The crystal-structural transformation NaCl-type → CsCl-type: analysis by martensite theory

Abstract

Orientation relations, interface (habit plane) orientations, and shape changes have been computed by martensite theory for the crystal-structural transformation NaCl-type (f.c.c.) → CsCl-type (primitive cubic), especially in alkali halides. The lattice correspondence used involves contraction along the three-fold axis of a primitive rhombohedron of f.c.c. For the matrix analysis a computer program was written in FORTRAN. Mathematical solutions were obtained using two lattice-invariant shears based on slip and one based on transformation twinning. There are three types of solution. Within each of the three types multiplicity due to symmetry leads to 24 variants. Possible habit planes are near to {\bar 111}I, {210}I and {310}I. Shape changes are large. Predictions agree with observations within experimental error. As the principles applied need not exclude some changes of stoichiometry, they may be relevant to topotaxy.