We report novel Raman measurements made on 'Pr₀.5Ca₀.5MnO₃' thin films grown on 'SrTiO₃' and 'LaAlO₃' (PCMO/STO and PCMO/LAO respectively). The measurements were performed for a temperature range 78 K < T < 295 K and under hydrostatic pressures up to 5 GPa. In addition, different scattering polarizations (xx, xz, x'x' and x'z') have been selected for certain temperatures. In order to examine the effect of the strain from the substrate three films of different thickness have been used (50 nm, 100 nm and 180 nm). X-ray diffraction measurements have shown that the PCMO/STO film is [010]-oriented (i.e. with the [010] axis perpendicular to the substrate plane) and the PCMO/LAO film is [101]-oriented due to the lattice mismatch. The measurements have disclosed differences for the high-energy phonons related with the Jahn-Teller distortions of the octahedra. Close to the transition temperature to the charge ordered state, 'T_CO', and as the temperature decreases, an increase of the relative intensities of Jahn-Teller modes has been observed. From the different growth orientation of the two films the symmetry of several modes has been identified. No shift or splitting of the modes was observed due to the strains from the different substrates. Furthermore, two rotational modes of vibration of the octahedra were found to exhibit an anharmonic behavior.


Acknowledgments
This work was partially financially supported by the General Secretary of Research and Technology (GSRT) under the project 01EP13.

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