The samson phase, β-Mg2Al3, revisited

The Al-Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, beta-Mg2Al3 (cF1168). For the composition Mg38.5Al61.5, this cubic phase, space group Fd (3) over barm (no 227), a = 28.242(1) angstrom, V = 22526(2) angstrom(3), undergoes at 214 degrees C a first-order phase transition to rhombohedral beta'-Mg2Al3 (hR293), a 19.968(1) angstrom, c = 48.9114(8) angstrom, V = 16889(2) angstrom(3), (i.e. 22519 angstrom(3) for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd (3) over barm. The structure of the beta-phase has been redetermined at ambient temperature as well as in situ at 400 degrees C. It essentially agrees with Samson's model, even in most of the many partially occupied and split positions. The structure of beta'-Mg2Al3 is closely related to that of the beta-phase. Its atomic sites can be derived from those of the beta-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the beta'-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the beta- to beta'-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.