STUDIES ON AB(2)-TYPE INTERMETALLIC COMPOUNDS .5. THE CRYSTAL-STRUCTURE OF MG9SN5, A SUPPOSED HIGH-PRESSURE MODIFICATION OF MG2SN

Single crystals of Mg9Sn5 have been obtained from the elements by high -pressure high-temperatures reaction in a modified Belt-type apparatus (Ta capsule, 50 kbar, 1400-->500 degrees C, and subsequent quenching to ambient conditions). The compound crystallizes trigonally, space gr oup R3, with a = 13.222(2), c = 13.150(2) Angstrom, c/a = 0.9946 and Z = 6. Final residuals after refinement against F-2 were R(1) = 0.0297, wR(2) = 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 com prises a framework of magnesium atoms with an arrangement similar to t hat found in Fe2P. The tin atoms are accommodated in tunnels running a long the c-axis. Mg9Sn5 is at least very similar, perhaps even identic al, with the ''dense form of Mg2Si'', Mg2Si-II, described by Cannon an d Conlin (1964) and Dyuzheva et al. (1972, 1995).