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We report inelastic neutron scattering measurements over 7-1251 K in CaMnO3covering various phase transitions, and over 6-150 K in PrMnO3 covering themagnetic transition. The excitations around 20 meV in CaMnO3 and at 17 meV inPrMnO3 are found to be associated with magnetic origin. In spite of similarityof the structure of the two compounds, the neutron inelastic spectrum of PrMnO3exhibits broad features at 150 K unlike well-defined peaks in the spectrum ofCaMnO3. This might result from the difference in nature of interactions in thetwo compounds (magnetic and Jahn-Teller distortion). The interpretation andanalysis of the observed phonon spectra have been performed using ab-initiophonon calculations. We also discuss the effect of pressure on the structuraldistortions in the orthorhombic phase of CaMnO3 and PrMnO3. On application ofpressure, we found that the variations of Mn-O distances are isotropic forCaMnO3 and highly anisotropic for PrMnO3. The calculated structure as afunction of pressure in PrMnO3 shows that suppression of Jahn-Teller distortionand insulator to metal transition occurs simultaneously. Our calculations showthat this transition may not be associated with the occurrence of thetetragonal phase above 20 GPa as reported in the literature, since thetetragonal phase is found to be dynamically unstable although it is found to beenergetically favored over the orthorhombic phase above 20 GPa. CaMnO3 does notshow any phase transition up to 60 GPa.

Spin-Phonon Coupling and High Pressure Phase Transitions of RMnO3 (R= Ca and Pr): An Inelastic Neutron Scattering and First Principle Studies