A MODEL OF DISPLACIVE CRYSTALLOGRAPHIC PHASE-TRANSITION FOR A15-TYPE SUPERCONDUCTOR ALLOYS AND FOR ICOSAHEDRAL-TYPE PEROVSKITES

A continuous polyhedral evolution of octahedron through icosahedron to cuboctahedron, previously described, is extended in the present paper to primitive and body-centered cubic crystal lattices. The evolution of these lattices describes displacive crystallographic phase transiti ons, with a monodimensional order parameter. It is shown that both the superconductor alloys of A15 type and some perovskite structures of i cosahedral type can be described as phases in these transitions. Besid es, some of these compounds present two or more phases, where the abov e evolution exists in between, for example, the observed quasi-continu ous transition from Pm (3) over bar m to Im (3) over bar in the perovs kite NaxWO3. In other cases, such transition has not been observed, bu t we suggest that it could exist, as for instance between the phases P m (3) over bar m and Pm (3) over bar observed in the A15-type supercon ductor HgTi3. (C) 1995 Academic Press, Inc.