By implanting Co and Fe in sequence into Si (111), metastable ternary Co1-xFexSi2 phases were formed. Mossbauer effect measurements showed three resonance line components in the spectrum. Comparison of the central shift (CS) values of the components with those appearing in the stable ternary phases indicated that iron atoms are positioned in the substitutional Co site, in the empty cube of the fluorite-type lattice and in CsCl-like B2 structures. It was found that the CS values of two components are in the velocity range of the values obtained for the metastable gamma-FeSi2 synthesized using various methods. This result suggests the existence of a similar structure. (C) 2000 American Institute of Physics. [S0003-6951(00)01414-5].