Ge1-xSnx alloys were grown on Ge(001) and GaAs(001) substrates in a conventional R. F. sputtering system with two independents plasmas not simultaneous focus to substrate. We determined the in-plane and in-growth lattice parameters, as well as the alloy bulk lattice parameter of the alloys for different Sn concentrations by high resolution x-ray diffraction. At low concentrations, we observed that Ge1-xSnx layers have pseudomorphic characteristics. We also determine the band gap of these alloys from transmittance measurements at different temperatures, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap it is observed to lie between 0.10< xc < 0.13.