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Colour centres in conducting SrTiO3
K. Blazey, R. Koch, J. Bednorz
To cite this version:
Colour centres in conducting SrTi0
3K. W. Blazey, R. Koch and J. G. Bednorz (*)
IBM Zurich Research Laboratory, 8803 Ruschlikon, Switzerland (*) Laboratorium fur Festkorperphysik ETH, Honggerberg, 8093 Zurich.
Abstract. — Colour centres induced by reduction of SrTi03 have been found with an isotropic EPR absorption at
g = 1.977 and an optical absorption band at 576 nm. These centres are not related to the free carriers but are made possible by the extraction of oxygen from the lattice.
When SrTi03 is reduced, its electrical conductivity
increases [1] with the temperature of reduction and even becomes superconducting on cooling to low temperatures [2]. This conductivity is due to electrons liberated by the removal of oxygen from the crystal lattice. Highly-conducting SrTi03 may also be
pre-pared by doping, for example, with niobium. We have measured the EPR and optical spectra of such conducting SrTi03. The results show the existence
of intrinsic colour centres in conducting reduced SrTiOj.
Crystals of nominally-pure SrTi03 were reduced
between 1 100 °C and 1 400 °C in a Formier gas atmosphere (94 % N2, 6 % H2) and quickly cooled
on withdrawal from the oven. The crystals were then cut into three for EPR, optical and Hall measurements. Due to the high conductivity of the reduced SrTi03
single crystal, EPR was not always possible at our frequency of 13 GHz and therefore powder spectra were measured. In each powder, an isotropic EPR signal was observed at g = 1.997 ± 0.000 4 and 4.2 K. This signal was not seen at room temperature or 77 K, neither in powders of the as-grown material. It could, however, be induced in various other differ-ently-doped and undoped crystals from various sources. Therefore, this signal is thought to be due to an intrinsic centre of reduced SrTi03. In
single-crystal EPR, the isotropic line appeared 44 Oe higher in field at g = 1.977. No line was seen at g = 1.997 until the crystal was crushed when the g = 1.977 signal disappeared. The signals due to the powders are larger than those of the single crystals owing to their larger surface area. Since the powder particle size was less than the skin depth, the approximate concentration of these centres could be determined by comparing the signal strength with that due to a calibrated ZnS : Mn powder, also in the microwave cavity. The variation
of centre concentration with reduction temperature was very rapid below about 1 300 °C and saturated at around 1018 c m- 3 above 1 300 °C.
The optical absorption spectra of these crystals were taken on a Beckmann Acta MVII spectrophoto-meter with the sample mounted in a CTI Spectrim cryocooler. The spectra observed were similar to those reported by Perluzzo and Destry [3]. The infra-red absorption is dominated by the charge carriers,
and the band gap cuts off all transmission beyond 3.25 eV. The window between these two features contained the absorption band at 576 nm which Perluzzo and Destry suggest is due to oxygen diva-cancies. This absorption band was found to have a Lorentzian lineshape of the form
centred on e0 = 2.153 eV and with halfwidth r = 0.341 eV at 25 K. The temperature dependence
of the linewidth r is given by
where the phonon energy hvp = 38.8 meV. Measure-ments on a mono-domain crystal at 25 K show the band is not polarized.
A plot of the maximum absorption coefficient, «ma» against concentration of the EPR centres, N, shows a linear relation between the two, as seen in figure 1. Applying Smakula's formula to these centres,
requires an oscillator strength / x 1 for the optical
JOURNAL DE PHYSIQUL Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-511
Résumé. — Les centres colorés produits par réduction du SrTi03 sont observés par une absorption isotrope en
RPE avec g = 1,977 et une bande d'absorption optique à 576 nm. Ces centres qui sont obtenus par réduction ne sont pas directement liés à la présence de porteurs libres.
C6-512 K. W. BLAZEY, R. KOCH AND J. G. BEDNORZ
absorption band observed, which corresponds to the solid line in figure 1 .
Fig. 1. - Variat~on of optical absorption coefficient, oc,,, with concentration of paramagnetic centres.
To ascertain whether these centres were related with the charge carriers formed in the reduction process, measurements were also made on niobium- doped SrTiO,. The oxidized crystals are highly conducting with about 1020 carriers per cm3, but the g = 1.977 EPR line was at least two orders of magni- tude weaker than that in the reduced samples. Neither was the 576
nm
optical absorption band observable due to the presence of a stronger absorption band centred on 498 nm. Of course, both the 576 nrn optical absorption band and the g = 1.977 EPR line could be induced in the niobium-doped crystals by reduction in just the same way as they could be induced in anyother doped SrTiO, crystal.
Further evidence that these colour centres are independent of the charge carriers was provided by the Hall-effect measurements. A plot of the colour- centre density against carrier concentration as deter- mined by the Hall effect shows neither linear relation nor quadratic dependence, as might be expected if the colour centres are due to divacancies [3]. Therefore, it is concluded that the colour centres in reduced SrTiO, are not determined by the charge carriers but rather the oxygen vacancies play an important role. Even the oxygen vacancies themselves are unlikely to be the actual centres, since they should then appear in greater numbers. Further, it is hard to imagine how a complex such as an oxygen divacancy with a trapped electron can give rise to a single isotropic EPR line. A possible source of these colour centres involves a redistribution of the metal ions during oxygen vacancy formation. This redistribution is then frozen in the crystal with quick cooling. If SrTiO, is only mildly reduced and quenched, EPR shows the pre- sence of Ti3+ on Sr2+ sites, as investigated by Schir- mer and Miiller [4] as well as the g = 1.977 isotropic line. With increasing reduction temperature and quicker cooling rates, the g = 1.977 isotropic line increases, and the Ti3+ on the Sr2+ site EPR absorp- tion decreases. The isotropic centre could, therefore, be due to a greater deviation from the ideal perovskite at high temperatures than the antistructure defect of Ti3+ on the Sr2+ site.
The presence of paramagnetic colour centres in reduced SrTi03 and not in niobium-doped crystals may account, in part, for their different superconduct- ing properties [5].
We should like to thank J. M. Rigotty at IBM T. J. Watson Research Center for making the Hall mea- surements. Stimulating discussions with K. A. Miiller,
G . Binnig, H. Rohrer and W. Berlinger are gratefully
acknowledged. DISCUSSION
Question.
-
J . F . POURADIER.Have any results about other alkaline-earth tita- nates been obtained ? How do they compare to SrTiO, ?
Reply. - K . W . BLAZEY.
Similar centres may be induced in TaTiO,.
Question. -
1. Could you tell me whether your reduction
process is at well-defined conditions of oxygen partial pressure ?
2. Probably not too much weight should be at- tached to the fact that the optical band does not fit the Ivey relation - it would not be the first excep- tion !
Reply. - K . W . BLAZEY.
1. Reduction was also carried out in a H2 atmo- sphere and in ultra-high vacuum. The results are the same in both cases.
References
[ l ] FREDERIKSE, H. P. R., THURBER, W. R . and HOSLEK, W. R., [5] BINNIG, G. and HOENIG, H. E., Verhandl. DPG VZ14 (1979) 403 : Phys. Rev. 134 (1964) A442. BINNIG, G., BARATOW, A. and HOENIG, H. E., Verhandl. DPG VI 121 SCHOOLEY, J. E., HOSLER, W. R . and COHEN, M. L., Phys. Rev. 15 ( 1 980) 424 ;
Lett. 12 (1 974) 474. BINNIG, G., BARATOFF, A., HOENIG, H. E. and BEDNORZ, J. G
.
131 PERLUZZO, G. and DESTRY, J., Can. J. Phys. 56 (1978) 453. to be published.
