Comment on “Conduction states in oxide perovskites:  Three manifestations ofTi3+Jahn-Teller polarons in barium titanate”

Abstract

In this comment to S. Lenjer, O. F. Schirmer, H. Hesse, and Th. W. Kool, Phys. Rev. B 66, 165106 (2002), we discuss the electronic structure of oxygen vacancies in perovskites. First-principles computations are in favor of rather deep levels in these vacancies, and Lenjer et al. even suggest that the electrons' interaction energy is negative. However, data on electroconductivity are against both these scenarios.It has been well established by gravimetric measurements that the thermal annealing of BaTiO 3 results in reversible changes of the oxygen content. 1,2 Computations in the framework of the shell model 3,4 as well as the quasichemical reactions approach (based on the dependence of conductivity on oxygen partial pressure 5-7 ) showed that, after annealing at 1000 K Ͼ T Ͼ 500 K, the main defects are of Shottky type, and the appearance of oxygen vacancies ͑V O ͒ is much more probable than the vacancies on other sites. This has been confirmed, for instance, by experiments on electroconductivity (see Ref. 8 and references therein) and oxygen diffusion (with isotopes). 9 The increase of the V O concentration increases n-type conductivity. At a large V O concentration, their ordering by the formation of shear planes can take place. 10 Reduction of the samples changes the color from transparent light-yellow to blue, and, at large concentration, to black. There appears characteristic absorbtion of light. 11 Doping the samples with p-type impurities is compensated by the appearance of V O at annealing.Annealing in the oxygen atmosphere decreases n-type conductivity and p-type conductivity can appear at a large exposure time. This is usually interpreted as the presence in the samples (even single crystals) of unavoidable p-type impurities. Depending on the degree of the compensation of V O and p-type impurities, V O can accept two electrons (neutral vacancy), single charged V O + has one electron, and, finally, V O ++ without electrons is double charged. Also, V O + and V O can have a high degree of compensation (hdc) (the Fermi level coincides with the energy level position) or low degree of compensation (ldc) (the Fermi level is at the middle between the energy level and the conduction band bottom. Thus, the oxygen vacancy is a multiple-charge center and, in order to treat the thermodynamics of electrons on such centers one has to consider the grand canonical ensemble. 8 Electroconductivity data show that all these states can be observed if one varies the degree of the defects' compensation 8 (see Table I). Note that these energies were observed in samples having different values of conductivity: very high for V O and much smaller for V O + . These samples had different color: black or gray (very small activation energies inherent to V O ) or light-yellow (comparatively deep levels corresponding to V O + ). By using electron paramagnetic resonance (EPR) spectroscopy, Scharfschwerdt et al. 12 found a signal in reduced BaTiO 3 which they attributed to a symmetry broken state of Ti 3+ bound to V O ++ ͑V O ++ Ti 3+ ͒. S...

Self-Nanostructuring in SrTiO3: A Novel Strategy for Enhancement of Thermoelectric Response in Oxides

“…The air sintered L10 samples were light gray in color; after subsequent annealing under reducing conditions, the samples were black in color with marginally lower density, but generally 95% theoretical and above. The change in color from gray to black reflects the increasing content of Ti 3+ , as a result of sintering under reducing conditions.…”