Fe-17.5Cr alloys have been grown on chemically polished silicon using co-sputter deposition with and without the use of 200 eV argon ion assistance during deposition. Thin film glancing incidence X-ray analysis was then used to characterise the microstructure of the film, As expected for the co-sputter deposited film (grown without ion-assistance) the d-spacings corresponded closely to those of ferritic stainless steel (Fe-18Cr) which has a bcc Fe structure. Surprisingly however, the film deposited using ion assistance had a structure of a metastable chi-phase, similar to that of alpha-manganese. Normally large impurity species are considered essential for the stabilisation of this structure. The possible reasons for the formation of the chi-phase are discussed in terms of the effects of the ion assistance on film growth.