Abstract
Several metal oxide compounds, especially those containing metals possessing several valence states, are able to absorb or release oxygen under suitable thermodynamic conditions. Such behavior is found often in systems containing oxides of transition metals. It is important to note that the equilibrium oxidation level of those metal oxides can depend on the aggregation state, which may significantly impede crystal growth processes from the melt. If during the melt growth of such oxide crystals, the average valence state of the oxides is different in the molten and solid state, then crystallization is connected with the absorption of free oxygen from the ambient gas, or with the release of free oxygen into it. This phenomenon can be detected by simultaneous DTA/TG measurements and can deteriorate the stability of crystal growth. This holds especially if the average valence in the solid is smaller than in the melt, because oxygen release can lead to bubble formation at the crystallization front.
Highlights
IntroductionEquilibria between oxides in the solid (crystalline) and molten state will be described, which are important for the growth of crystals from the melt, e.g., using the Czochralski, Bridgman, or float-zone methods [2,3]
It is important to note that the equilibrium oxidation level of those metal oxides can depend on the aggregation state, which may significantly impede crystal growth processes from the melt
If during the melt growth of such oxide crystals, the average valence state of the oxides is different in the molten and solid state, crystallization is connected with the absorption of free oxygen from the ambient gas, or with the release of free oxygen into it
Summary
Equilibria between oxides in the solid (crystalline) and molten state will be described, which are important for the growth of crystals from the melt, e.g., using the Czochralski, Bridgman, or float-zone methods [2,3]. The Czochralski method is demonstrated first for CaWO4, which is often used for different materials such as LiNbO3, YAG (Y3Al5O12) and other garnets, and partially for α-Al2O3 (usually named sapphire, but “corundum” would be more accurate) [4]. These materials can be grown in different atmospheres, ranging from air to “pure” argon or nitrogen with ca. Of these different conditions, the constituent metal cations prevail almost completely in one oxidation state. These are Ca2+, W6+, Li+, Nb5+, Y3+, or Al3+, respectively
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