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Valence band photoemission behavior of Pr1−xSrxMnO3
Materials Science and Engineering B76 (2000) 14 – 17 www.elsevier.com/locate/mseb
Valence band photoemission behavior of Pr1 − x Srx MnO3 Ibrahim Kurash a,*, Liu Fengqin a, Qian Haijie a, Guo Lin a, Xian Dingchang a, Xiong Guangcheng b, Wu Sicheng b a
Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China b Department of Physics, Peking Uni6ersity, Beijing 10087, PR China
Abstract Valence band photoemission experiments on Pr1 − x Srx MnO3 polycrystalline system with x ranging from 0.0 to 0.4 show that the density of states of the bands appearing in the ranges between Fermi level and 12 eV below Fermi level have a substantial change with the doping level x. These features are discussed in terms of charge transfer in the ground states of Pr1 − x Srx MnO3 system. A nonlinear charge transfer with the doping amount x is evidenced and a possible second order phase transition mediated by the charge transfer is proposed. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Valence band; Photoemission; Fermi level
1. Introduction The discovery of negative colossal magnetoresistance [1,2] in rare-earth manganates with the perovskite structure, R1 − x Ax MnO3 with R representing a rare-earth such as La, Pr, Nd, and A a divalent cation such as alkaline earths Ca, Sr, Ba, has attracted wide attention.Without hole doping, the parent compounds at x =0.0, and the end members hole-doped with x= 1.0, viz RMnO3 and AMnO3 are paramagnetic insulators at all temperatures above the antiferromagnetic transition Ne´el points. By substituting trivalent R with a divalent ion A, in Pr1 − x Srx MnO3 for instance it has a fraction of (1−x) in Mn3 + ion state and a fraction of x in Mn4 + state. This mixed valent crystal is metallic because the eg electron can hop from a Mn3 + site to a neighboring Mn4 + site. Due to strong Hund’s rule coupling, the hopping is proportional to the probability amplitude that the initial and final states have the maximum spins. Thus the eg electron kinetic energy favors parallel orientation of the nearest neighbor spins. This is the double exchange of Zener, and of Anderson and Hasegawa [3,4]. The manganates exhibit high resistivities even in the metallic state at low T. Although several microscopic * Corresponding author. E-mail address: [email protected] (I. Kurash)
mechanisms have been proposed to explain the unusual physical properties, a clear self-consistent picture of the electrons responsible for the features such as high resistivity at low T and large magnetoresistance in a small magnetic field at a relatively high T is as yet to emerge [5]. Here we report photoemission results for the valence band of the Pr1 − x Srx MnO3 polycrystalline system with x ranging from 0.0 to 0.4. The results show that at x= 0.3 the electron density of states near Fermi level decrease, but the total density of d electron states increase remarkably by shifting the center of the spectral weight 0.4 eV relative to that of the neighbor compounds toward the higher binding energy side. It seems to exhibit a discontinuous point around x=0.3. Preliminarily, we propose a second order phase transition mechanism, for the appearance of the singularity point for the Pr0.7Sr0.3MnO3 compound, which is associated with the redistribution of electron density of states. 2. Experimental The measurements were performed at the photoemission station at Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences. The experimental setup used for the photoemission investigation has been described in detail
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 9 1 - 3
I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
elsewhere[6]. The ultrahigh vacuum chamber background pressure was at 8 ×10 − 10 Torr and it was at 1× 10 − 9 Torr during the measurement. The Pr1 − x Srx MnO3 polycrystalline sample were prepared using conventional method by mixing appropriate molar amounts of Pr2O3, Mn2O3 and SrCO3 powders. The mixtures were ground and calcined several times and were shaped into rods with Sr substitution level x =0, 0.1, 0.2, 0.3 and 0.4. The X-ray diffraction measurements showed that they were in single phase state and the samples prepared in the same procedure were subject to use as target for thin film preparation and properties investigation [7]. The sample surfaces were scraped before transfer into fast sample entrance chamber and they were again scraped in the sample preparation chamber before transfer into the analytical chamber. The valence band photoemission experiments of Pr1 − x Srx MnO3 samples were performed at 66.2, 82, 127 and 200 eV photon energies using synchrotron radiation as the excitation source. The experimental resolution at these photon energies were estimated to be 0.3 to 0.5 eV for the valence band photoemission.
3. Results and discussion According to the Zaanen – Sawatzky [8] definition on the transition-metal compounds, it has been thought
Fig. 1. (A) Shows PrMnO3 valence band photoemission spectrum measured at 66.2 eV photon energy. The solid circles are experimental data, and the solid line accross through the solid circles is the fitted spectrum. The experimental spectrum is decomposed into four constituents using the Shirley background. The spectrum in (B) is obtained by subtracting the background from Fig. 1A.
15
that Mn oxides are intermediate between the typical Mott–Hubbard-type compounds such as Ti oxides and charge-transfer-type compounds such as Cu and Ni oxides. Nowadays, there are considerable number of experimental results and theoretical calculations on the perovskite-type Mn oxides [9,10] prove that they are located in the charge-transfer-type insulator regime. As it has been shown elsewhere [11, this volume] that the behavior of O 1s core level XPS is conducted by charge transfer character, the valence band photoemission behavior of the Pr1 − x Srx MnO3 system also indicates prominent charge redistribution properties with the doping level x. The valence band photoemission spectrum of PrMnO3 at 66.2 eV, as a prototype of the Pr1 − x Srx MnO3 series, and its decomposed parts are shown in Fig. 1A. A further treated spectra by subtracting the Shirley background from the Fig. 1A are given in Fig. 1B. The following presentation and discussion are all based on the results obtained through the spectra as shown in Fig. 1B and further treatment to them. The net d-electron numbers of the two perovskite end members LaMnO3 and SrMnO3 are 4.5 and 3.8 on their ground states, respectively. This has been reported in a configuration interaction calculation [10] including anisotropy in the metal-ligand hybridization. These appreciable deviations from the nominal d-electron numbers 4 and 3 indicate that the p–d hybridization induces a considerable amount of charge transfer from ligand to metal ions. The hybridization and charge transfer lead to the ground state of LaMnO3 with 50% d4, 40% d5 L6 , and 10% d6L6 2 states and that of SrMnO3 with 40% d3, 50% d4L6 , and 10% d5L6 2 states, showing a predominant p character of doped holes in LaMnO3 and both p and d character to a comparable extent of doped electrons in SrMnO3 [10]. The assignments for peaks a, b, g and their origin are essentially clear from the valence band photoemission experiments and calculations [10,12–14]. The valence band photoemission experiments and theoretical calculation results in manganese perovskites show the contributions from Mn 3d eg and t2g orbitals to the measured density of states near to the Fermi level, correspondingly [10,15]. The eg is about 1 eV wide [15], which makes essential contribution to the peak a. The t2g −eg separation is about 1.5 eV, and the Fermi level is 3.0 eV above the top of the O 2p [15]. The states with mainly p character lie in the region from 4 to 8 eV binding energy below the Fermi level. This spread of oxygen levels over about 4 eV is due to the intrinsic width of oxygen band [16]. In the system of Pr1 − x Srx MnO3, the peak b at about 4 eV is attributed to the electron density coming from t2g states plus contribution from p-like states due to charge transfer. The contribution to the density of state of peak b from the Pr 4f 2 orbitals superimposes with that from the t2g states estimated by
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I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
Fig. 2. Relative ratios of a, b, g and d peaks out of total intensities at 66.2 eV with Sr doping level x. Fig. 2(a) are the a and g peaks’ relative intensities along with the Sr doping level, and in Fig. 2(b) are the b and d peaks’ percentage values. The corresponding peak positions of a, b, g and d intensities in Fig. 2(a, b) are shown in insets of both figures as a guidence. The data of MnO2 are also added to the figures as a reference point for the tetravalent Mn ion.
taking account of the 3.2 eV binding energy of the Pr 4f [17]. The density of state forming the peak g at 6 eV is from the O 2p states. The identification of peak d at 9 eV below the Fermi level is a knotty problem. This is because there have been a lot of experimental results that attribute this peak to different origins. A valence band photoemission study [12] of La1 − x Cax MnO3 films ascribes the peak to the s-bonding state due to Mn – O pds hybridization. Here the covalency behavior of the extensively hybridized Mn 3d-O 2p states and the bonding structure for this oxide that basically involves the pds
interaction of Mn 3d and O 2p electrons are taken into account. The s-type overlap between O ps and Mn eg orbitals results in separated bonding and antibonding state at the extremes in the valence band. A similar peak after the argon ion sputtering for a La0.65Ba0.35MnO3 single crystal film is found due to surface disorder effects [14]. Valence band XPS investigation of La1 − x Srx MnO3 system reveals a peak at 9.5 eV and it is attributed to Mg Ka3,4 satellite of Sr 4p signal at 18.6 eV binding energy [13]. Except for the above mentioned results, there is also experiments that take the 9 eV peak as an indication of the surface contamination of samples [10]. The peak d in the present case is ascribed to the intrinsic feature of the Pr1 − x Srx MnO3 system rather than the extrinsic effects or contaminations. The peak intensity has a slight increase following the increase of the excitation photon energies. The photoionization cross section data of Mn 3d and O 2p [18] at different energies can be used for identification. The O 2p to Mn 3d relative cross section decreases from 141% at 66 eV to 63% at 200 eV. If this peak were from oxygen contamination, the experimental result would take a similar trend to the numerical estimation. The average ratios of the peak at different doping level x to total intensity change from 12.7% at 66.2 eV to 16.4% at 82 eV, and then drop to 13.6% at 200 eV. This shows that the intensity variation with excitation energies is not in a monotone mode. The relatively higher ratio at 82 eV is very likely due to Mn 3s2 – – 3s14p1 photoabsorption resonance enhancement. If so, the peak d has no reason to be excluded from the whole valence band photoemission density-of-states. It is well known that XPS and UPS spectra can be compared directly with valence band density of states dN V 2m 3 D(o) = 2( 2 ) /2.o 1/2, where the o is the energy of d o 2p ' electrons with wavevector k. The experimental D(o) is directly comparable to each other in a valence band photoemission spectrum from different orbitals when the probed sample is monoelement. In a multi-element system such as Pr1 − x Srx MnO3, the experimentally measured density of states (Fig. 1B) must be further corrected for each element’s partial photoionization cross section at determined excitation energy and for element’s content involved in the charge neutralized stoichiometric formula such as Pr1 − x Srx MnO3. In the following discussion the contribution from Pr 4f2 to the valence band photoemission has been estimated and corrected, which varies from 13 to 7% out of total photoionization cross sections ( the sum of Mn 3d, O 2p and Pr 4f photoionization cross sections contributing to the valence band photoemission) at 66 eV with divalent Sr doping amount x. The correction is not performed for the contribution from O 2p around the Pr and Sr atoms to the valence band photoemission
I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
intensity. The charge transfer effects are assumed to play roles only between Mn 3d and O 2p bands, and the density of states contributed from O 2p of Pr–O and Sr–O bands superimpose to the charge transfer. An obvious feature in both Fig. 2(a) and Fig. 2(b) is that the intensities of the a, b, g and d peaks vary not linearly with the Sr doping level. Particularly, as shown in Fig. 2 the intensity of the four peaks of Pr0.7Sr0.3MnO3 varies toward two extrema. In Fig. 2(a) the intensities of a and g peaks drop to the minimum at x =0.3, whereas the intensities of b and d peaks reach to the maximum at the same point. As it has been assumed that the g peak was from ‘pure’ O 2p states and the d peak is of s-bonding state due to Mn–O pds hybridization. The intensity variation of the g and d peaks coincide so well with each other that the decrease of g peak intensity at x = 0.3 corresponds to a parallel increase in intensity of d peak, furthermore these increase and decrease of intensities are far higher or lower than their neighboring points. Simultaneously, the d peak’s binding energy (BE) decreases 0.4 eV and the g peak’s BE increases 0.5 eV relative to their neighboring points. This is in agreement with the intensity variation of g and d peaks that increase of density of states will screen the hole potential which induces a shifting of binding energy toward Fermi level and vice versa. At all other doping levels, as at the x = 0.3, the g and d peaks’ intensities have a one-to-one correspondence relation. The intensity of the b peak reaches the maximum at x =0.3, this can also be understood in terms of charge transfer. A subtle problem is to determine the origin of inducing the a peak intensity variation. Its intensity varies more like that of g peak with O 2p character rather than that of having Mn 3d character. It seems very likely that the a peak’s intensity were directly regulated by the intensity variation of the g peak.
4. Conclusion The valence band photoemission spectra of the Pr1 − x Srx MnO3 system reveal that the intensities of the peaks below the Fermi level change nonlinearly with the Sr doping level x. In addition, there exists an obvious singularity point for all peaks at x = 0.3. This
.
17
was proposed due to charge transfer effects. The nonlinearity of the peaks at x= 0.3 was suggested to be a subsequent effect that mediated by the non-uniform charge transfer process with the divalent metal ion doping, which was preliminarily due to the possibility of a second order phase transition at x = 0.3.
Acknowledgements We are grateful to the Chinese Academy of Sciences for financial support under the grant No. KJ952-S1-418 and National Natural Science Foundation of China (NSFC) under No. 19834050. GCX and SCW would express their gratitude to the support from NSFC.
References [1] R. von Helmholt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990. [3] C. Zener, Phys. Rev. 82 (1951) 403. [4] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [5] C.N.R. Rao, B. Raveau (eds), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific, 1998. [6] S.C. Wu, K. Xun, J.Z. Deng, J. Yao, F.Q. Liu, Z.Q. Wang, R.S. Han, Z.N. Gu, Phys. Rev. B 47 (1993) 13830. [7] G.C. Xiong, B. Zhang, S.C. Wu, Z.X. Lu, G.J. Lian, D.S. Dai, Sol. State Commun. 97 (1996) 777. [8] J. Zaanen, G.A. Sawatzky, J.W. Allen, Phys. Rev. Lett. 55 (1985) 418. [9] E.Z. Kurmaev, M.A. Korotin, V.R. Galakhov, et al., Phys. Rev. B 59 (1999) 12799. [10] T. Saitoh, A.E. Bocquet, T. Mizokawa, et al., Phys. Rev. B 51 (1995) 13942. [11] K. Ibrahim, F.Q. Liu, H.J. Qian, L. Guo, D.C. Xian, G.C. Xiong, S.C. Wu, Mater. Sci. Eng. B76 (2000) 26. [12] J. Zhang, D.N. McIlroy, P.A. Dowben, S.H. Liou, R.F. Sabirianov, S.S. Jaswal, Sol. State Commun. 97 (1996) 39. [13] A. Chainani, M. Mathew, D.D. Sarma, Phys. Rev. B 47 (1993) 15397. [14] C. Waldfied, D.N. McIlroy, S.-H. Liou, R. Sabiryanov, S.S. Jaswal, P.A. Dowben, J. Phys.: Condens. Matter 9 (1997) 1031. [15] J.M.D. Coey, M. Viret, L. Ranno, K. Ounadjela, Phys. Rev. Lett. 75 (1995) 3910. [16] H. Eskes, G.A. Sawatzky, Phys. Rev. B 43 (1991) 119. [17] D. van der Marel, G.A. Sawatzky, Phys. Rev. B 37 (1988) 10674. [18] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tab. 32 (1985) 1.
Valence band photoemission behavior of Pr1 − x Srx MnO3 Ibrahim Kurash a,*, Liu Fengqin a, Qian Haijie a, Guo Lin a, Xian Dingchang a, Xiong Guangcheng b, Wu Sicheng b a
Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China b Department of Physics, Peking Uni6ersity, Beijing 10087, PR China
Abstract Valence band photoemission experiments on Pr1 − x Srx MnO3 polycrystalline system with x ranging from 0.0 to 0.4 show that the density of states of the bands appearing in the ranges between Fermi level and 12 eV below Fermi level have a substantial change with the doping level x. These features are discussed in terms of charge transfer in the ground states of Pr1 − x Srx MnO3 system. A nonlinear charge transfer with the doping amount x is evidenced and a possible second order phase transition mediated by the charge transfer is proposed. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Valence band; Photoemission; Fermi level
1. Introduction The discovery of negative colossal magnetoresistance [1,2] in rare-earth manganates with the perovskite structure, R1 − x Ax MnO3 with R representing a rare-earth such as La, Pr, Nd, and A a divalent cation such as alkaline earths Ca, Sr, Ba, has attracted wide attention.Without hole doping, the parent compounds at x =0.0, and the end members hole-doped with x= 1.0, viz RMnO3 and AMnO3 are paramagnetic insulators at all temperatures above the antiferromagnetic transition Ne´el points. By substituting trivalent R with a divalent ion A, in Pr1 − x Srx MnO3 for instance it has a fraction of (1−x) in Mn3 + ion state and a fraction of x in Mn4 + state. This mixed valent crystal is metallic because the eg electron can hop from a Mn3 + site to a neighboring Mn4 + site. Due to strong Hund’s rule coupling, the hopping is proportional to the probability amplitude that the initial and final states have the maximum spins. Thus the eg electron kinetic energy favors parallel orientation of the nearest neighbor spins. This is the double exchange of Zener, and of Anderson and Hasegawa [3,4]. The manganates exhibit high resistivities even in the metallic state at low T. Although several microscopic * Corresponding author. E-mail address: [email protected] (I. Kurash)
mechanisms have been proposed to explain the unusual physical properties, a clear self-consistent picture of the electrons responsible for the features such as high resistivity at low T and large magnetoresistance in a small magnetic field at a relatively high T is as yet to emerge [5]. Here we report photoemission results for the valence band of the Pr1 − x Srx MnO3 polycrystalline system with x ranging from 0.0 to 0.4. The results show that at x= 0.3 the electron density of states near Fermi level decrease, but the total density of d electron states increase remarkably by shifting the center of the spectral weight 0.4 eV relative to that of the neighbor compounds toward the higher binding energy side. It seems to exhibit a discontinuous point around x=0.3. Preliminarily, we propose a second order phase transition mechanism, for the appearance of the singularity point for the Pr0.7Sr0.3MnO3 compound, which is associated with the redistribution of electron density of states. 2. Experimental The measurements were performed at the photoemission station at Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences. The experimental setup used for the photoemission investigation has been described in detail
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 9 1 - 3
I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
elsewhere[6]. The ultrahigh vacuum chamber background pressure was at 8 ×10 − 10 Torr and it was at 1× 10 − 9 Torr during the measurement. The Pr1 − x Srx MnO3 polycrystalline sample were prepared using conventional method by mixing appropriate molar amounts of Pr2O3, Mn2O3 and SrCO3 powders. The mixtures were ground and calcined several times and were shaped into rods with Sr substitution level x =0, 0.1, 0.2, 0.3 and 0.4. The X-ray diffraction measurements showed that they were in single phase state and the samples prepared in the same procedure were subject to use as target for thin film preparation and properties investigation [7]. The sample surfaces were scraped before transfer into fast sample entrance chamber and they were again scraped in the sample preparation chamber before transfer into the analytical chamber. The valence band photoemission experiments of Pr1 − x Srx MnO3 samples were performed at 66.2, 82, 127 and 200 eV photon energies using synchrotron radiation as the excitation source. The experimental resolution at these photon energies were estimated to be 0.3 to 0.5 eV for the valence band photoemission.
3. Results and discussion According to the Zaanen – Sawatzky [8] definition on the transition-metal compounds, it has been thought
Fig. 1. (A) Shows PrMnO3 valence band photoemission spectrum measured at 66.2 eV photon energy. The solid circles are experimental data, and the solid line accross through the solid circles is the fitted spectrum. The experimental spectrum is decomposed into four constituents using the Shirley background. The spectrum in (B) is obtained by subtracting the background from Fig. 1A.
15
that Mn oxides are intermediate between the typical Mott–Hubbard-type compounds such as Ti oxides and charge-transfer-type compounds such as Cu and Ni oxides. Nowadays, there are considerable number of experimental results and theoretical calculations on the perovskite-type Mn oxides [9,10] prove that they are located in the charge-transfer-type insulator regime. As it has been shown elsewhere [11, this volume] that the behavior of O 1s core level XPS is conducted by charge transfer character, the valence band photoemission behavior of the Pr1 − x Srx MnO3 system also indicates prominent charge redistribution properties with the doping level x. The valence band photoemission spectrum of PrMnO3 at 66.2 eV, as a prototype of the Pr1 − x Srx MnO3 series, and its decomposed parts are shown in Fig. 1A. A further treated spectra by subtracting the Shirley background from the Fig. 1A are given in Fig. 1B. The following presentation and discussion are all based on the results obtained through the spectra as shown in Fig. 1B and further treatment to them. The net d-electron numbers of the two perovskite end members LaMnO3 and SrMnO3 are 4.5 and 3.8 on their ground states, respectively. This has been reported in a configuration interaction calculation [10] including anisotropy in the metal-ligand hybridization. These appreciable deviations from the nominal d-electron numbers 4 and 3 indicate that the p–d hybridization induces a considerable amount of charge transfer from ligand to metal ions. The hybridization and charge transfer lead to the ground state of LaMnO3 with 50% d4, 40% d5 L6 , and 10% d6L6 2 states and that of SrMnO3 with 40% d3, 50% d4L6 , and 10% d5L6 2 states, showing a predominant p character of doped holes in LaMnO3 and both p and d character to a comparable extent of doped electrons in SrMnO3 [10]. The assignments for peaks a, b, g and their origin are essentially clear from the valence band photoemission experiments and calculations [10,12–14]. The valence band photoemission experiments and theoretical calculation results in manganese perovskites show the contributions from Mn 3d eg and t2g orbitals to the measured density of states near to the Fermi level, correspondingly [10,15]. The eg is about 1 eV wide [15], which makes essential contribution to the peak a. The t2g −eg separation is about 1.5 eV, and the Fermi level is 3.0 eV above the top of the O 2p [15]. The states with mainly p character lie in the region from 4 to 8 eV binding energy below the Fermi level. This spread of oxygen levels over about 4 eV is due to the intrinsic width of oxygen band [16]. In the system of Pr1 − x Srx MnO3, the peak b at about 4 eV is attributed to the electron density coming from t2g states plus contribution from p-like states due to charge transfer. The contribution to the density of state of peak b from the Pr 4f 2 orbitals superimposes with that from the t2g states estimated by
16
I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
Fig. 2. Relative ratios of a, b, g and d peaks out of total intensities at 66.2 eV with Sr doping level x. Fig. 2(a) are the a and g peaks’ relative intensities along with the Sr doping level, and in Fig. 2(b) are the b and d peaks’ percentage values. The corresponding peak positions of a, b, g and d intensities in Fig. 2(a, b) are shown in insets of both figures as a guidence. The data of MnO2 are also added to the figures as a reference point for the tetravalent Mn ion.
taking account of the 3.2 eV binding energy of the Pr 4f [17]. The density of state forming the peak g at 6 eV is from the O 2p states. The identification of peak d at 9 eV below the Fermi level is a knotty problem. This is because there have been a lot of experimental results that attribute this peak to different origins. A valence band photoemission study [12] of La1 − x Cax MnO3 films ascribes the peak to the s-bonding state due to Mn – O pds hybridization. Here the covalency behavior of the extensively hybridized Mn 3d-O 2p states and the bonding structure for this oxide that basically involves the pds
interaction of Mn 3d and O 2p electrons are taken into account. The s-type overlap between O ps and Mn eg orbitals results in separated bonding and antibonding state at the extremes in the valence band. A similar peak after the argon ion sputtering for a La0.65Ba0.35MnO3 single crystal film is found due to surface disorder effects [14]. Valence band XPS investigation of La1 − x Srx MnO3 system reveals a peak at 9.5 eV and it is attributed to Mg Ka3,4 satellite of Sr 4p signal at 18.6 eV binding energy [13]. Except for the above mentioned results, there is also experiments that take the 9 eV peak as an indication of the surface contamination of samples [10]. The peak d in the present case is ascribed to the intrinsic feature of the Pr1 − x Srx MnO3 system rather than the extrinsic effects or contaminations. The peak intensity has a slight increase following the increase of the excitation photon energies. The photoionization cross section data of Mn 3d and O 2p [18] at different energies can be used for identification. The O 2p to Mn 3d relative cross section decreases from 141% at 66 eV to 63% at 200 eV. If this peak were from oxygen contamination, the experimental result would take a similar trend to the numerical estimation. The average ratios of the peak at different doping level x to total intensity change from 12.7% at 66.2 eV to 16.4% at 82 eV, and then drop to 13.6% at 200 eV. This shows that the intensity variation with excitation energies is not in a monotone mode. The relatively higher ratio at 82 eV is very likely due to Mn 3s2 – – 3s14p1 photoabsorption resonance enhancement. If so, the peak d has no reason to be excluded from the whole valence band photoemission density-of-states. It is well known that XPS and UPS spectra can be compared directly with valence band density of states dN V 2m 3 D(o) = 2( 2 ) /2.o 1/2, where the o is the energy of d o 2p ' electrons with wavevector k. The experimental D(o) is directly comparable to each other in a valence band photoemission spectrum from different orbitals when the probed sample is monoelement. In a multi-element system such as Pr1 − x Srx MnO3, the experimentally measured density of states (Fig. 1B) must be further corrected for each element’s partial photoionization cross section at determined excitation energy and for element’s content involved in the charge neutralized stoichiometric formula such as Pr1 − x Srx MnO3. In the following discussion the contribution from Pr 4f2 to the valence band photoemission has been estimated and corrected, which varies from 13 to 7% out of total photoionization cross sections ( the sum of Mn 3d, O 2p and Pr 4f photoionization cross sections contributing to the valence band photoemission) at 66 eV with divalent Sr doping amount x. The correction is not performed for the contribution from O 2p around the Pr and Sr atoms to the valence band photoemission
I. Kurash et al. / Materials Science and Engineering B76 (2000) 14–17
intensity. The charge transfer effects are assumed to play roles only between Mn 3d and O 2p bands, and the density of states contributed from O 2p of Pr–O and Sr–O bands superimpose to the charge transfer. An obvious feature in both Fig. 2(a) and Fig. 2(b) is that the intensities of the a, b, g and d peaks vary not linearly with the Sr doping level. Particularly, as shown in Fig. 2 the intensity of the four peaks of Pr0.7Sr0.3MnO3 varies toward two extrema. In Fig. 2(a) the intensities of a and g peaks drop to the minimum at x =0.3, whereas the intensities of b and d peaks reach to the maximum at the same point. As it has been assumed that the g peak was from ‘pure’ O 2p states and the d peak is of s-bonding state due to Mn–O pds hybridization. The intensity variation of the g and d peaks coincide so well with each other that the decrease of g peak intensity at x = 0.3 corresponds to a parallel increase in intensity of d peak, furthermore these increase and decrease of intensities are far higher or lower than their neighboring points. Simultaneously, the d peak’s binding energy (BE) decreases 0.4 eV and the g peak’s BE increases 0.5 eV relative to their neighboring points. This is in agreement with the intensity variation of g and d peaks that increase of density of states will screen the hole potential which induces a shifting of binding energy toward Fermi level and vice versa. At all other doping levels, as at the x = 0.3, the g and d peaks’ intensities have a one-to-one correspondence relation. The intensity of the b peak reaches the maximum at x =0.3, this can also be understood in terms of charge transfer. A subtle problem is to determine the origin of inducing the a peak intensity variation. Its intensity varies more like that of g peak with O 2p character rather than that of having Mn 3d character. It seems very likely that the a peak’s intensity were directly regulated by the intensity variation of the g peak.
4. Conclusion The valence band photoemission spectra of the Pr1 − x Srx MnO3 system reveal that the intensities of the peaks below the Fermi level change nonlinearly with the Sr doping level x. In addition, there exists an obvious singularity point for all peaks at x = 0.3. This
.
17
was proposed due to charge transfer effects. The nonlinearity of the peaks at x= 0.3 was suggested to be a subsequent effect that mediated by the non-uniform charge transfer process with the divalent metal ion doping, which was preliminarily due to the possibility of a second order phase transition at x = 0.3.
Acknowledgements We are grateful to the Chinese Academy of Sciences for financial support under the grant No. KJ952-S1-418 and National Natural Science Foundation of China (NSFC) under No. 19834050. GCX and SCW would express their gratitude to the support from NSFC.
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