  We present a minimal Landau theory of plastic-to-crystal phase transitions in
which the key components are a multipole-moment order parameter that describes
the orientational ordering of the constituent molecules, coupling between such
order parameter and elastic strains, and thermal expansion. We illustrate the
theory with the simplest non-trivial model in which the orientational ordering
is described by a quadrupole moment, and use such model to calculate
barocaloric effects in plastic crystals that are driven by hydrostatic
pressure. The model captures characteristic features of plastic-to-crystal
phase transitions, namely, large changes in volume and entropy at the
transition, as well as the linear dependence of the transition temperature with
pressure. We identify temperature regions in the barocaloric response
associated with the individual plastic and crystal phases, and those involving
the phase transition. Our model is in overall agreement with previous
experiments in powdered samples of fullerite C$_{60}$, and predicts peak
isothermal entropy changes of $\sim90 \,{\rm J K^{-1} kg^{-1}}$ and peak
adiabatic temperature changes of $\sim35 \,{\rm K}$ under $0.60\,$GPa at
$265\,$K in fullerite single crystals.
