Interpretation of the BCCD-phase diagram within the frame of an ANNNI-model with pseudo-spin-phonon-coupling

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Abstract
The experimentally observed T, p-phase diagram of betaine-calcium-chloride-dihydrate is interpreted in terms of an ANNNI-madel with interactions JO, J1, and J2 depending on pressure and temperature. The assumption that the Jv depend on T and p via the average lattice constant leads to an understanding of the main characteristics of the phase diagram, including the occurrence and sequence of modulated and incommensurate phases, structure branchings etc. The lattice theoretical basis for the T- and p-dependence of the interactions is discussed.