A Double Ising Spin model for uniaxially structurally modulated materialsexhibits as a special feature phase transitions between phases with equal wavenumbers but different pseudo spin configurations. The character of these`internal' transitions is investigated in mean field approximation, with themean field transfer matrix method, and in Monte Carlo simulations. Thestructural changes at the transitions are characterized by different strengthsof harmonics in a Fourier analysis of the spatial modulation. A dielectricanomaly in the phase diagram of betaine calcium chloride dihydrate (BCCD) andseemingly contradictory structure analyses are explained.
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