Raman, infrared and x-ray diffraction study of phase stability in La_1-xBa_xMnO₃ doped manganites

Abstract

Formation of different crystallographic phases and their characteristic Raman and infrared spectra are studied in the manganite system La_1-xBa_xMnO₃ for x in the range of 0 × 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 lt;x β 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 (p6₃mc noncentrosymmetric group) whose presence is not seen in X-ray diffraction data. For x gt;0.35, all three techniques used here show the formation of a two-phase mixture of La_0.65Ba_0.35MnO₃ 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 δ with the reported electrical and magnetic behavior of this hole-doped manganite.