Crystal Chemistry of alpha-AlB12 and gamma-AlB12

The structures of alpha-AlB12 and gamma-AlB12 are both characterized by a rigid boron framework derived from a combination of three-dimensional networks of icoasahedral B12 and B20 units. Crystals of gamma-AlB12 (P21212; a=16.57, b=17.51, c=10.14 +) are almost always obtained as syntactically intergrown crystals with alpha-AlB12 (P41212 or P43212; a= 10.16, c=14.27 +) from high-temperature Al-B solutions. Examination of structural details of alpha-AlB12 and gamma-AlB12 demonstrated that there are close similarities between them, notwithstanding marked differences in the crystal data. The syntactic intergrowth between alpha-AlB12 and gamma-AlB12 is always effected so as to have the (101)alpha and the (100)gamma planes parallel to each other and the aalpha or balpha axis and the cgamma axis in the same direction; these planes are identical in unit area and in the orientation inside the intergrown crystals. This fact, as well as the fact that the icosahedral B12 networks of both crystals which are parallel to the syntactic intergrowth plane are identical, may facilitate the formation of the intergrown crystal. Structural details of one crystal are compared to the corresponding ones of the other.