Raman, infrared and x-ray diffraction study of phase stability in La1-xBaxMnO3 doped manganites

Formation of different crystallographic phases and their characteristic Ram an and infrared spectra are studied in the manganite system La1-xBaxMnO3 fo r x in the range of 0 less than or equal to x less than or equal to 1, and with variations in oxygen stoichiometry. Synthesis of the end member LaMnO3 in pure argon environment leads to the formation of Jahn-Teller distorted, antiferromagnetic orthomanganite. While the observed Raman modes in this c ompound are primarily due to internal vibrations of MnO6 octahedra, the inf rared (IR) spectra show an absorption edge in addition to the IR active pho nons. The oxygen rich LaMnO3 is rhombohedral and has fewer zero-wave-vector phonon modes. In the barium substituted compositions with 0<x less than or equal to 0.25, a single phase rhombohedral compound of decreasing rhombici ty is formed. A further increase in x leads to the ideal cubic perovskite s tructure for which a factor group analysis yields no Raman active and three IR active phonons of F-1u symmetry. The compound with x = 0.35, shows fain t Raman modes of hexagonal BaMnO3 (P6(3)mc noncentrosymmetric group) whose presence is not seen in x-ray diffraction data. For x>0.35, all three techn iques used here show the formation of a two-phase mixture of La0.65Ba0.35Mn O3 and hexagonal BaMnO3. An attempt has been made to correlate the progress ive increase in symmetry of the crystal structure, shift of IR absorption e dge to lower energies, and disappearance of phonon modes with x and delta w ith the reported electrical and magnetic behavior of this hole-doped mangan ite. (C) 1999 American Institute of Physics. [S0021-8979(99)01006-3].