At present, there is an increasing interest in the iron di-silicide phase β-FeSi2, which is supposed to be a direct band gap semiconductor and one of the most promising materials for silicon-based optoelectronics, e.g., light-emitting devices, solar cells, and photo detectors. But this phase is very difficult to be produced. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 400 to 600°C is reported. The aim of the experiments was to achieve a complete reaction of the deposited Fe layer with the Si substrate that results in the formation of a pure, single-phased β-FeSi2 surface layer. The obtained silicide layers, their structure and composition are investigated by conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The fraction of the β-FeSi2 formed is determined by CEMS as function of ion species, energy, fluence and temperature. Complete growth and formation of a single-phased β-FeSi2 layer was achieved by 205 keV Xe ion irradiation at a fluence of 2 × 1016 ions/cm2 at 600°C.