Raman phonons and the Jahn-Teller transition in RMnO3 manganites

We have studied the behavior of the Raman phonons through the transition from a static to dynamic Jahn-Teller order in stoichiometric RMnO3 samples, tin particular, LaMnO3 single crystal and PrMnO3 and YMnO3 polycrystalline pellets). Thermal expansion effects on the phonon frequencies are not relevant above 300 K in La and Pr samples. The intensity of the spectrum decreases drastically as the temperature increases but is still observable in the pseudo-cubic phase. The remaining spectral weight, above the Jahn-Teller transition, can be explained by the presence of dynamic Jahn-Teller distortions that lower the symmetry of the cubic perovskite. The Raman spectra of LaMnO3 and PrMnO3, which presents a similar structural transition temperature, shows the same behavior while that of YMnO3 is clearly different. This is explained by the fact that this compound presents a stronger cooperative Jahn-Teller distortion. Anharmonic scattering of the phonons are the origin of the phonon softening and partially that of the strong width increase as temperature reaches that of the Jahn-Teller transition. (C) 2001 Elsevier Science B.V. All rights reserved.