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Effect of the Ca content on the electronic structure of Pb 1x Ca x TiO 3 perovskites

J. C. Jan, K. P. Krishna Kumar, J. W. Chiou, H. M. Tsai, H. L. Shih, H. C. Hsueh, S. C. Ray, K. Asokan, W. F. Pong, M.-H. Tsai, S. Y. Kuo, and W. F. Hsieh

Citation: Applied Physics Letters 83, 3311 (2003); doi: 10.1063/1.1618375 View online: http://dx.doi.org/10.1063/1.1618375

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/16?ver=pdfcov

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Effect of the Ca content on the electronic structure

of Pb

1Àx

Ca

x

TiO

3

perovskites

J. C. Jan, K. P. Krishna Kumar, J. W. Chiou, H. M. Tsai, H. L. Shih, H. C. Hsueh, S. C. Ray, K. Asokan,a)and W. F. Pongb)

Department of Physics, Tamkang University, Tamsui, Taiwan 251, Republic of China

M.-H. Tsai

Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China

S. Y. Kuoc)and W. F. Hsieh

Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 300, Republic of China

共Received 20 June 2003; accepted 22 August 2003兲

This study performs O K- and Ti L3,2-edge x-ray absorption near-edge structure 共XANES兲 measurements and first-principles pseudopotential calculations for the electronic structures of ABO3-type Pb1⫺xCaxTiO3(x⫽0 – 1) perovskites. The features in the O K-edge XANES spectra are found to be contributed primarily by hybridization between O 2 p and Ti 3d, Pb 6 p, and Ca 3d orbitals. The O K-edge XANES spectra reveal that partial substitution of A cations, Pb, by Ca not only decreases O 2 p – Pb 6 p but also O 2 p – Ti 3d hybridization. The Ti L3,2-edge measurements find that the off-center displacement of Ti, and hence, ferroelectricity persist up to a Ca concentration between 0.3 and 0.4. © 2003 American Institute of Physics.

关DOI: 10.1063/1.1618375兴

Ferroelectric ABO3 perovskites have been the subject of extensive studies since they exhibit rich electric characteris-tics potentially useful in fundamental research and techno-logical applications.1Among them lead titanate, PbTiO3 共de-noted as PTO兲 has a characteristic unit cell that contains a highly polarizable TiO6 octahedron, which gives rise to ferroelectric characteristics.1,2 Theoretical calculations showed that Ti 3d and O 2 p hybridized states and the Pb–O covalent bonding crucially cause the ferroelectric instability in PTO.3–5 While previous extended x-ray absorption fine structure studies of ferroelectric titanates revealed the off-center displacement of Ti.6Introducing Ca was found to re-duce both the tetragonal distortion and the transition tem-perature of PTO.7Recently, compositional substitution of the A cation, Pb, in PTO by Ca and the effect on its electrical and structural characteristics have been studied by x-ray dif-fraction and Raman measurements.8,9 The x-ray diffraction measurement performed by Kuo et al. showed that as the Ca content in Pb1⫺xCaxTiO3 (PCxTO) increased from 0 to 1, phase transitions from tetragonal to cubic and then to ortho-rhombic occurred.9In this study, O K- and Ti L3,2-edge x-ray absorption near-edge structure 共XANES兲 measurements and first-principles electronic structure calculations10 are per-formed to better understand the influence of Ca substitution on the electronic structure of PCxTO perovskites.

Room temperature XANES spectra at O K and Ti L3,2 edges were obtained using a high-energy spherical grating monochromator beamline by the fluorescence and sample

current modes, respectively, at the National Synchrotron Ra-diation Research Center, Hsinchu, Taiwan. Powdered PCxTO

samples were synthesized using the sol-gel technique. Theo-retical calculations of Pb0.5Ca0.5TiO3were based on the first-principles pseudopotential method with the local-density approximation,11 the computation details are given elsewhere.10,12

Figure 1 presents the O K-edge XANES spectra of PCxTO (x⫽0.2–0.9兲, PbTiO3 (x⫽0, PTO兲, CaTiO3 (x⫽1, CTO兲 and the reference TiO2. These spectra are normalized to the same area in the energy range between 550 and 570 eV

共not fully shown兲. The features marked by A1 to D1 in the spectra of PCxTO and TiO2 are centered at ⬃530.8, 532.4, 533.6, and 536.0 eV, respectively. The inset in Fig. 1 presents a magnified view to better resolve features A1– D1. In these spectra, the background intensity was subtracted from a best-fitted Gaussian curve as indicated by the dashed lines. These features are best resolved with two peaks for CTO and TiO2. Similar two-peak structures at the threshold of the O K edge were also observed for other 3d-transition metal oxides.13 The two-peak structures, marked by A1 and C1, in the CTO and TiO2 spectra, are contributed by O 2 p and Ti t2g and eg

antibonding orbitals, respectively.12,13

Figure 2 shows the O K-edge XANES spectrum and the calculated O 2 p-, Ti 3d-, Pb 6s p-, and Ca 3d-derived states of the Pb0.5Ca0.5TiO3 to elucidate O 2 p – Ti 3d, O 2 p – Pb 6s p, and O 2 p – Ca 3d hybridization and to identify the contributions to the O K-edge XANES features. Figure 2 reveals strong hybridization between O 2 p and Ti 3d t2gand

eg orbitals, which gives rise to features A1 and C1, respec-tively. Features B1 and D1 arise predominantly from the hy-bridization between O 2 p and Pb 6 p and Ca 3d orbitals, respectively.

Figure 3 presents the difference curves of the O K-edge

a兲Present address: Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi 110067, India.

b兲Author to whom correspondence should be addressed; electronic mail: wfpong@mail.tku.edu.tw

c兲Present address: Precision Instrument Development Center, Hsinchu, Taiwan.

APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 16 20 OCTOBER 2003

3311

0003-6951/2003/83(16)/3311/3/$20.00 © 2003 American Institute of Physics

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XANES spectra of PCxTO and PTO to illustrate the

depen-dence of the hybridization between O 2 p and Ti 3d, Pb 6 p and Ca 3d orbitals on x. The darkened areas under the dif-ference curve 共hereafter denoted as ⌬A1, ⌬B1, ⌬C1, and

⌬D1) are proportional to the difference between the densities of the unoccupied O 2 p-derived states of PCxTO and those

of PTO. Figure 3 reveals that the areas under ⌬A1 to⌬C1 (⌬D1) are always negative共positive兲 and increase with the Ca content, indicating reduced 共increased兲 O 2p – Ti 3d and O 2 p – Pb 6 p 共O 2p – Ca 3d) hybridization with the Ca sub-stitution. Roughly three different regions can be identified for x between 0 and 0.3, 0.4, and 0.6, and 0.7 and 1 accord-ing to the variation of the intensities and line shapes of the difference spectra. The trend of the line shapes/intensities of

⌬A1 to ⌬C1 exhibits some changes at x⫽0.4 and 0.7

共marked by arrow bars兲, which is consistent with previous

observation of a phase transition from tetragonal to cubic at

x⫽0.4 and then to orthorhombic at x⫽0.7.9

The ranges of ⌬A1 共from ⬃529.0 to 531.4 eV兲, ⌬C1

共from ⬃533.4 to 535.8 eV兲, ⌬B1 共from ⬃531.4 to 533.4 eV兲, and ⌬D1 共from ⬃535.8 to 539.8 eV兲 are attributable to O 2 p – Ti t2g, O 2 p – Ti eg, O 2 p – Pb 6 p, and O 2 p – Ca 3d

antibonding orbitals. The dependence of the O–Ti, O–Pb, and O–Ca hybridization on the Ca concentration can be es-timated qualitatively from Fig. 4, which presents the inte-grated areas of⌬A1⫹⌬C1,⌬B1, and⌬D1. The decrease of the ⌬B1 area and the increase of the ⌬D1 area with the increase of x are approximately linear. In contrast, the varia-tion of the ⌬A1⫹⌬C1 area can be roughly separated into three regions, which suggests that the structural transitions of PCxTO are correlated primarily with the O–Ti hybridization.

Figure 4 shows that the substitution of the A cation, Pb, by Ca not only decreases the O 2 p – Pb 6 p but also O 2 p – Ti 3d hybridization. Ca has a smaller electronegativity than other two cations, Pb and Ti共1.0 vs 2.33 and 1.54, respectively兲.14

Thus, electron charge is transferred from the Ca cation to both Pb and Ti cations in PCxTO, which reduces the positive

effective charges on Pb and Ti ions. Thus, the attractive Cou-lomb potentials at the Pb and Ti sites are reduced, which raises Pb and Ti orbital energies and reduces both O 2 p – Pb 6 p and O 2 p – Ti 3d hybridization.

Earlier theoretical calculations suggested that the ferro-electric transition occur as a result of a balance between the long-range Coulomb interaction and the short-range force.3 Zhong et al. also discussed the correlation between Coulomb interaction and the ferroelectric states15 and argued that the Coulomb interaction leads to the splitting of LO and TO ferroelectric phonon modes. In recent studies of Pb-based PCxTO9 and Pb1⫺xSrxTiO3,16 Kuo et al. reported that the frequency difference between LO and TO modes decreases when PCxTO and Pb1⫺xSrxTiO3 transit from the tetragonal structure to the high-symmetry cubic structure. Kuo et al. interpreted their results as related to the reduction of long-range Coulomb interactions by enhanced Pb–O covalent bonding. However, based on present O K-edge results, it is argued in the following that the softening of the TO mode is not due to the enhanced Pb–O covalent bonding, but due to the reduction of Ti–O hybridization and Ti effective charge. The vibration frequency of the TO phonon mode is propor-tional to the square root of the force constant between cions and ancions, which contains contributcions from the at-tractive electrostatic Coulomb energies and hybridization. If the TO mode is contributed dominantly by the force constant between O and A cations, i.e., Pb and Ca, the TO mode will not soften by the substitution of Pb by Ca because Ca has a larger effective charge and enhanced Ca–O hybridization. Thus, the observed decrease of the LO–TO splitting suggests that the TO mode be contributed dominantly by the force constant between O and B cations, i.e., Ti ions. This argu-ment is consistent with the understanding that ferroelectricity in PTO is due to the off-center displacement of B cation, Ti. Figure 5 displays the Ti L3,2-edge XANES spectra of PCxTO. These spectra are split into L3and L2regions by the

FIG. 1. Normalized O K-edge XANES spectra of the PCxTO (x⫽0 – 1) samples. The dashed line represents a best-fitted Gaussian shape ground. The inset shows the magnified near-edge feature after the back-ground subtraction.

FIG. 2. The O K-edge XANES spectrum共upper solid-line兲 and the calcu-lated O 2 p-, Ti 3d-, Pb 6s p-, and Ca 3d-derived states for Pb0.5Ca0.5TiO3.

The first O K-edge feature has been aligned with that of the calculated O 2 p-derived states. Zero energy is the Fermi level.

3312 Appl. Phys. Lett., Vol. 83, No. 16, 20 October 2003 Janet al.

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spin-orbit interaction and each region contains t2g and eg

splitting by approximately 2 eV due to the crystal-field ef-fect. In these spectra, features A2 and B2 (C2 and D2) cor-respond to the L3 (L2) edge with t2g and eg symmetries,

respectively.17,18 Feature B2 has a splitting of about 0.5 eV

共indicated by vertical lines兲 for PCxTO for x between 0 and 0.3. The inset in Fig. 5 highlights the splitting by subtracting the background using two arctangent functions shown by the dashed lines. Feature B2is contributed by Ti 3d egsubband,

which contains 3dx2⫺y2and 3dz2orbitals. Since 3dx2⫺y2and

3dz2orbitals point to the four side-corner and the two apex O

ions of the octahedron, respectively, the variation of the Ti–O bond lengths due to Ti off-center displacement cause the splitting of feature B2. In contrast, the 3dxy, 3dy z, and

3dzxorbitals of the t2g subband point in directions between O ions, so that feature A2 is not affected. The lack of split-ting of feature D2 may be due to the broadening of this higher energy feature. The similar splitting of 0.5 eV of the

eg subband for x⫽0 – 0.3 suggests that the Ti off-center dis-placement persists up to between x⫽0.3 and 0.4. When x

⫽0.4 and larger, the splitting of feature B2disappears, which suggests the restoration of the Ti ion to the center of the

octahedron. The Ti L3,2-edge XANES results suggest that the ferroelectric property in PCxTO persists only up to a Ca

con-centration between 0.3 and 0.4.

This work was supported by the National Science Coun-cil共NSC兲 of the Republic of China under Contract Nos. NSC 91-2112-M-032-015 and NSC 91-2112-M-032-020.

1

For example, see M. E. Lines and A. M. Glass, Principles and

Applica-tions of Ferroelectrics and Related Materials共Clarendon, Oxford, 1979兲;

J. F. Scott and C. A-Paz de Araujo, Science共Washington, DC, U.S.兲 246, 1400共1989兲.

2C. Kittel, Introduction to Solid State Physics, 7th ed.共Wiley, New York, 1996兲.

3R. E. Cohen, Nature共London兲 358, 136 共1992兲.

4Y. Kuroiwa, S. Aoyagi, A. Sawada, J. Harada, E. Nishibori, M. Takata, and M. Sakata, Phys. Rev. Lett. 87, 217601共2001兲.

5

K. Miura and M. Tanaka, Jpn. J. Appl. Phys., Part 1 35, 3488共1996兲; H. Miyazawa, E. Natori, S. Miyashita, T. Shimoda, F. Ishii, and T. Oguchi,

ibid. 39, 5679共2000兲.

6B. Ravel, Ph.D. thesis, University of Washington, 1997. 7K. Okazaki, Ferroelectrics 41, 77共1981兲.

8

F. M. Pontes, D. S. L. Pontes, E. R. Leite, E. Longo, E. M. S. Santos, S. Mergulha˜, A. Chiquito, P. S. Pizani, F. Lanciotti, Jr., T. M. Boschi, and J. A. Varela, J. Appl. Phys. 91, 6650共2002兲.

9S. Y. Kuo et al.共unpublished兲. 10H. C. Hsueh et al.共unpublished兲. 11

M. C. Payne, M. P. Teter, D. C. Allen, T. A. Arias, and J. D. Joannopoulos, Rev. Mod. Phys. 64, 1045共1992兲.

12K. Asokan, J. C. Jan, J. W. Chiou, W. F. Pong, M.-H. Tsai, H. L. Shih, H. Y. Chen, H. C. Hsueh, C. C. Chuang, Y. K. Chang, Y. Y. Chen, and I. N. Lin, J. Phys.: Condens. Matter 13, 11087共2001兲.

13

F. M. F. de Groot, M. Grioni, J. C. Fuggle, J. Ghijsen, G. A. Sawatzky, and H. Petersen, Phys. Rev. B 40, 5715共1989兲; 48, 2074 共1993兲.

14Table of Periodic Properties of the Elements 共Sargent-Welch Scientific, Skokie, IL, 1980兲.

15

W. Zhong, R. D. King-Smith, and D. Vanderbilt, Phys. Rev. Lett. 72, 3618

共1994兲.

16S. Y. Kuo, C. T. Li, and W. F. Hsieh, Appl. Phys. Lett. 81, 3019共2002兲. 17G. van der Laan, Phys. Rev. B 41, 12366共1990兲.

18

F. M. F. de Groot, J. C. Fuggle, B. T. Thole, and G. A. Sawatzky, Phys. Rev. B 41, 928共1990兲.

FIG. 3. The O K-edge XANES difference intensity curves between PCxTO and PTO.

FIG. 4. Integrated difference intensity curves, over⌬A1⫹⌬C1,⌬B1, and

⌬D1regions vs the Ca concentration.

FIG. 5. Normalized Ti L3,2-edge XANES spectra of PCxTO. The dashed lines represent two best-fitted arctangent functions of the continuum step centered at the maximum height. The inset shows the magnified A2and B2

features after the background subtraction.

3313

Appl. Phys. Lett., Vol. 83, No. 16, 20 October 2003 Janet al.

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數據

Figure 4 shows that the substitution of the A cation, Pb, by Ca not only decreases the O 2 p – Pb 6 p but also O 2 p – Ti 3d hybridization
FIG. 3. The O K-edge XANES difference intensity curves between PC x TO and PTO.

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