  We report on a high-pressure Raman study on two members of the
La$_{1-x}$Ca$_x$MnO$_{3-\delta}$ manganite family ($x=0.20$, $\delta=0$ and
$\delta=0.08$). The results obtained for the $\delta=0$ sample show a different
behavior in the low and high pressure regimes ascribed to the onset of a new
pressure-activated interaction previously invoked in other manganite compounds.
The comparison of our results with literature data gives further support to the
identification of the Jahn-Teller active stretching mode and shows that
pressure-induced octahedral symmetrization is more effective in systems
exhibiting a lower metallic character. On the contrary the new interaction sets
in at pressure which decreases on increasing the metallic character of the
system indicating a relevant role of the Mn-Mn hopping integral in its
activation.
