Mössbauer studies of Fe1–xO. Part II. Disproportionation between 300 and 700 K

Abstract

A Mössbauer investigation of the disproportionation of quenched samples of Fe_1–xO between 300 and 700 K has been carried out. At temperatures above 500 K the prior rearrangement of Fe_1–xO into Fe_1–x–yO and Fe_1–x+yO (see Part I) was too rapid to be recorded on the Mössbauer spectra, but the observed reduction in the quadrupole splitting as a function of time at constant temperature can be discussed in terms of the reaction: (1–4z)Fe_1–xO →(1–4x)Fe_1–zO +(x–z)Fe₃O₄ In the limit, z becomes very small and the monoxide approaches stoicheiometry. The phase Fe_0·99O had a Néel temperature of 196 ± 3 K and a range of hyperfine magnetic fields of 340 ± 20 kG at 77 K. At temperatures above about 570 K further disproportionation of Fe_1–zO itself becomes rapid and the spectrum of metallic iron begins to appear, according to the reaction: 4Fe_1–zO →(1–4z)Fe + Fe₃O₄ The rate of precipitation of Fe₃O₄ at a given temperature was found to depend not only on the initial composition of Fe_1–xO, but also on the temperature from which the initial samples were quenched during preparation. The effect is interpreted in terms of the defect aggregations present in the initial samples.