  The organic mesogens RM734 and DIO are members of separate molecular families
featuring distinct molecular structures. These families, at the present time,
are the only ones known to exhibit a ferroelectric nematic liquid crystal (LC)
phase. Here we present an experimental investigation of the phase diagram and
electro-optics of binary mixtures of RM734 and DIO. We observe paraelectric
nematic (N) and ferroelectric nematic (NF) phases in both materials, each of
which exhibits complete miscibility across the phase diagram, showing that the
paraelectric and ferroelectric are the same phases in RM734 as in DIO.
Remarkably, these molecules form ideal mixtures with respect to both the
paraelectric-ferroelectric nematic phase behavior and the ferroelectric
polarization density of the mixtures, the principal order parameter of the
transition. Ideal mixing is also manifested in the orientational viscosity, and
the onset of glassy dynamics at low temperature. This behavior is attributable
in part to the similarity of their overall molecular shape and net longitudinal
dipole moment (~ 11 Debye), and to a common tendency for head-to-tail molecular
association. In contrast, the significant difference in molecular structures
leads to poor solubility in the crystal phases, enhancing the stability of the
ferroelectric nematic phase at low temperature in the mixtures and making
possible room temperature electro-optic effects. In the mixtures with excess
DIO, an intermediate phase appears via an ultraweak, first-order transition
from the N phase, in a narrow temperature range between the paraelectric and
ferroelectric nematics.
