Electrical and optical characterization of undoped BaTiO(3) in the quenched state

On undoped polycrystalline BaTiO(3-delta) (99.9% nominal purity), that had been equilibrated and quenched at 1000 degrees C under different oxygen activities in the range of -19.4 < or = log a(O(2)) < 0.1, were measured optical reflectance spectra at 298 K and ac conductivity at 473, 523 and 573 K, respectively. It was observed that all specimens quenched at log a(O(2)) > -17 are electrically insulating and white in color, once powdered, and show optical absorption only at the absorption edge at approximately 3 eV. In contrast, those quenched at log a(O(2)) < -17 are electronically conducting and dark and exhibit a significant absorption in the IR region. This demarcating oxygen activity for the conductivity transition is ca. 13 orders of magnitude lower than that for the n-to-p type transition (delta = 0) in the equilibrium state, contrary to expectation. It is, thus, suggested that contrary to the textbook knowledge, the hole traps responsible for the conductivity transition in undoped BaTiO(3) may be variable-valent acceptor defects like Mn(Ti). The strong IR absorption in the semiconducting state is ascribed to small-polaron (Ti'(Ti)) hopping processes.