Anomalous high pressure dependence of the Jahn-Teller phonon in La0.75Ca0.25MnO3

Raman spectra of La0.75Ca0.25MnO3 have been collected for the first time ov er a wide pressure range (0-14 GPa) using a diamond anvil cell. The frequen cy range explored (200-1100 cm(-1)) and the very good quality of the data a llowed us to carefully analyze the pressure evolution of the phonon modes o f the MnO6 octahedra. The results show an abrupt transition at similar to7. 5 GPa with an evident deviation from the linear trend of the frequency of t he Jahn-Teller phonon versus the applied pressure, accompanied by a strong phonon broadening. This behavior disagrees with the predicted insulator to metal transition and, on the contrary, indicates the occurrence of a new un predicted phase in the very high pressure regime.