Mössbauer spectroscopic studies of the disintegration of hexavalent iron compounds (BaFeO4 and K2FeO4)

Mössbauer spectroscopic studies of BaFeO4 and K2FeO4 as prepared, then either sealed, or exposed to air, or exposed to moist air for a period up to more than one year, were performed at room temperature as a function of time. Some of the samples were studied as a function of temperature down to 4.2 K. K2FeO4 and BaFeO4 after preparation, exhibit a pure Fe6+ spectrum. K2FeO4 shows low stability. After a period of 14 months in a sealed sample holder, the spectrum exhibits 83% noncrystalline Fe3+, as Fe2O3 nanoparticles, and only 17% of the original Fe6+. BaFeO4 sealed, or exposed to dry air disintegrates slowly, exhibiting a spectrum composed of three subspectra. In addition to the original Fe6+ and final Fe3+ subspectra, a subspectrum, of an intermediate stage of a crystalline Fe4+ system, is present. In the first month the increase of the Fe3+ subspectrum is 15%, and that of the Fe4+ is 8%. BaFeO4 exposed to moist air, disintegrates at a very fast rate. The Fe3+ subspectrum, due to Fe2O3 nanoparticles, increases in the first days at the rapid rate of ∼13%/day, and there is no evidence for Fe4+ in the spectrum. The Fe6+ in BaFeO4, Fe3+ and Fe4+ in the disintegrated systems are all magnetically ordered at 4.2 K. Above 90 K the Fe3+ subspectra exhibit a superposition of a paramagnetic doublet and a diffuse magnetic sextet, with relative intensities changing with temperature, and changing from sample to sample according to their blocking temperatures, which are determined by the distribution in size of the nanoparticles.