Electrochemical Single-Crystal Growth of Nonstoichiometric Terbium Oxide

Abstract

Single crystals of some nonstoichiometric terbium oxide phases such as Tb16O30 (TbO1.875; π-phase), Tb24O44 (TbO1.833; β2-phase), and Tb11O20 (TbO1.818; δ-phase) were artificially grown by dc electrolysis of a Tb3+ ion conducting Tb2(MoO4)3 solid electrolyte. Selective growth of a specific phase has been realized by controlling the oxygen partial pressure, the electrolysis temperature, and the voltage applied during the electrolysis. The formation of high-quality single crystals has been evidenced by the selected-area electron diffraction analysis. These nonstoichiometric phases of fluorite-related rare earth oxides have been difficult to grow in a single crystal form because they are usually obtained as mixtures of some RnO2n−2m (n = 7, 9, 11, 12, 16, 19, 24, 29, 39, 40, 48, 62, and 88; m = 1, 2, 3, 4, 6, and 8) phases. However, our dc electrolysis method can be simply applied to grow a specific nonstoichiometric phase in a single crystal form selectively at moderate temperatures at 800 or 900 °C, which had been unrealistic by the conventional growth processes via solidification of the melt at high temperatures above 2300 °C.