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

摘要

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 transitions, with a monodimensional order parameter. It is shown that both the superconductor alloys of A15 type and some perovskite structures of icosahedral type can be described as phases in these transitions. Besides, some of these compounds present two or more phases, where the above evolution exists in between, for example, the observed quasi-continuous transition from Pm (3) over bar m to Im (3) over bar in the perovskite NaxWO3. In other cases, such transition has not been observed, but we suggest that it could exist, as for instance between the phases Pm (3) over bar m and Pm (3) over bar observed in the A15-type superconductor HgTi3. (C) 1995 Academic Press, Inc. [References: 24]