  Linear dielectric thermodynamics are formally developed to explore the
isothermal and adiabatic temperature - pressure dependence of dielectric
constants. The refractive index of optical materials is widely measured in the
literature: it is both temperature and pressure dependent. The argument to
establish the dielectric constant's isentropic temperature dependence is a
thermodynamic one and is thus applicable to all physical models that describe
electron clouds and electronic resonances within materials. The isentropic
slope of the displacement field versus the electric field at all temperatures
is described by an adiabatic dielectric constant in an energy-per-unit mass
system. This slope is shown through the electronic part of the entropy to be
unstable at high temperatures due to the change in the curvature of the
temperature dependence of the dielectric constant. The electronic entropy
contribution for optical, thermo-electro materials has negative heat capacities
which are unacceptable. The dielectric constant's temperature and pressure
dependence is predicted to be only dependent on the specific volume so
isentropes are always positive. A new universal form for the dielectric
constant follows from this hypothesis: the dielectric constant is proportional
to the square root of the specific volume.
