Structural Unit in Icosahedral MnAlSi and MnAl

Experimental evidence from extended x-ray-absorption fine-structure measurements indicates that a cage of Mn atoms at the vertices of an icosahedron is the structural unit in the icosahedral MnSiAl and MnAl phases. The connections among these icosahedral units and between them and the Al atoms are different in the icosahedral and $\alpha$ phases. It is suggested that the $i$ phase grows by the random nucleating together of Mn icosahedra along their twenty threefold directions, as allowed by local steric constraints.