Electronic structure of the valence band for perovskite-type titanium double oxides studied by XPS and DV-Xα cluster calculations

Journnl of Electron Spectroscopy and Related Phenomena, 24 (1981) 149-159 Elsev1er Sclenttf1c Publlsblng Company, Amsterdam - Printed 1n The Netherlands

ELECTRONIC STRUCTURE OF THE VALENCE BAND FOR PEROVSKITE-TYPE TITANIUM DOUBLE OXIDES STUDIED BY XPS AND DV--Xa! CLUSTER CALCULATIONS

HIROHIDE

NAKAMATSU,

HIROHIKO

ADACHI*

and SHIGERO

IKEDA

Department of Chemrstry, Faculty of Sczence, Osaka Unwerszty, Toyonaka, Osaka 560 (Japan) (Fu-st received 1 December 1980,1n

final form 20 April 1981)

ABSTRACT XPS spectra of the valence bands for the perovsklte-type titanium double oxides BaT103, SrnOg and CaTlO have been measured and analyzed by means of DV-XCY calculations for the T106 embedded cluster model The theoretical photoelectron spectra modulated by the photolonlzatlon cross-sections are m good agreement v&h expenment The XPS results show that the 0 2p valence band 1s constructed of two peaks whose spacing becomes larger 1n the order BaTlO < SrT103 < CaT103 The DV-Xa results indicate that the greater part of the lower-energy peak 1s attributed to the levels which have 0 2p orbltals pointing to T1 cations and are thus stab111zed by the electrostatic potential concomitant mth a decreased T1-0 bond distance The electrostatic potential also reduces the 0 2p-T1 3d mixing and, together with the repulsion of the electron cloud between the T1 and 0 ions, makes the T1-0 bond more ionic

INTRODUCTION

The nature of the bondmg between tltamum and oxygen m perovskltetype tltanlum double oxides MT103 1s of great interest because of the relationship between electromc structure and ferroelectrlclty These compounds have been studied by means of various expenmental techniques In general, measurement of core-electron bmdmg eneraes by XPS 1s useful for elucldatmg the chemical states of matenals However, m the present case the chemical shifts of the core levels for the series of compounds did not provide direct mformatlon about the charge dlstrrbutlon and bonding character because of the large relaxation effects [l] Investigations of valence XPS spectra for tltamum double oxides MT103 have been performed by several authors [ 2-431 The XPS spectra show appreciable changes m the *Department

of Nuclear Enpneerlng, Osaka University, Sulta, Osaka 565, Japan

036%2048/81/0000-0000/$02

50 0 1981

Elsevler Scientific Publlshlng Company

150

den&y of states mth different catron M In order to interpret these changes it ISnecessary to study the detailed electronic structures of these compounds Some theoretical studies have been previously reported The general properties of the band structure for perovsklte-type compounds have been dlscussed [ 7-111 and some band calculations have been performed for SrTiO, However, systematic mvestlgatlons for the valence and BaTlO [ 12-141 bands of the series of perovshte-type oxides MT103 have not previously been performed Recently, the discrete vmat1ona.l (DV)--XCY cluster method has been shown vahd for evaluating accurate electron bmdrng energies, and successfully applied m theoretical studies of surface states of SrT103 [ 151 and m photoelectron spectroscopy of various mater&s [16--181 Intensity analyses of valence-band XPS peaks for vmous oxyamons have also shown good agreement with expenment [ 191 In the present paper, we report a measurement of the 0 2p valence-band XPS spectra for the semes of perovsklte-type oxides CaT103, SrT103 and BaTlO Theoretical analyses hf the XPS spectra are also carried out by use of the DV-Xcu cluster method to @ve an interpretation of the variation of the valence-band shape The detzuled electromc structures of these compounds are also discussed

METHODS

Expenmen tal An AEI ES-200 spectrometer was used as the photoelectron analyzer, vvlth an Al Kcu X-ray source, The Kcu3,4 peaks overlapped mth the lowenergy tall m the valence-band spectrum of SrTlO, and mth the high-energy tall m that of BaTlO The mtensltles of the KCX, and Kcq components were estimated as 7% and 3% respectively of that of the Ka,,, mam peak The KCX, and Ka, peaks and the linear background were subtracted from each spectrum In the present work, the full urldth at half-maximum (FWHM) of the Tl 2psf2 peak was 1 4 eV for CaTlO 3 and BaTlO and 16 eV for SrTlO 3 The smgle-crystal SrTlO 3 samples were obtained commercially The samples of BaTlO and CaTlO powder (99 9% purity) were pressed mto discs, and were then smtered at 1620K Before insertion into the spectrometer, each sample disc was scraped mth another of the same composltlon, then subjected to ultrasomc cleanmg m dehydrated hexane, and annealed to red heat m a crucible for more than 20 mm When annealing of the samples was not performed, broadening or wing appeared as shown in Fig 1 for the Sr 3d peaks Other effects on the spectra resulting from these pretreatments have been reported elsewhere [ 5, 201 The samples were not perfectly free from Hz0 and/or O2 contammatlon and ylelded background 0 1s peaks mth 20-30% of the mtenslty of the bulk 0 Is signal This does not affect the valence-band spectra 133

151

l

.

.

’

’ .

(b)

l-

1

I

I

I

-6 -4 -2 Relative Energy

Fig 1 Sr 3d spectra of SrTlO,

I

0 / eV (a) sample without annealmg, (b) annealed sample

Compu tu tlons

The computational delis of the SCC DV-Xa method used m this work have been described elsewhere [ 21-231 Molecular-orbital calculations were made usmg a cluster model for TlO$- v&h Oh symmetry and embedded m a potential field constructed from those of the surroundmg ions mcludrng most of the nearest neighbors of the T10, cluster (MsT160Z4, M = Ca, Sr and Ba) The levels of the simple cluster TlOg’ excluding the surroundmg potent& were also calculated m order to examine the potential-effects from nelghbonng ions Numerical atomic orbltals (ls-4p for T1 and ls-2p for 0) were ut&zed for all of the present calculations The distances between the Tl and 0 ions were taken to be 0 200, 0 195 and 0 192 nm for BaT103, SrTlO, and CaTr03, respectively [ 24, 251 The crystal structure of SrTlO, 1s known to be cubic, while that of BaTlO IS tetragonal (c/a ratio = 1 Ol), and CaTlO forms an orthorhomblc lattice devlatmg slightly from cubic The cluster structure of the sm oxygen ions coordmated to the central TI ion has nearly Oh symmetry Thus we used the TlOt- cluster with Oh symmetry 111calculations for all of these compounds, 1 e assuming that the effects of the lattice dlstortlons were very small For comparison mth expenmenta XPS spectra, the local densltles of

152

states were obtamed by replacing MO levels by dlstrlbutlon functions It IS, however, more reasonable to modulate the den&y of states by takmg account of the photolomzatlon cross-sectlons A simple semlempmcal intensity formula proposed by Gehus [ 261 was used

where Il 1s the probability of photolomzatlon from the Zth MO level, and 0, and P,l are respectively the photolomzatlon cross-section and the gross population of the lth atomic orbital Atomic cross-sectlons for the neutral free atoms are available [ 271 and PZrvalues were obtamed from the DV--Xa cluster calculations RESULTS

AND

DISCUSSION

The 0 2p valence-band spectra for CaT103, SrT103 and BaTlO, are shown m Fig 2 In all cases, the valence-band spectra are composed of two peaks

,fL l

.c

5

.

. 5

.

.

..

. 0.

0.’

.-

h

. . : .

I -8 Fig

2

I

l.

I

I

I

-6 -4 -2 Relative Energy

Valence-band

.

0 I eV

spectra of tltanmm

double

oxldes

153

The peak at high energy 1s straightforward, while that at low energy has shoulders The mtensltles normalized with reference to the 0 Is peak decrease m the order BaTlO > SrT103 > CaTlO The widths of the valence band and the Ti 2p lme for SrT103 are larger than those for the other compounds The spacing between the two overlappmg peaks for BaTlO, 1s clearly small The spacmg for CaTlO 1s a little larger than that for SrT103 The spacmg becomes larger m the order BaTlO < SrT103 < CaTlO Calculations for T106 clusters have been reported previously m studies of vmous compounds [28-301 In order to obtain more accurate electronic structures for these compounds, the potentials of ions surroundmg the cluster should be mcluded The valence levels for the simple cluster TlOzand those obtained m the present calculations for the embedded cluster for SrT103 are compared m Fig 3 However, the effects of the wavefunctlons of the surroundmg ions on the electronic structure of the cluster are consldered to be small m these loruc crystals The valence band of the cluster 1s composed mamly of 0 2p orbltals The highest ltl, and the followmg It?, levels are nonbondmg m character bonding mteractlons between Tl 3d and 0 2p occur m lower levels It 1s found that n-type bondmg 1s involved m ltZg and u-type m 3e, Weak interactions between Ti and 0 orbltals are also found m 6tl, (ppn-type), 5tl,(ppo) and 6a1,(spo) The levels v&h m-type mteractlons are located at higher energies than the u-type levels The addition of the surroundmg potential causes the o-type 5t1,, 3e, and 6a1, levels to shift downwards markedly, as shown m Fig 3 On the other hand, vmatlons of the level energies of the x-type orbltals are rather small The

?il

;o-

(a)

(b)

-

--1

tjg

F

ii!

W z % xi

m-5-

Fig 3 Calculated valence levels for SrTlO 3 (a) TIOZ- cluster, (b) TIO~-cluster m the potential field of SrsTl, 0%

embedded

154 TABLE

1

VALENCE-ORBITAL COMPONENTS FOR EMBEDDED Ti06 CLUSTER AND XPS INTENSITIES CALCULATED FOR SrTlOs

Level

Orbdal

components

0

(%)

Intensity

Tl

2p

2s

3d

ltlg

100

-

-

-

1t2U 6t1, lh 5f1, 3eg 6alg

100 981 92 100 87 1040

-

-0 8 12 --

-

2 1

3p

2 -

60 60 67 69 12 8 52 28

shift of the u-type levels results m a change m the ordenng of the m-type ltzg and the a-type levels, and the band 1s markedly broadened For compmson filth the expemmental spectra, the XPS mtensltles were evaluated usmg the mtenslty formula (1) The gross orbital-population and the mtenslty calculated for each level are given m Table 1 The components of the Tl orbltals are welghted by a factor of two m the mtenslty calculations, because the Tl/O atomic ratio 1s l/3 for the compounds, double that for the present cluster The Tl 3d orbital populations are rather large m the tzg and eg levels but make smaller contmbutlons to the mtensltles, because the okbltal’s cross-section 1sabout the same as that of 0 2p [ a(T13d)/a(O 2p) = 1 4] In previous stu&es, all orbltals except 0 2p and Tl 3d have generally been neglected m the estlmatlon of XPS mtensltles [3, 5, 111, and the o(T13d)/ o(0 Zp) ratio has been estimated to be 3-4 by comparmg the theoretical and expenmental spectra Even the small amounts of 0 2s and Tl 3p components present contnbute effectively to the peak mtensltles, especially for [a,,.(0 2s) = 14 6 Qlu 9 because of the large cross-sections of these orbit& and u,,,(Tl3p) = 28 1, when ~(0 2p) = I] Figure 4(a) shows the density of states (DOS) obtained by convolution of the MO levels replaced by Gausslans w&h FWHM 1 6 eV The theoretical XPS spectrum obtamed by modulatmg the density of states unth the photolomzatlon cross-sections 1s illustrated m Fig 4(b) The expenmental spectrum IS also shown for companson, m Fig 4(c) The spectra of CaTlO and BaTlO, calculated hkemse are similar m shape to that of SrT103 The DOS feature obtamed m this calculation 1s slmllar to that calculated by Matthelss [ 131 The present theoretical XPS spectra are m better agreement with the expenmental spectra than are those from other theories [3,61

155

Cd) lg -10

-5 Relative

Energy

0 / eV

Fig 4 Valence-band spectra of SrTlO 3 (a) calculated den&y of states convoluted by Gausslans (FWHM = 1 6 eV), (b) calculated spectrum modulated by photolomzatlon cross-sections, (c) expenmental spectrum, and (d) calculated levels (lme lengths proportional to state densities)

The expenmental mtensltles of the valence-band spectra decrease m the order BaTlO > SrT103 > CaTlO,, but the calculated mtensltles show neghgble change In the present calculations, atomic orbltals for the Ca, Sr and Ba cations are ignored These orbltals may not affect the level structure However, the cross-sections of catlon mp orbltals (m = 3, 4 and 5 for Ca, Sr and Ba) are about twenty times those of the 0 2p orb&k Thus it 1s possible that they may contribute to the XPS mtenslty of the valence band The decrease of the measured valence-band mtenslty 1s associated urlth the departure of the catlon mp level from the 0 2p level The calculated results concemmg the T1-0 bond character for the embedded T10, cluster are tabulated m Table 2 In E, ,Tz8 representations, the overlap populations become larger as the distance between the Tl and 0 ions decreases (BaTlO 3 + CaTlO 3 ), while the ?1 3d orb&al populations decrease As the distance decreases, the electrostatic field becomes larger This lowers

156 TABLE

2

Tl-0 OVERLAP, TI ORBITAL POPULATIONS AND NET CHARGE ON T1 ION FOR EMBEDDED TlOz- CLUSTER CaTlO 3 Net charge on Tr Tz--0 dlstance(nm)

SrT103

BaTlO

321 0192

315 0195

3 00 0200

El3

Tl 3d Tl-0

0266 0 264

0270 0261

0 280 0259

Tzs

Tl 3d Tl-0

0163 0130

0163 0124

0164 0119

Al,

T14s T1-0

-009 -079

-006 -064

-002 -042

Tt,

Tl4~ Tl-0

-007 -047

-005 -041

-003 -031

the potential at the 0 anion site and rases that at the T1 cation site, which brmgs about an increase of the spacing between the 0 2p and T1 3d levels and then a reduction of the 0 2p-T1 3d muE1ngdespite the mcrease of the orbital overlap On the other hand, m A Ig,T1, representations, bond overlap and T1 orbital populations decrease together The repulsion of the electron cloud between the T1 and 0 ions reduces the T1 4s, 4p populatrons, which have large electron densities 1n the mteratomlc reDon Thus the net charge on the ?k cation becomes larger (1 e , the compounds become more 1omc) as the T1-0 distance decreases The calculated level enerees are shown 1n Table 3 Referred to the nonbondmg levels, the levels, especially those of CJtype such as 6a1,, 3e, and 5tl,, are shifted to lower energes The magnitudes of the shifts are 1n the order BaTlO 3 < SrT103 < CaT10 3, correspondmg to decreasmg T1* distance The level shift 1s due to the strength of the covalent mteractlon and to the potential effect of the neighboring 10ns As discussed above, the electrostatic potent& effects a downward shift of the 0 2p levels Furthermore, 1t stablllzes the o-type 0 2p orb1tals pomtmg to T1 cations more than the other orbltals The greater part of the lower-energy peak may be attributed to the a-type levels The change 1n spacmg of the two peaks which constitute the valence band m the calculated spectra corresponds very well to that m the experimental spectra 1n the order of decreasmg T1-0 distance X-ray emission spectra (XES) are complementary to XPS spectra XES measurements of the T1 K& spectra of the tltaruum double oxides have been reported [ 311 It 1s suggested the spectra consist of two components, namely 5Ll +- T1 1s and 6t,, + T1 1s transitions [ 321 The present calculatxon has

157

TABLE

3

VALENCE ENERGIES

LEVELS FOR EMBEDDED TxO;REFERRED TO THE It,, LEVEL

Level

WeV

)

CaTlO 1t1,

smog

0 00

lhu

-0

6tlu

-139 -2 90 -3 94 -5 01 -5 07

1t2g

5t1, 3% 6a tg

CLUSTER

0 00

-0

84

-1 -2 -3 -4 -4

81 32 79 69 63 87

BaTlO 3 0 00

-0 76 -122 -2 63 -3 11 -4 14 -4 44

revealed that the 5t,, level 1s mcluded m the lower-energy peak of the valence band and the 6t,, level m the higher-energy peak The 5t1, MO has mamly o-type mteractlon between the Tl and 0 ions and the 6t,, MO has mamly P-type The change m valence-band structure observed m the present work should be also found m the Ti Kfl, spectra The Tl K spectra approxlmately reflect the p-state densities according to the selection rule for dipole transltlons From the cluster calculations, it IS found that the Tl p orbltal components m 5t,, and 6t,, are very small, as shown m Table 1 However, it 1s likely that the mteratonuc overlap re@ons 41 contibute considerably to the peak intensity [33]. The Kfl, spectrum of TlO, (rutile) separates rnto two peaks of slmllar mtenslty The Tl ion m rutlle 1s coordmated mth suc 0 ions, sunllar to the arrangement m perovsklte-type oxides If the two translhons have the same probablllty, the peak urldths m the Tl Kps spectra

CaTi Fig

SrTi03

5 (a) Vanatlons

separation

between

5tl,

BaTi

of lmewldth and 6tl,

m TI K@s

emlsslon

spectra,

and (b)

calculated

level

158

should parallel the level spacing between 5tl, and 6t,, This IS supported by the relation between the calculated level separations and the expemmental peak widths shown m Fig 5

ACKNOWLEDGEMENTS

The authors express their thanks to Dr M Murata for helping in the preparation of the samples This work was supported by Toray Science and Technology, Grants 72-1302 The numerical calculations were carned out usmg ACOS NEAC 700 at the Computer Center of Osaka Unlverslty

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429