The thermodynamics and the decomposition behavior of an MgH2 fiber of a single crystal synthesized by chemical vapor deposition were investigated using in-situ atomic force microscopy and transmission electron microscopy. The MgH2 fiber without oxidation decomposed into polycrystalline Mg with a crystalline size of 20-50 nm. The polyciystalline Mg absorbed hydrogen at a pressure similar to the equilibrium pressure of bulk Mg. On the other hand, the MgH2 fiber with surface oxidation decomposed into Mg crystals larger than 100 nm. The surface oxide layer affected the decomposition behavior of the MgH2 fiber and prevented polycrystallization. These results suggest that surface oxidation increases the equilibrium pressure for hydrogen absorption of the MgH2 fiber.
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