Intermediate phases of the α-σ phase transition in the Fe-Cr system

(Fe_100-x{Cr}ₓ)_100-y{Sn}y$ alloys with 42.0{≤}x{≤}44.7 and 0{≤}y{≤}5.6 were annealed at 700 ^∘{}C for a period of up to 1000 h in order to cause a transformation of the {α} phase into the {σ} phase. Room temperature ⁵⁷Fe M\"ossbauer spectra registered on the samples consist of a magnetic subspectrum, related to the {α} phase, and a nonmagnetic subspectrum, related to the {σ} phase. Evidence is found that the isomer shift of the nonmagnetic subspectrum changes continuously with the amount of its relative contribution in the spectrum and it reaches the value characteristic of a pure {σ} phase only when its contribution in the two-phase sample is larger than about 30%.