On the Temperature Dependence of an Elastic Constant in the Layered Perovskite-Type Crystals of the (CH₃NH₃)₂MCl₄-Family

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The temperature dependence of the elastic constant c₆₆ of the (CH₃NH₃)₂MCl₄ (M=Mn, Fe, Cu, Cd) is discussed in the relationship with the successive structural phase transitions on the basis of a simple Landau-type thermodynamic potential. It has turned out that a sixth order term in the transition parameters plays an important role in determining the temperature behavior of c₆₆.