E. SCHULZ et al.: X-Ray Spectroscopic Investigations of Titanium Oxides 361

phys. stat. sol. (a) 111, 361 (1989)

Subject classification: 78.70; 71.25; S10

Technikuin Suhl der Technischen Hochschule Ilmenau, Suhl') ( a ) and Sektion Physik der i iartin-Luther- Universitat Halle- Wittenberg, Halle (Saale)2) (b )

X-Ray Spectroscopic Investigations of Titanium Oxides and Basic Materials for Ferroelectric Ceramics

BY E. ScriuLz (a), G. DRAGER (b), W. CZOLBE (b), and 0. BRUMMER (b)

The Ti I< valence band emission spect'ra (VHXES) and the Ti K near edge absorption spectra (XANES) of some polycrystalline titanium oxidm arid perovskit>es, including also the polytitanate Ba,Ti,O,,, are investigated. Characteristic relations between the spectra and the structure and composition of the materials are found. They can be used for fundament'al research of the elec- tronic structure but also for analytical applications a t working with these compounds which are basic mat,erials for ferroelectric and semiconducting ceramics.

Die Ti K-Valenzbandemission (VBXES) und die kantennahen Ti K-Absorptionsspektren (XAKES) von einigen polykristallinen Titanoxiden und -perowskiten, einschlieBlich auch des Polytitanats Ba,Ti,O,,, werden untersucht. Charakteristische Zusammenhange zwisehen den Spektren und der Struktur und Ziisammensetzung der Materialien werden gefunden. Sie kiinnen fur Grundlagen- untersuchungen zur elektronischen Struktur genutzt werden, aber auch fur analytische Zweckc bei der Arbeit mit diesen Verbindungen, welche Basismaterialien fur ferroelektrische und halb- leitende Iieramiken darstellen.

1. Introduetion

Ceramic materials enter perniarieritly new fields of practical application. For a better understanding of their fundamental properties basic research on these materials is necessary. The objects of our investigations are some polycrystalline titanium oxides and perovskites serving as basic materials for ferroelectric arid semiconducting ceram- ics. In particular, these compounds are TiO, Ti,O,, TiO, (rutile), BaTiO,, SrTiO,, Ba, $r, $a, ,TiO,, Ba'Fi,.,,,O,, and also the polytitanate Ba,Ti,O,,.

T n all of these compounds the Ti atoms are coordinated by six nearest oxygen atoms in an ideal or distorted octahedron. Greater differences exist in the arrangement and in the species of the next nearest and further neighbour atoms. These differences cause also a different electronic behaviour.

Some remarkable properties of these coinpounds are the existence of a ferroelectric phase and the so-called PTCR-effect (positive temperature coefficient of electric resistance [l]) in some of these materials. It is of theoretical and practical interest to explain such phenomena and to make proposals for realizing these and other effects in practice. An important step in this direction is the investigation of the crystal structure and of the electronic properties. Different methods are used for this purpose which complement each other. In this paper some results will be obtained by means of the high resolving X-ray emission and absorption spectroscopy. .~

l) PSF 142, DDR-6000 Suhl, GDR. 2, PSF, DDR-4010 Halle (Saale), GDR.

362 E. SCHULZ, G. DRKGER, IT. CZOLBE, and 0. BR~~MRIER

2. Mothod

The method is based on the measurement of the valence band X-ray emission spectra (VBXES) and of the X-ray absorption near edge structure (XANES) of polycrystallirie materials. In our paper we will concentrate on the Ti K spectra.

For the VBXES the intensity I ( w ) of the emitted radiation with frequency (u can be written in the form

I ( w ) - C O ~ M Z , ~ ( E ) N p ( E ) S ( E - E K - hw) , (1) where E denotes the binding energy of the valence states and E, that one of the K- core level, respectively. The formula means that the intensity I ( w ) provides a repro- duction of the local partial density of states N J E ) of the valence band states with p-symmetry modulated by the radial transition probability M:, J E ) . The latter changes slowly for emission spectra in the energy range of interest and produces no additional structures of the spectra. For the XANES the absorption coefficient p(w) can be written in a form similar to (I),

( 2 ) p(u1) - wM:,p(E) N p ( E ) 8(E - E K - am) . But there exist some differences between the emission and absorption spectra. First of all the latter ones reflect the local partial density of unoccupied states in the en- vironment of the absorbing atom. Furthermore, because of the missing K-electron, this density of states need not agree with that of the ground state band structure. There exists no simple rule to estimate the influence of the remaining core hole on the density of empty states. l n some cases also the so-called core excitons are discussed which can be found experimentally below the conduction band.

Secondly in the absorption spectra the energy dependence of the transition prob- ability cannot be neglected in general. Only for the first 10 to 15 eV above the ab- sorption threshold the fine structure can be interpreted unambiguously in terms of the local density of states. At higher energies the influence of the transition probability increases and the interpretation of the experimental spectra on the basis of ( 2 ) is no longer correct. On the other hand, the absorption spectra depend on the final state wave functions and, therefore, they contain information on the arrangement of the atoms in the sample material. So the extended X-ray absorption fine structure (EXAFS) enables to investigate the local environments of the absorbing atoms by using special mathematical methods. From the XANES region further information can be obtained such as bond angles and the symmetry of the arrangement of the adjoining atoms. TO get this information i t is necessary either to compare the spectra with those of well-known substances or to interpret them by means of theoretical spectra calculated for model substances.

3. Experimental

The spectra were measured with a Johann-type spectrometer [a ] suitable now for recording VBXES and XANES. An X-ray tube with a chromium anode (40 kV, 45 m-4) and a tungsten tube (7.4 keV, 35 mA) served as radiation sources for the excitation of the emission spectra and for the absorption spectra, respectively. The radiation was analyzed with a quartz crystal (101 1) in the second order of reflection. The spectrometer operated in a vacuum vessel. The absorption spectra were taken in an air atmosphere of about 1 3 kPa causing a sufficient discrimination of the continuous radiation obtained from the first-order reflection. The samples for the emission ex- periments were pellets of polycrystalline material, pressed and partly sintered. For

X-Ray Spectroscopic Jnvestigntions of Ti Oxides for Ferroelcctric Ceramics 363

the XANES measixremeiits thin absorber foils were prepared by spreading fine powder materials onto adhesive tape. The spectra recorded on X-ray film were transformed into digital intensities and further treated by means of a photometer coupled with a control and computer unit. For better comparability all emission and absorption spectra were normalized in the same way. This means for the VBXES that all spectra were fitted on the high-energy tail of the normalized Ti KP,,, peak which appears a t the low energy side of the TiKP,,, valence band spectra. On the other hand, all XANES data were normalized to the same absorption step which is the difference iri the average absorption over a wide range of energies below and above the absorption edge.

4. Results and Discussion

In Fig. 1 the measured Ti K VBXES and XANES data are presented in a commoii energy scale.

The low-energy satellite Ti KP" represents a tightly bound subband with mainly 0 2s character and some content of T i4p states. In terms of the MO theory these levels have tl, symmetry [3]. For the compounds with Ti4+ ions and having a Ti-O distance of 0.2 nm arid smaller the mixing of Ti 4p states into this band is considerable (for example in BaTiO,, TiO,, and Ba,Ti 0 ). In the spectra of the other compounds with a lower oxidation state of Ti the Ti KP satellite is not clearly visible iridicaatirig 29, that the 0 %-Ti 4p mixing is only n-eak.

I 1 I 1 I I i n

KPZ5 K/3" A

4970 4990 500 hw (eV) _ _ ~ - 4953

c

Fig. 1. The Ti Is. valence band emis- sion (VBXES) and the X-ray absorp- tion near edge structure (SASES) of some titanium oxides and titanatrs

364 E. SCHULZ, G. DRHGER, 'CV. CZOLBE, and 0. BRUMMER

The Ti KP,,, band represents the states with most Ti p character within the valence band. The peak shifts to higher energies when the ionic charge increases from Ti2' to T i 4 ~ . This chemical shift is caused by the reduced screening charge around the Ti core orbitals. The small peak and the shoulder a t the top of the Ti KP,,, emission of T i0 indicates a strong p-d mixing as a result of destroying the inversion symmetry of the Ti lattice sites a t higher oxygen defect concentration. For some compounds the experimental VBXES data were fitted to theoretical band calculations 14 to 71. The positions of the valence and conduction hand edges E, and E,, respectively, and of the Fermi level EB found by this procedure are marked in the spectra of Pig. 1. They are in realistic positions confirming by this the results of the band calculations.

Within the first conduction band states extending over a few eV the Ti 3d character dominates. Therefore, the absorption near E, or EF is weak. Under the influence of the crystal field the Ti 3d bands are split. I n the case of TiO, and the perovskites the resulting higher teg and lower eR bands corresponding to a nearly octahedral crystal field can be clearly distinguished. They are reproduced in the spectra by quadrupole transitions and/or dipole transitions where the latter ones can result from a p-d mixing by hybridization. Additionally, for these compounds a weakly pronounced shoulder nearly 1 eV below E, is visible. According to Balzarotti et al. [S] and Grunes [9] i t may be a result of the so-called core excitons.

The less ionic compounds Ti,O, and Ti0 have no clearly resolved fine structures a t the begiuning of absorption. This fact can be qualitatively explained by the smaller band splitting due to the lower bonding ionicity and the greater atomic distances. Moreover, the Ti d-like bands are filled with one or two additional electrons, which will reduce the absorption strength because of the decreased number of free electron states in this region.

The absorption structure a t higher energies is very different even for substances with similar electronic structure and with siiiiilar first neighbour coordination. For example, the pronounced absorption peak a t 4985.5 eV exists only for the IJa perovs- kites and may result from the existence of Ba 4f states a t this conduction band energy [lo]. It must be concluded that the structures in the energy region from 10 to 40 eV above the absorption threshold are strongly affected by the species, number, and arrangement of atoms in the second and possibly further coordination shells. The same result was obtained from theoretical calculations : only the complete considera- tion of the second shell atoms makes it possible to reproduce the experimental Ti K absorption spectra of Ti0 theoretically (Kutzler and Ellis [ll]). Another question is, how do little variations of the composition influence the X-ray spectra. Because of the wealth of resolved structures in the XANES region this energy range seems to be suitable for giving an answer to the question. An additional Ti content of 40,; in BaTiO,, for example, causes a significant decrease of the peak a t 4955.5 eV and a smearing of the absorption valleys at 4981 and 4990 eV. There is a similar effect of the substitution of Ba atoms hy Sr and Ca atoms (Fig. 1). A quantitative analysis has shown that the normalized Ti K XANER of Ba,,,Sr,,,Ca, ,TiO, can be reproduced iii a good approximation by a weighted superposition of the normalized spectra of BaTiO,, SrTiO,, and CaTiO,. If we suppose an ideal random distribution of the Ba, Ca, and Si atoms on their lattice sites in this mixed compound, the additivity of the spectra confirms the following two assumptions : Firstly, the little differences in the crystal structure (BaTiO,, CaTiO,, and SrTiO, crystallize in three different space groups) have no significant influence on the spectra. Secondly, the final state wave function of the excited electron is riot strongly affected by multiple scattering proces- ses between two atoms of the group Ha, Ca, Sr.

X-Ray Spectioscopic Investigations of Ti Oxides for Ferroelectric Ceramics 365

Fig. 2. Tlie measured Ti K XBXES of Ba,Ti,O,, (a) in comparison to the

BaTiO, and TiO, (rutile) XASES (h )

1---. + r-r-- 1 I neighted average of the experimental I

a

I n Fig. 1 there are also seen the spectra of the polytitariate Ba,Ti,O,,. The sirnilari- ties of the Ti VBXES and of the t z r c and eK pre-edge absorption striictures to those of Ti02 (rutile) and the perovskites point to corresponding sirnilarities of the electronic structure and therefore also of the Ti4+ coordination. The conclusion is obvious that the first ueighbours of the Ti atoms are also oxygen atoms in more or less distorted octahedra. But what can be concluded about the arrangeiiicnt of the inore distant ncighbour atoms '8

T n Fig. 2 the XANES of the polytitanate Ba,?'i,O,,, is conipared with that of a weighted average of the TiO, arid BaTi0,XAXES in the ratio 7 : 2. This corresponds to a formal composition of Ba,Ti,O,,, from seven parts of TiO, and two parts of BaTiO,. Especially in the energy range froin 4985 to 4995 eV there are some pronounced differences in the absorption structures, lying beyond the error bars. They should be the result of a modified configuration of the next nearest neighbours of Ti, which are the Ba and other Ti atoms [12]. In this way from the experimental Ti K XAKES follows that the considered polytitanate will not be a simple mixing of Ba'l'iO, and TiO, phases but represents a new phase. For getting inore information about this i t should be successful to compare the experimental spectra with the results of XANES model calculations a t clusters containing the first and several different second coordi- nation shells around the Ti atoms. By this not only EXAFS but also XXNES can contribute to investigate complicated crystal structures or other atomic arrangements.

References

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[9] L. A. GRUNES, Phys. Rev. B 2 i , 2111 (1983). State Commun. 36, 145 (1980).

[lo] V. V. NEMOSHKALENXO and A. N. TIMOSHEVSKII, phys. stat. sol. (b) 127, 163 (1985). [ll] F. W. KUTZLER and D. E. ELLIS, Phys. Rev. €3 29, 6890 (1984). [12] E. TILLMANNS and W. HOFMEISTER, J. Amer. Ceram. Soc. 66, 268 (1983).

(Received September 13, 1988)