  The current intense study of ferroelectric nematic liquid crystals was
initiated by the observation of the same ferroelectric nematic phase in two
independently discovered organic rod-shaped mesogenic compounds, RM734 and DIO.
We recently reported that the compound RM734 also exhibits a monotropic,
low-temperature, antiferroelectric phase having reentrant isotropic symmetry
(the $I_A$ phase), the formation of which is facilitated to a remarkable degree
by doping with small (below $1\%$) amounts of the ionic liquid ${\rm
BMIM-PF}_6$. Here we report similar phenomenology in DIO, showing that this
reentrant isotropic behavior is not only a property of RM734 but is rather a
more general, material-independent feature of ferroelectric nematic mesogens.
We find that the reentrant isotropic phases observed in RM734 and DIO are
similar but not identical, adding two new phases to the ferroelectric nematic
realm. The two $I_A$ phases exhibit similar, strongly peaked, diffuse x-ray
scattering in the WAXS range $(1<q<2 \, \AA^{-1})$ indicative of a distinctive
mode of short-ranged, side-by-side molecular packing. The scattering of the
$I_A$ phases at small $q$ is quite different in the two materials, however,
with RM734 exhibiting a strong, single, diffuse peak at $q \sim 0.08 \,\AA
^{-1}$ indicating mesoscale modulation with $\sim 80 \,\AA$ periodicity, and
DIO a sharper diffuse peak at $q \sim 0.27 \,\AA ^{-1} \sim (2\pi/{\rm
molecular length})$, with second and third harmonics, indicating that in the
$I_A$ phase of DIO, short-ranged molecular positional correlation is smectic
layer-like.
