On interatomic mixing and demixing phenomena in Cr-Fe: Statistical-mechanical calculations based on composition-dependent interaction energy model

Variations of Fe(1 0 0) surface composition with Cr coverage at 53 K were evaluated by means of the "free-energy concentration expansion method" (FCEM). The calculations, complementing our previous study [Phys. Rev. B 65 (2002) 205404], are based on four energetics models: composition-dependent (or independent) and surface-enhanced (or uniform) interactions. A linear dependence of the mixing enthalpy and of the corresponding effective interactions on temperature was taken into account. Numerical free-energy minimization showed that only the first of the four models, namely composition-dependent surface-enhanced interactions that change sign at ∼10% Cr, is consistent with scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) studies reported before. Effects of temperature on alloying at the Cr/Fe(1 0 0) surface are weak in the mixing region of low Cr concentrations, while for higher coverages the transition to Cr segregated/separated surface layer, induced by the mixing/demixing interaction reversal, is diminished considerably at elevated temperatures. Mixing/demixing effects related to the interaction change seem to be manifested also in model calculations of the bcc part of the Cr-Fe bulk phase diagram.