  Adiabatic scanning calorimetry (ASC) is capable of providing simultaneously
the specific enthalpy h(T) and the specific heat capacity cp(T), and is an
important tool to determine the order of transitions and to render
high-resolution information on pretransitional thermal behavior. Here we report
on ASC results on the compound DIO and on mixtures with RM734. Both compounds
exhibit a low-temperature ferroelectric nematic phase (NF) and a
high-temperature paraelectric nematic (N). In DIO these two phases are
separated by an intermediate phase (Nx). Detailed data of h(T) and cp(T),
indicated that the intermediate phase was present in all mixtures over the
complete composition range, albeit with strongly decreasing temperature width
with decreasing mole fraction of DIO (xDIO). The xDIO dependence of the two
transitions could be well described by a quadratic function and both
transitions were weakly first order. The true latent heat of the Nx-N
transition of DIO was as low as 0.0075 +/- 0.0005 J/g and 0.23 +/- 0.03 J/g for
the NF-Nx transition, about twice the previously reported value of 0.115 J/g
for the NF-N transition in RM734. In the mixtures both transition latent heats
decrease gradually with decreasing xDIO. At all the Nx-N transitions
pretransition fluctuation effects are absent and these transitions are purely
but very weakly first order. As in RM734 the transition from the NF to the
higher temperature phase exhibits substantial pretransitional behaviour. Power
law analysis of cp(T) resulted in an effective critical exponent of 0.88 +/-
0.10 for DIO and this value decreased in mixtures with decreasing xDIO towards
0.50 +/- 0.05 reported for RM734. Ideal mixture analysis of the phase diagram
was consistent with ideal mixture behavior for the total transition enthalpy
change
