Anomalous High Pressure Dependence of the Jahn-Teller Phonon in La_0.75Ca_0.25MnO₃

Raman spectra of La_0.75Ca_0.25MnO₃ have been collected for the first time over a wide pressure range (0–14GPa) using a diamond anvil cell. The frequency range explored (200–1100cm^-1) and the very good quality of the data allowed us to carefully analyze the pressure evolution of the phonon modes of the MnO₆ octahedra. The results show an abrupt transition at ∼7.5 GPa with an evident deviation from the linear trend of the frequency of the 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 unpredicted phase in the very high pressure regime.