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Photoelectron studies of the BaTiO3 and SrTiO3 valence states
Solid State Communications,Vol. 19, pu. 269—271, 1976.
Pergamon Press.
Printed in Great Britain
PHOTOELECTRON STUDIES OF THE BaTiO3 AND SrTiO3 VALENCE STATES F.L. Battye, H. Höchst and A. Goldmann Fachbereich Experimentaiphysik, Universität des Saarlandes, 66 Saarbrucken, Federal Republic of Germany (Received 25 February 1976 by M. Cardona) The densities of valence states of the ferroelectric materials BaTiO3 and SrTiO3 have been investigated by high resolution X-ray photoelectron spectroscopy. Comparison of the experimental results with various theoretical predictions shows no satisfying agreement. APPLICATION of the photoelectron spectroscopic technique to athe occupied valence states of solid provides means for testing theoretical band materials structure calculations. In this note we report the results of an X-ray photoelectron spectroscopic (XPS) study of the outer electron states of BaTiO 3 and SrTiO3. Both materials crystalize in the cubic perovskite form although BaTiO3 develops slight (~-‘1%) tetragonal distortions on passing into its ferroelectric phase below the Curie ternperature at 120°C.The unusual electric properties of these materials have instigated a number of band structure calculations, thus enabling a comparison of the experimental results with the density of occupied electron states predicted theoretically. Moreover, theoretical studies of surface states on d-band perovskites such as SrTiO3, which are based mainly on the methods for calculating thespeculations bulk electronic properties, provide a foundation for concerning catalytic properties of perovskite surfaces.1 It is therefore of considerable interest to check band structure calculations for this class of compounds. The relevant calculations have been largely based on the linear combination of atomic orbitals (LCAO) scheme, the pioneering work being that of Kahn and Leyendecker2 on SrTiO 3 in which empirical data were used wherever possible to define the parameters. More 3 have performed an “ab initio” recently, Soules eta!. LCAO calculation for SrTiO 3 using a self consistant field procedure. Their basis functions included all Ti is through 3d and 0 is through 2p orbitals, a subsequent investigation indicating negligible error associated with the neglect of Sr orbitals. The number 4ofinbasis functions a treatment in has been further reduced by Mattheiss which various LCAO parameters have been first determined in an augmented plane wave calculation. The final results for the density of states followed variation of the parameters determining the valence/conduction band gap to fit experimental data. A similar procedure was later used by Zook and Casselman.5 The band structure of BaTiO 3 for both cubic and tetragonal phases has been 269
detemiined in a modified 6 LCAO calculation by MichelCalendini Mesnard. In theand present work, experimental data were obtamed using a HP 5950A spectrometer employing monochromatized A1Ka radiation. The instrumental line width was 0.55 eV. The BaTiO3 data were taken from ceramic discs sintered for about 3 hr at 13 50°C from 99.5% BaTiO3 powder. The SrTiO3 samples were cut from commercially available (99.5%) single crystal material. Spectra were acquired after three stages of sample cleaning; firstly, after degreasing with organic solvents, next, after the samples had been scraped prior to insertion into the spectrometer and finally, after scraping or argon ion sputtering in the spectrometer sample preparation chamber. During the in situ scraping procedure, the sample chamberwas operated a pressure 8torr. Argon ion sputtering carriedatout at an Arof lO pressure of about 5 x iO~torr, typically for a few minutes with an ion current of< lOpA. Following cleaning, the sample was transferred to the main chamber at a pressure better than iO—~torr. Since thick (~-~ 0.5 mm) solid samples were used, the maintenance of stable spectra and, hence, of the optimum resolution, required elimination of the effects of charging of the sample surfaces. This was achieved using an electron flood gun. Quite currents (< 50/AA), of thermal energy electrons werelow found adequate to prevent charging induced shifts in the binding energies of several eV as measured with the flood gun switched off. Sample cleanliness was measured at certain stages of the experiments by monitoring the 018 and C~photoelectrons. was found two that018 those BaTiO3 samples not scraped inItsitu yielded peaks separated in energy by 1.8(2) eV which are readily interpreted as arising from the oxygen of the bulk sample and from the oxygen of 02 or water vapor adsorbate respectively. This is indicated by the almost complete disappearance of the peak of higher binding energy following in situ scraping of the sample (the intensity of that remaining was less than 3% of that of the bulk 01~signal). Our
270
BaTiO3 AND SrTiO3 VALENCE STATES Ti3p Sr4s
Sr4p
o
Zook arid Cassetman VB
Ti3p
°-
Bo5s
SrTiO3
SrTiO3
0 2s
_j20
(~I
Bo5p
Soutes et at.
(b) BaTiO3
02s
~ 20
VB
S
Experiment
Ic) 0
40
30 20 10 BINDING ENERGY (eV)
0
F~5
x1 r~r25 ~
Fig. 1. Photoelectron energy distribution curves for SrTiO3 and BaTiO3, obtained with monochromatized A1K,~radiation (raw data). •
Vol. 19, No.3
•
.
.
•
.
interpretation is in 7agreement withofthat by distrion the basis XPSgiven angular Brunnermeasurements. and ThUler It is quite significant that we have bution observed no change in the BaTiO 3 valence band spectrum following the removal of the adsorbate. In contrast, the use of Ar ion sputtering for cleaning the sample surfaces was always accompanied by rather drastic changes of the spectra which were not reversible after re-exposure of the samples to atmosphere. It was thus concluded that the sputtering resulted in a change of surface morphology and/or stoichiometry and hence that the technique was unsuitable for these materials. Adsorption on SrTiO3 surfaces was similarly observed but as a result of the mechanical hardness of the single crystals, its removal by scraping presented greater difficulty. The adsorbate signal therefore remained in this case as large as 30% of that from the bulk. However, as was the case for BaTiO3, the valence band spectrum of SrTiO3 was observed to be insensitive to the extent of adsorption under our experimental conditions. It can therefore be concluded that the samples were always sufficiently clean for the purposes of the XPS experiment, The raw spectra obtained from each material for the binding energy range 0 to 40eV are shown in Fig. 1. No smoothing or background subtractions has been applied, The peak assignments and positions for SrTiO3 agree with those of an earlier XPS determination by Board et a!. 8 although there is a much greater statistical accuracy in the present results. In the case of BaTiO3, the peak assignments based on energyappears values of 9 The are spectrum in the this binding energy region Sevier. qualitatively similar to that reported in reference 7.
Mattheiss
Id)
________________________________ 8
6 ‘. 2 0 BINDING ENERGY (eV)
Fig. 2. Comparison of various density of valence states calculations forwith SrTiO3 (a: reference 5 b: result reference d: reference 4) present experimental (c); 3for details see text. ____________________________ BoTiO3 ~
Michel-C. and Mesnard -
Ib) Experiment
~ BINDING ENERGY 1eV) .
Fig. 3. Comparison of theoretical density of valence states for BaTiO3 (a: reference 6) with present experimental result (b); see text for details. Binding energies of core levels derived from our data are in quantitative agreement with references 8 and 10. Detailed spectra of the SrTiO3 and BaTiO3 valence bands are shown in Figs. 2 and 3 respectively. A simple background subtraction procedure has been used in which the number of scattered electrons at any point is assumed tohigher the integrated numberThe of unscatteredproportional electrons with kinetic energy. spectra have not been smoothed. Strong similarities between
Vol. 19, No.3
BaTiO3 AND SrTiO3 VALENCE STATES
the two valence electron spectra are immediately apparent. In each case, the distribution consists of a central peak with shoulders to each side, although, for BaTiO3, the shoulder of higher binding energy is not strongly pronounced. Although 10 incharge SrTiOtransfer shake-up satellites have been observed 3 the energy separation associated with them is in all cases sufficiently large that we expect no interference with the features of the valence bands now observed, Also shown in Figs. 2 and 3 are3 the density of states Mattheiss,4 and (DOS)and calculations of Soules eta!., Zook Casselman5 for SrTiO 6 for BaTiO 3 and of Michel-Calendini and Mesnard 3. To facilitate comparison between the theory and experiment, the calculated DOS histograms have been convoluted with the spectrometer resolution function and have been positioned on the energy scale to give the best visual fitting to the features of the measured valence bands. For SrTiO3, considerable differences are observed between theory and experiment and, moreover, also be. tween the various theoretical5 distributions. thehave reand of SoulesBoth et a!.3 sults of Zook and Casselman structural features much more widely separated in energy than those of the experimental data. Better agreement with the photoelectron distribution is shown by the result of Mattheiss.4 It has three peaks which can be exactly aligned with the experimental structures although the relative heights are different. For BaTiO 3, the theoretical density of states curve shown features similar to that obtained in reference 4 for SrTiO3 although in this case the energy separation of the peaks is somewhat larger than in the experimental data, and the disagreement of relative intensities is more pronounced. •
.
271
In general, with valence bands of mixed orbital character, photoionization cross section variations can result in intensity modulations which prevent an exact duplication of the density of occupied states. It seems, however, most unlikely, that in the present case this effect can explain the different relative heights in theory and experiment. All calculations for SrTiO3 and BaTiO3 predict a valence band derived from the 0 2p orbitals with the Ti 3d orbitals forming the empty conduction band. Only away from by the symmetry. I’ point areDue mixtures different orbitals allowed to theoflarge energy separation (~4 eV) of the 0 2p and Ti 3d states, one expects, averaged over the whole Brillouin zone, only a very small p—d hybridization. Furthermore, from the data of reference 11 one can estimate that the ionization cross sections for both orbital symmetries do not differ by a great amount. Thus the disagreement of relative intensities in theory and experiment cannot be explained as a result of cross section modulation effects. Instead, it is most likely due to the critical dependence of band integrals. widths and densities of states on the various overlap
Acknowledgement
—
This work was supported by the
Deutsche Forschungsgemeinschaft. One of us (F.L.B.) also acknowledges personal financial support from this organisation. We thank Dr. H. Schmitt and other members thesupply Sonderforschungsbereich 130samples. (Ferroelek. trika)offor and preparation of the Thanks are also due to Prof. Dr. S. HUfner for his continuous support and a critical reading of the manuscript.
REFERENCES 1.
WOLFRAM 1. & MORIN F.J.,App!. Phys. 8, 125 (1975), and references quoted therein.
2.
KAHN A.H. & LEYENDECKER A.J.,Phys. Rev. 135, A132l (1964).
3. 4.
SOULES T.F., KELLY E.J., VAUGHT D.M. & RICHARDSON J.W.,Phys. Rev. B6, 1519 (1972). MATTHEISS L.F., Phys. Rev. B6, 4718 (1972).
5.
ZOOK iD. & CASSELMAN T.N., Surf Sci. 37, 244 (1973).
6.
MICHEL.CALENDINI F.M. & MESNARD G., Phys. Status Solidi. 44, K 117 (1971); J. Phys. C6, 1709 (1973).
7. 8. 9.
BRUNNER J. & THULER M., Helv. Phys. Acta 48, 23 (1975). BOARD R., WEAVER H. & HONIG J.M., in Electron Spectroscopy (Edited by SHIRLEY D.A.). North Holland, Amsterdam (1972). SEVIER K.D., Low Energy Electron Spectrometry. Wiley Interscience, New York (1972).
10.
KIM K.S. & WINOGRAD N., Chem. Phys. Lett. 31, 312 (1975).
11.
HUFNER S. & WERTHEIM G.K., Phys. Rev. B8, 4857 (1973).
Pergamon Press.
Printed in Great Britain
PHOTOELECTRON STUDIES OF THE BaTiO3 AND SrTiO3 VALENCE STATES F.L. Battye, H. Höchst and A. Goldmann Fachbereich Experimentaiphysik, Universität des Saarlandes, 66 Saarbrucken, Federal Republic of Germany (Received 25 February 1976 by M. Cardona) The densities of valence states of the ferroelectric materials BaTiO3 and SrTiO3 have been investigated by high resolution X-ray photoelectron spectroscopy. Comparison of the experimental results with various theoretical predictions shows no satisfying agreement. APPLICATION of the photoelectron spectroscopic technique to athe occupied valence states of solid provides means for testing theoretical band materials structure calculations. In this note we report the results of an X-ray photoelectron spectroscopic (XPS) study of the outer electron states of BaTiO 3 and SrTiO3. Both materials crystalize in the cubic perovskite form although BaTiO3 develops slight (~-‘1%) tetragonal distortions on passing into its ferroelectric phase below the Curie ternperature at 120°C.The unusual electric properties of these materials have instigated a number of band structure calculations, thus enabling a comparison of the experimental results with the density of occupied electron states predicted theoretically. Moreover, theoretical studies of surface states on d-band perovskites such as SrTiO3, which are based mainly on the methods for calculating thespeculations bulk electronic properties, provide a foundation for concerning catalytic properties of perovskite surfaces.1 It is therefore of considerable interest to check band structure calculations for this class of compounds. The relevant calculations have been largely based on the linear combination of atomic orbitals (LCAO) scheme, the pioneering work being that of Kahn and Leyendecker2 on SrTiO 3 in which empirical data were used wherever possible to define the parameters. More 3 have performed an “ab initio” recently, Soules eta!. LCAO calculation for SrTiO 3 using a self consistant field procedure. Their basis functions included all Ti is through 3d and 0 is through 2p orbitals, a subsequent investigation indicating negligible error associated with the neglect of Sr orbitals. The number 4ofinbasis functions a treatment in has been further reduced by Mattheiss which various LCAO parameters have been first determined in an augmented plane wave calculation. The final results for the density of states followed variation of the parameters determining the valence/conduction band gap to fit experimental data. A similar procedure was later used by Zook and Casselman.5 The band structure of BaTiO 3 for both cubic and tetragonal phases has been 269
detemiined in a modified 6 LCAO calculation by MichelCalendini Mesnard. In theand present work, experimental data were obtamed using a HP 5950A spectrometer employing monochromatized A1Ka radiation. The instrumental line width was 0.55 eV. The BaTiO3 data were taken from ceramic discs sintered for about 3 hr at 13 50°C from 99.5% BaTiO3 powder. The SrTiO3 samples were cut from commercially available (99.5%) single crystal material. Spectra were acquired after three stages of sample cleaning; firstly, after degreasing with organic solvents, next, after the samples had been scraped prior to insertion into the spectrometer and finally, after scraping or argon ion sputtering in the spectrometer sample preparation chamber. During the in situ scraping procedure, the sample chamberwas operated a pressure 8torr. Argon ion sputtering carriedatout at an Arof lO pressure of about 5 x iO~torr, typically for a few minutes with an ion current of< lOpA. Following cleaning, the sample was transferred to the main chamber at a pressure better than iO—~torr. Since thick (~-~ 0.5 mm) solid samples were used, the maintenance of stable spectra and, hence, of the optimum resolution, required elimination of the effects of charging of the sample surfaces. This was achieved using an electron flood gun. Quite currents (< 50/AA), of thermal energy electrons werelow found adequate to prevent charging induced shifts in the binding energies of several eV as measured with the flood gun switched off. Sample cleanliness was measured at certain stages of the experiments by monitoring the 018 and C~photoelectrons. was found two that018 those BaTiO3 samples not scraped inItsitu yielded peaks separated in energy by 1.8(2) eV which are readily interpreted as arising from the oxygen of the bulk sample and from the oxygen of 02 or water vapor adsorbate respectively. This is indicated by the almost complete disappearance of the peak of higher binding energy following in situ scraping of the sample (the intensity of that remaining was less than 3% of that of the bulk 01~signal). Our
270
BaTiO3 AND SrTiO3 VALENCE STATES Ti3p Sr4s
Sr4p
o
Zook arid Cassetman VB
Ti3p
°-
Bo5s
SrTiO3
SrTiO3
0 2s
_j20
(~I
Bo5p
Soutes et at.
(b) BaTiO3
02s
~ 20
VB
S
Experiment
Ic) 0
40
30 20 10 BINDING ENERGY (eV)
0
F~5
x1 r~r25 ~
Fig. 1. Photoelectron energy distribution curves for SrTiO3 and BaTiO3, obtained with monochromatized A1K,~radiation (raw data). •
Vol. 19, No.3
•
.
.
•
.
interpretation is in 7agreement withofthat by distrion the basis XPSgiven angular Brunnermeasurements. and ThUler It is quite significant that we have bution observed no change in the BaTiO 3 valence band spectrum following the removal of the adsorbate. In contrast, the use of Ar ion sputtering for cleaning the sample surfaces was always accompanied by rather drastic changes of the spectra which were not reversible after re-exposure of the samples to atmosphere. It was thus concluded that the sputtering resulted in a change of surface morphology and/or stoichiometry and hence that the technique was unsuitable for these materials. Adsorption on SrTiO3 surfaces was similarly observed but as a result of the mechanical hardness of the single crystals, its removal by scraping presented greater difficulty. The adsorbate signal therefore remained in this case as large as 30% of that from the bulk. However, as was the case for BaTiO3, the valence band spectrum of SrTiO3 was observed to be insensitive to the extent of adsorption under our experimental conditions. It can therefore be concluded that the samples were always sufficiently clean for the purposes of the XPS experiment, The raw spectra obtained from each material for the binding energy range 0 to 40eV are shown in Fig. 1. No smoothing or background subtractions has been applied, The peak assignments and positions for SrTiO3 agree with those of an earlier XPS determination by Board et a!. 8 although there is a much greater statistical accuracy in the present results. In the case of BaTiO3, the peak assignments based on energyappears values of 9 The are spectrum in the this binding energy region Sevier. qualitatively similar to that reported in reference 7.
Mattheiss
Id)
________________________________ 8
6 ‘. 2 0 BINDING ENERGY (eV)
Fig. 2. Comparison of various density of valence states calculations forwith SrTiO3 (a: reference 5 b: result reference d: reference 4) present experimental (c); 3for details see text. ____________________________ BoTiO3 ~
Michel-C. and Mesnard -
Ib) Experiment
~ BINDING ENERGY 1eV) .
Fig. 3. Comparison of theoretical density of valence states for BaTiO3 (a: reference 6) with present experimental result (b); see text for details. Binding energies of core levels derived from our data are in quantitative agreement with references 8 and 10. Detailed spectra of the SrTiO3 and BaTiO3 valence bands are shown in Figs. 2 and 3 respectively. A simple background subtraction procedure has been used in which the number of scattered electrons at any point is assumed tohigher the integrated numberThe of unscatteredproportional electrons with kinetic energy. spectra have not been smoothed. Strong similarities between
Vol. 19, No.3
BaTiO3 AND SrTiO3 VALENCE STATES
the two valence electron spectra are immediately apparent. In each case, the distribution consists of a central peak with shoulders to each side, although, for BaTiO3, the shoulder of higher binding energy is not strongly pronounced. Although 10 incharge SrTiOtransfer shake-up satellites have been observed 3 the energy separation associated with them is in all cases sufficiently large that we expect no interference with the features of the valence bands now observed, Also shown in Figs. 2 and 3 are3 the density of states Mattheiss,4 and (DOS)and calculations of Soules eta!., Zook Casselman5 for SrTiO 6 for BaTiO 3 and of Michel-Calendini and Mesnard 3. To facilitate comparison between the theory and experiment, the calculated DOS histograms have been convoluted with the spectrometer resolution function and have been positioned on the energy scale to give the best visual fitting to the features of the measured valence bands. For SrTiO3, considerable differences are observed between theory and experiment and, moreover, also be. tween the various theoretical5 distributions. thehave reand of SoulesBoth et a!.3 sults of Zook and Casselman structural features much more widely separated in energy than those of the experimental data. Better agreement with the photoelectron distribution is shown by the result of Mattheiss.4 It has three peaks which can be exactly aligned with the experimental structures although the relative heights are different. For BaTiO 3, the theoretical density of states curve shown features similar to that obtained in reference 4 for SrTiO3 although in this case the energy separation of the peaks is somewhat larger than in the experimental data, and the disagreement of relative intensities is more pronounced. •
.
271
In general, with valence bands of mixed orbital character, photoionization cross section variations can result in intensity modulations which prevent an exact duplication of the density of occupied states. It seems, however, most unlikely, that in the present case this effect can explain the different relative heights in theory and experiment. All calculations for SrTiO3 and BaTiO3 predict a valence band derived from the 0 2p orbitals with the Ti 3d orbitals forming the empty conduction band. Only away from by the symmetry. I’ point areDue mixtures different orbitals allowed to theoflarge energy separation (~4 eV) of the 0 2p and Ti 3d states, one expects, averaged over the whole Brillouin zone, only a very small p—d hybridization. Furthermore, from the data of reference 11 one can estimate that the ionization cross sections for both orbital symmetries do not differ by a great amount. Thus the disagreement of relative intensities in theory and experiment cannot be explained as a result of cross section modulation effects. Instead, it is most likely due to the critical dependence of band integrals. widths and densities of states on the various overlap
Acknowledgement
—
This work was supported by the
Deutsche Forschungsgemeinschaft. One of us (F.L.B.) also acknowledges personal financial support from this organisation. We thank Dr. H. Schmitt and other members thesupply Sonderforschungsbereich 130samples. (Ferroelek. trika)offor and preparation of the Thanks are also due to Prof. Dr. S. HUfner for his continuous support and a critical reading of the manuscript.
REFERENCES 1.
WOLFRAM 1. & MORIN F.J.,App!. Phys. 8, 125 (1975), and references quoted therein.
2.
KAHN A.H. & LEYENDECKER A.J.,Phys. Rev. 135, A132l (1964).
3. 4.
SOULES T.F., KELLY E.J., VAUGHT D.M. & RICHARDSON J.W.,Phys. Rev. B6, 1519 (1972). MATTHEISS L.F., Phys. Rev. B6, 4718 (1972).
5.
ZOOK iD. & CASSELMAN T.N., Surf Sci. 37, 244 (1973).
6.
MICHEL.CALENDINI F.M. & MESNARD G., Phys. Status Solidi. 44, K 117 (1971); J. Phys. C6, 1709 (1973).
7. 8. 9.
BRUNNER J. & THULER M., Helv. Phys. Acta 48, 23 (1975). BOARD R., WEAVER H. & HONIG J.M., in Electron Spectroscopy (Edited by SHIRLEY D.A.). North Holland, Amsterdam (1972). SEVIER K.D., Low Energy Electron Spectrometry. Wiley Interscience, New York (1972).
10.
KIM K.S. & WINOGRAD N., Chem. Phys. Lett. 31, 312 (1975).
11.
HUFNER S. & WERTHEIM G.K., Phys. Rev. B8, 4857 (1973).