Electrochemical single-crystal growth of nonstoichiometric terbium oxide

Single crystals of some nonstoichiometric terbium oxide phases such as Tb16O30 (TbO1.875; pi-phase), Tb24O44 (TbO1.833; beta 2-phase), and Tb11O20 (TbO1.818; delta-phase) were artificially grown by dc electrolysis of a Tb3+ ion conducting Tb-2(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; In = 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 degrees C, which had been unrealistic by the conventional growth processes via solidification of the melt at high temperatures above 2300 degrees C.