Al-doped SrTiO3: Part I, anomalous oxygen nonstoichiometry

Oxygen nonstoichiometry (δ) was measured on the system of SrTi1−xAlxO(3−x/2)−δ (x = 0.018) as a function of oxygen activity in the range of 10− 18 ≤ Po2/atm ≤ 1 at elevated temperatures of 800° to 1000 °C by a coulometric titration technique with a resolution of ∼ 10 ppm. It has been found that in the stoichiometric regime (δ ≈ 0) where oxygen vacancy concentration is essentially fixed by the majority dopant concentration (2[VO] ≈ [AlTi′]), the isothermal variation of nonstoichiometry deviates anomalously (apparently with three inflections) from the usual, sinehyperbolic trend with a single inflection. This is interpreted as being due to a valence change of variable–valent, background impurities or the like in the minority, and their amount (∼ 80 ppm) estimated from the deviation. Subsequently, the related defect chemical parameters, including the equilibrium constants for the ionization of the unknown variable–valent entities and the hole trapping by the majority dopants AlTi′, are evaluated.