  The phase diagram of the prototype antiferroelectric liquid crystal MHPOBC in
dependence of enantiomeric excess was measured. It was shown that the SmCbeta
phase in very pure samples is the SmCFI2 phase with a four layer structure, and
only after small racemization it transforms into the ferroelectric SmC phase.
The phase diagram was theoretically explained by taking into account longer
range bilinear and short range biquadratic interlayer interactions, that lead
to the distorted clock structures and first order transitions between them.
