  The structure of La$_{1-x}$Ca$_{x}$MnO$_{3}$ solid solutions (x=0, 0.25,
0.50, 0.67, 1) under high pressure (up to 40-45 GPa) has been investigated by
synchrotron X-ray powder diffraction (XRD) in order to characterize their
volume \emph{vs.} pressure (P-V) equation of state. All the members of the
solid solution, except the extreme compounds: LaMnO$_3$ and CaMnO$_{3}$,
present low pressure orthorhombic P$nma$ phase evolving toward an high pressure
tetragonal (I$4/mcm$) symmetry. The details of this transition are related to
the Ca doping: on increasing the Ca content, the critical pressure of the
transition decreases, but the energetic cost of the transition increases. This
study depicts a peculiar evolution of structural distortions as a function of
pressure and concentration.
