Landau-Ginzburg model of interphase boundaries in CsCl-type ferroelastics due to M(5)(-) mode instability: LaAg1-xInx

We have constructed a Landau-Ginzburg model of ferroelastic domain walls in the tetragonal phase of CsCl-type intermetallic compounds. This improper ferroelastic phase transition is driven by the condensation of a degenerate zone-edge phonon mode of M(5)(-) symmetry and can be described by a six-component order parameter. Analytic and numerical kink-type soliton solutions for the order parameter profile and the strain distribution are obtained for three different interphase boundaries: a twin boundary and two antiphase boundaries. The stability, merging and splitting of various domain types are also studied. Specifically, a symmetry-allowed product phase of I4/mmm (D-4h(17)) symmetry has been experimentally observed in pseudobinary rare earth alloys of composition RAg(1-x) In-x (R=La, Ce, Pr) and in related systems (YCu, LaCd).