Lattice dynamics representation theory versus isotropy subgroup method with application to M5mode instability in CsCl structure

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Abstract
The Landau theory for ferroic phase transitions occurring in materials with CsCl structure (space group O1 h, Pm m) induced by M− 5 mode softening is developed by two different procedures: (i) The physically transparent lattice dynamical approach in conjunction with standard group representation theory, and (ii) the abstract, powerful group theoretical approach employing the concepts of isotropy subgroups and subduction frequencies. These two methods are outlined and their respective merits compared. The Landau free energy is given up to sixth order in the six-component primary order parameter. All 26 symmetry allowed product phases are identified and described in terms of their atomic displacements. Specifically, an improper ferroelastic transition to a tetragonally distorted low symmetry phase (D1 4, 14/mmm) and its atomic displacements observed in x-ray and neutron scattering experiments on LaAg1-x Inx (x ∼ 0.2) are confirmed. For this transition another symmetry allowed secondary order parameter (other than strain) is found that could describe local ordering. Including strain, all 19 symmetry allowed secondary order parameters are identified and their possible physical origin is discussed. In addition, the ‘linear-polynomial’ coupling invariants for each of the secondary order parameters with the primary order parameter are listed.

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