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

Formation of different crystallographic phases and their characteristic Raman and infrared spectra are studied in the manganite system La1-xBaxMnO3 for 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 compound are primarily due to internal vibrations of MnO6 octahedra, the infrared (IR) spectra show an absorption edge in addition to the IR active phonons. 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 rhombicity is formed. A further increase in x leads to the ideal cubic perovskite structure 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 faint 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 techniques used here show the formation of a two-phase mixture of La0.65Ba0.35MnO3 and hexagonal BaMnO3. An attempt has been made to correlate the progressive increase in symmetry of the crystal structure, shift of IR absorption edge to lower energies, and disappearance of phonon modes with x and delta with the reported electrical and magnetic behavior of this hole-doped manganite. (C) 1999 American Institute of Physics. [S0021-8979(99)01006-3].