NOVELTY - An optical material comprises an iron silicide in the form of dots, islands, or a film. The iron silicide has a monoclinic crystalline structure which is synthesized at the surface or in the interior of the crystalline silicon. The monoclinic structure corresponds to a deformed structure of a beta-iron silicide.    USE - For use in a light-emitting layer or photo-receiving layer of an optical element.    ADVANTAGE - The optical material which is monoclinic FexSi2 is less symmetrical than the orthorhombic beta-FeSi2 so that dipole transitions are allowed between may electronic states and the oscillator strength which determines characteristics of the material is larger than that of beta-FeSi2. The optical material operates at wavelength of 1.5 mum which corresponds to the low-loss wavelength of optical fibers. Since the luminescent decay time is 10 nanoseconds or less and is shorter than that of conventional iron silicide material, the optical material can be applied to high-speed communications. Since a layer of the optical material can be deposited on the (111) surface of the crystalline silicon, a light-emitting/photo-receiving layer can be manufactured at reduced costs. Because the monoclinic FexSi2 can be made by an epitaxial growth method, defect density at the interface between the silicon and the FexSi2 is reduced and the resulting optical material is suitable for a current injection element. The optical material is capable of emitting or receiving light in the infrared region of 1.5 mum at room temperature. Finally, because carrier mobility is enhanced, rapid and efficient response is possible when light is received.    DETAILED DESCRIPTION - An optical material comprises a crystalline silicon and iron silicide (FexSi2) in the form of dots, islands, or a film. The FexSi2 has a symmetrical monoclinic crystalline structure belonging to the P21/c space group or C2/c space group and is synthesized at the surface or in the interior of the crystalline silicon. The monoclinic structure corresponds to a deformed structure of beta-FeSi2 generated by heteroepitaxial stress between the (110) (for P21/c space group) or (101) (for C2/c space group) plane of the FexSi2 and the (111) plane of the crystalline silicon.    x = 0.85-1.1    An INDEPENDENT CLAIM is also included for an optical element comprising a p-type or n-type crystalline silicon and a light-emitting layer or a photo-receiving layer which comprises the FexSi2.    DESCRIPTION OF DRAWING(S) - The figure shows the state of heteroepitaxial relationship between FexSi2 having a beta'-I structure and monocrystalline silicon of the invention.