Yttrium is one of the transition metals and rare earth metals. It does not occur in nature in the elemental state. Minerals containing yttrium (ytter earths) are always conglomerated with other rare earth metals. Relatively stable in air, but darkens under light. At temperatures above 400 °C, fresh interfaces can ignite. Finely divided yttrium is relatively unstable. Yttrium has a low capture cross-section for neutrons.
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