Raman phonons and light scattering in RMnO3 (R=La, Pr, Nd, Ho, ErTb and Y) orthorhombic and hexagonal manganites

RMnO3 compounds present the Pbnm orthorhombic structure for R=La, Pr, Nd while compounds with smaller ionic radii (R=Ho, Er Tb and Y) can be obtained either in the Pbnm or the P6(3)cm hexagonal structure. Their magnetic and transport properties are strongly dependent on the rare earth and, therefore, on their detailed structure. We have studied these compounds through the Raman phonons of both structures as a function of the rare earth ion and temperature. The phonon frequencies of the main peaks of Pbnm structure have been correlated to structural characteristics, such as bond-lengths or bond angles, obtained from the analysis of neutron diffraction data. According to the structural and Raman behavior, these compounds can be classified into two groups: one includes La, Pr, and Nd, and the other includes the rest. This classification also correlates to the different behavior of their magneto transport properties when doped. A structural transition to a cubic or quasi-cubic perovskite, above 800 K, can be deduced from the phonon spectra for YMnO3 orthorhombic compound. (C) 2001 Elsevier Science B.V. All rights reserved.