High pressure Raman study of La${}_{1-x}$Ca${}_x$MnO${}_{3-\delta }$ manganites

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.