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Single crystals of Mg9Sn5 have been obtained from the elements by high-pressure high-temperature reaction in a modified Belt-type apparatus (Ta capsule, 50 kbar, 1400→500 °C, and subsequent quenching to ambient conditions). The compound crystallizes trigonally, space group R3d, with a = 13.222(2), c = 13.150(2) A , c/a = 0.9946 and Z = 6. Final residuals after refinement against F2 were R1 = 0.0297, wR2 = 0.0811 and S = 0.927 for 85 refined parameters and 975 unique reflections. The crystal investigated showed obverse-reverse twinning and a pronounced pseudo-symmetry. The crystal structure of Mg9Sn5 comprises a framework of magnesium atoms with an arrangement similar to that found in Fe2P. The tin atoms are accommodated in tunnels running along the c-axis. Mg9Sn5 is at least very similar, perhaps even identical, with the “dense form of Mg2Si”, Mg2Si-H, described by Cannon and Conlin (1964) and Dyuzheva et al. (1972, 1995). |
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