Twin wall of proper cubic-tetragonal ferroelastics

We derive solutions for the twin wall linking two tetragonal variants of pr oper cubic-tetragonal ferroelastics, including the dilatational and shear e nergies and strains. Our solutions satisfy the compatibility relations exac tly and are obtained at all temperatures. They require four nonvanishing st rains except at the Barsch-Krumhansl temperature T-BK (where only the two d eviatoric strains are needed). Between the critical temperature and T-BK, m aterial in the wall region is dilated, while below TBK it is compressed; we estimate a compression of similar to1% for Fe-Pd alloys. In agreement with experiment and more general theory, the twin wall lies in a cubic 110-type plane. We obtain the wall energy numerically as a function of temperature and we derive a simple estimate which agrees well with these values.