Electrotransport-induced unmixing and decomposition of ternary oxides

A general expectation is that in a uniform oxygen activity atmosphere, cation electrotransport induces a ternary or higher oxide, e.g., AB(1+xi)O(3+delta), to kinetically unmix unless the electrochemical mobilities of, say, A(2+) and B4+ cations are identically equal, and eventually to decompose into the component oxides AO and BO2 once the extent of unmixing exceeds the stability range of its nonmolecularity xi. It has, however, earlier been reported [Yoo et al., Appl. Phys. Lett. 92, 252103 (2008)] that even a massive cation electrotransport induces BaTiO3 to neither unmix nor decompose even at a voltage far exceeding the so-called decomposition voltage U-d, a measure of the standard formation free energy of the oxide (vertical bar Delta G(f)(o)vertical bar = nFU(d)). Here, we report that as expected, NiTiO3 unmixes at any voltage and even decomposes if the voltage applied exceeds seemingly a threshold value larger than U-d. We demonstrate experimentally that the electrochemical mobilities of Ni2+ and Ti4+ should be necessarily unequal for unmixing. Also, we show theoretically that equal cation mobilities appear to be a sufficiency for BaTiO3 only for a thermodynamic reason. (C) 2015 AIP Publishing LLC.