Structural transformation in Mg2NiH4 |
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Authors: | J Genossar PS Rudman |
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Institution: | Department of Physics, Israel Institute of Technology, Haifa, Israel |
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Abstract: | X-Ray diffraction measurements show that on heating Mg2NiH4 in a 1 atm pressure H2 atmosphere, above ~250°C it transforms into a cubic structure, metal atoms in CaF2 arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg2NiH4 is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg2NiH4??) phase is not resolved. |
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