Alpha manganese and the frank kasper phases

Abstract

Transition metal alloys often form in a group of structures, known as topologically-closepacked, TCP, systems. The TCP phases, which include the o, A15 and Laves systems among others, form in brittle complex structures. In a classic pair of papers Frank and Kasper defined the TCP systems by considering the packing of twelve and higher-fold triangulated coordination polyhedra, the higher-fold ones involving negative disclinations (''major ligands'' in Frank and Kaspers' notation). Frank and Kasper considered the crystal structures allowed subject to two principles: that lines of disclinations extend all the way across the crystal and that planar atomic layers containing certain patterns occur. The Frank-Kasper polyhedra which arise are the (0,0,12), (0,0,12,2), (0,0,12,3) and (0,0,12,4), i.e. there is no thirteen-fold polyhedron since it would appear that a disclination would have to halt at such a site. Frank and Kasper catalogued a number of structures then known, some subsequently observed and others which have yet to be observed. There is a particular problem with the ..cap alpha..Mn structure which is often lumped in with the TCP phases because of the way its occurrence is juxtaposed with them in sequences of transition metal alloys. However, there are MnIII sites which are reported tomore » have the not allowed 13-fold (as well as other sites with allowed 12 and 16-fold) coordination. Pearson has noted that the MnIII sites have a 14th neighbor at a distance which is intermediate between the first and second shells of neighbors and suggested that the MnIII might lie at a severely distorted Frank-Kasper (0,0,12,2) site.« less