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

Condensed Matter