A57Fe Mössbauer Study of the Cubic Perovskite-Type Phase LaBa2Fe3O8+w(−0.20

Abstract

Single-phase samples of LaBa2Fe3O8+wwith −0.20≤w≤0.83 were synthesized, characterized by oxygen-content analyses, powder X-ray diffraction, and magnetic susceptibility, and investigated using57Fe Mössbauer spectroscopy. At 296 K, antiferromagnetic ordering of the Fe sites was observed for samples havingw≤0.45. Whenw≤0.0, the iron atoms occupy predominantly an Fe3+,S=5/2 state, the rest being Fe2+,S=2. Forw≈0, the intensities of the components in the Mössbauer spectra correspond to random distribution of oxygen vacancies around Fe3+. When 0≤w≤0.45, increasing amounts of iron atoms enter the Fe4+,S=2 state. Forw≥0.45, which takes the paramagnetic state at room temperature, the amount of Fe4+increases accordingly. At 85 K, there is inconclusive evidence of disproportionation of Fe4+into Fe3+and Fe5+in the most oxidizedw=0.83 sample. The room-temperature Mössbauer data show that there occurs a gradual conversion from AF to P states with increasingwup tow≥0.45, when all AF components simultaneously disappear. This gradual frustration of the cooperative order has three contributing factors. First, an increasing proportion of the iron moments simply becomes paramagnetic in the AF arrangement. Second, the AF interactions become weakened upon disorder. Third, the AF interaction is obviously weakened by replacement ofS=5/2 withS=2. The room-temperature relationship between the average internal magnetic field and the oxygen content parameterwresembles the magnetic field vs temperature relationship for a cooperative magnetic material. The common mechanism is that both temperature and oxygen loading affect the ordered spin system.