We report the structural, optical, ferroelectric, and dielectric properties of reduced BaTiO3 samples. For this purpose, oxygen vacancies in BaTiO3 are created by heating these samples with a Ti metal in a vacuum environment at different temperatures. It is observed that with an increase in oxygen deficiencies, the c/a ratio decreases as compared to that of the oxygen treated sample. The ferroelectric properties of the oxygen deficient samples are visibly different as compared to those of the oxygen treated sample. The disappearance of the P-E loop and the anomaly in the temperature variation of the dielectric constant have been observed; however, the structural phase transition corresponding to ferroelectric phase transitions still persists. Thus, it appears that the anomaly in dielectric data and the presence of the P-E loop are getting masked possibly by the Maxwell-Wagner effect. The presence of Ti+3 states in the prepared samples has been confirmed by X-ray absorption near edge structure measurements. The Kubelka-Munk optical absorption shows the presence of extra states below fundamental transition, indicating the emergence of new electronic states within the bandgap, which might be due to Ti+3 states. These new states appear at different energy positions, and with different intensities for different samples, which are reduced in the presence of Ti. These new states within the bandgap appear to modify the electronic structure, thereby reducing the overall bandgap, and hence, they seem to modify the ferroelectric and dielectric properties of the samples. Our results may be treated as experimental evidence for theoretically proposed defect states in oxygen deficient or reduced BaTiO3.

No abstract

The origin of ferroelectricity in the cubic phase of BaTi1−x Hf x O3 has been investigated. The presence of well-defined ferroelectric polarization versus electric field (PE) hysteresis loop in the samples with global cubic symmetry suggests the presence of ‘local polar regions’, induced possibly due to the huge difference in the electronegativity and also difference in the ionic radii of Hf+4 and Ti+4 ions, which may lead to local structural disorder. The presence of polar regions is also supported through the appearance of A1(TO) polar mode in Raman spectra which in principle should be absent in the samples with cubic symmetry. The results are discussed in terms of disorder-induced local dipoles due to the electronegativity difference between Hf and Ti ions.