Phenomenological Versus Microscopic Interpretation of Incommensurate Systems -BCCD and the ANNNI-Model

Phenomenological and microscopic models to treat modulated crystal structures are reviewed. Their applicability to interpret the specific phase diagram of betaine-calciumchloride-dihydrate (BCCD), which is of the "incomplete devils,s staircase" type, is critically discussed. Some experimental observations, which emphasize the fractal topology of the BCCD phase diagram, have been compiled. They support the preference of microscopic approaches as the ANNNI-model over the classic Landau theory for this specific case.