The specific heat and the lattice constant of Fe–Co alloys in the bcc range have been measured. The specimens are prepared by the reduction of solid solution Fe_1−xCo_xC₂O₄·2H₂O in hydrogen at 800°C. It has been confirmed that the FeCo (CsCl type) superlattice exists over a wide range (30∼75 at% Co), and the so-called “550°C anomaly” of the Fe_0.5Co_0.5 superlattice has also been observed in the alloys containing 21∼60 at% Co. This anomaly appears at almost the same temperature in the composition range of 21∼50 at% Co (555°∼560°C), and its transition temperature falls abruptly when the cobalt content is greater than 50 atomic per cent. There is a broad peak at about 550°C in specific heat curves of 21, 23, 25, 27 and 27.5 at% Co alloys. These peaks have hitherto been considered due to the transition of the Fe₃Co superlattice. Judging from the composition dependence of its transition temperature and the shape of the specific heat curves, however, it is reasonable to consider that these peaks do not correspond to the Fe₃Co superlattice but to the extension of the 550°C anomaly.
The effect of ordering on the lattice constant causes an increase in cell size in the 40∼60 at% Co alloys and a decrease in the 23∼40 at% Co alloys.
The results of measurements by means of a neutron diffraction technique and the Mössbauer effect on a slow-cooled 25 at% Co alloy have shown no evidence for the existence of the Fe₃Co superlattice.