Raman phonons and the Jahn-Teller transition in RMnO3 manganites

We have studied the behavior of the Raman phonons through the transition fr om a static to dynamic Jahn-Teller order in stoichiometric RMnO3 samples, t in particular, LaMnO3 single crystal and PrMnO3 and YMnO3 polycrystalline p ellets). Thermal expansion effects on the phonon frequencies are not releva nt above 300 K in La and Pr samples. The intensity of the spectrum decrease s drastically as the temperature increases but is still observable in the p seudo-cubic phase. The remaining spectral weight, above the Jahn-Teller tra nsition, can be explained by the presence of dynamic Jahn-Teller distortion s that lower the symmetry of the cubic perovskite. The Raman spectra of LaM nO3 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 Ja hn-Teller distortion. Anharmonic scattering of the phonons are the origin o f the phonon softening and partially that of the strong width increase as t emperature reaches that of the Jahn-Teller transition. (C) 2001 Elsevier Sc ience B.V. All rights reserved.