A⁵⁷Fe Mössbauer Study of the Cubic Perovskite-Type Phase LaBa₂Fe₃O_8+w(-0.20<w<0.83)

Single-phase samples of LaBa₂Fe₃O_8+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.