  We report the properties of two new series of compounds that show the
ferroelectric nematic phase in which the length of a terminal chain is varied.
The longer the terminal chain, the weaker the dipole-dipole interactions of the
molecules are along the director, and thus the lower the temperature at which
the axially ferroelectric nematic phase is formed. For homologues of
intermediate chain length, between the non-polar and ferroelectric nematic
phases, there is a wide temperature range nematic phase with antiferroelectric
character. The size of the antiparallel ferroelectric domains critically
increases upon transition to the ferroelectric phase. In dielectric studies,
both collective ("ferroelectric" and non-collective fluctuations are present,
the "ferroelectric" mode softens weakly at the N-NX phase transition because
the polar order in this phase is weak. The transition to the NF phase is
characterized by a much stronger lowering of the mode relaxation frequency and
an increase in its strength, typical critical behavior is observed.
