Ca₇Ge-type hydride Mg₆VNa_xH_y (0 鈮?#xa0;x 鈮?#xa0;1): High pressure synthesis, synchrotron X-ray analysis and hydrogen storage properties

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• 作者：N. Takeichi ; J. Yan ; X. Yang ; K. Shida ; H. Tanaka ; T. Kiyobayashi ; ^{kiyobayashi-t@aist.go.jp} ; N. Kuriyama ; T. Sakai
• 关键词：High pressure ; Hydrogen storage ; Rietveld analysis ; Synchrotron X-ray
• 刊名：Journal of Power Sources
• 出版年：2012
• 期刊代码：58_03787753
• 类别：ce
• 出版时间：15 July, 2012
• 卷：210
• 期：Complete
• 页码：158-162
• 文件大小：580 K

摘要

A powder mixture of MgH₂:VH₂:NaH = 6:1:n is treated under 8 GPa at 873 K using an eight-anvil apparatus in order to investigate the influence of NaH addition to Mg₆VH_y, a Ca₇Ge-type FCC hydride. Synchrotron radiation X-ray diffraction (SR-XRD) of the obtained sample reveals that Na occupies the vacant 4b site in Mg₆VH_y to form Mg₆VNa_xH_y (0 鈮?#xA0;x 鈮?#xA0;1) as the main product while retaining its Ca₇Ge-type structure. The Rietveld analysis of the SR-XRD data suggests that the bond lengths between hydrogen and magnesium remain constant through the Na addition. All the samples reversibly desorb and absorb hydrogen at 620-630 and 590-600 K, respectively, under 0.5 MPa (H₂). These temperatures are, respectively, about 70 and 120 K lower than those of MgH₂. The hydrogen capacity of the main product phase, Mg₆VNa_xH_y, is estimated to be 5-6 mass%from the pressure-composition isotherms (PCIs) by taking its content rate in the specimen into account. The reaction enthalpies calculated from the van鈥檛 Hoff relation of the PCIs do not significantly differ from that of MgH₂. The bond lengths and energies between hydrogen and magnesium are not affected by the perturbation by the NaH addition in the lattice in Mg₆VNa_xH_y.