the ground state is a CoIII LS

state with heavily mixed d6 and d7

%L ðt62ge

1gÞ; the

thermally excited IS state at 300 K may be a mixed

t52ge1g and d7

%L ðt62ge

1g or t52ge

2gÞ; with almost filled t2gm

states. This indicates that the allowed 1s–3d dipole

transition is into unfilled t2gk and eg states. From

reported band structure calculation [1,2], the three peaks

P1;P2;P3 are probably corresponding to unfilled t2gk;egm and egk states, respectively.

This work was supported by NSC of R.O.C. under

contract Nos. NSC90-2112-M007-054 & -056.

References

[1] M.A. Korotin, et al., Phys. Rev. B 54 (1996) 5309.

0 50 100 150 200 250 3000

2

4

6

8

10

Ba = 3 T (ZFC)

Ts ~ 300 K

Ts ~105 K

RCoO3

Temperature T (K)

Mag

netic

sus

cept

ibili

ty χ

m(×

10-3 c

m3 /m

ol )

R = Eu (Pbnm) R = La (R3c)

Fig. 1. Molar magnetic susceptibility wmðTÞ in 3 T field (ZFC

mode) for rhombohedral LaCoO3 and orthorhombic EuCoO3:

7700 7710 7720 7730 7740 77500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

D

RCoO3

Co2O

3CoOCo

C

B

AP

Co K-edge XANES

R = La R = Eu

Nor

mal

ized

abs

orpt

ion

inte

nsity

Photon energy E (eV)

Fig. 2. Co K-edge XANES at 300 K for RCoO3 (R ¼ La; Eu).

The threshold edge energy of two standards (CoO, Co2O3) and

Co metal foil are indicated.

7704 7706 7708 7710 7712 7714 7716 77180.00

0.05

0.10

0.15

0.20

0.25

7708 7710 7712 77140.00

0.01

0.02

0.03

P3

RCoO3

P2

P1

R = La R = Eu

Nor

mal

ized

abs

orpt

ion

inte

nsity

Photon energy E (eV)

P3

P2

P1

R = La

Fig. 3. Low intensity pre-edge region of Co K-edge XANES

for RCoO3 (R ¼ La; Eu). The pre-edge P can be fitted with

three peaks P1; P2 and P3 (inset for LaCoO3).

J.Y. Chang et al. / Physica B 329–333 (2003) 826–828 827

[2] T. Saitoh, et al., Phys. Rev. B 55 (1997) 4257.

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[8] J.Y. Chang et al., Phys. Rev. B, 2002, submitted for

publication.

J.Y. Chang et al. / Physica B 329–333 (2003) 826–828828