A unique class of crystalline Ge1-ySny alloys with ultrahigh Sn concentrations reaching 33% as well as Si-doped analogues have been synthesized directly on large-area Si wafers by chemical vapor deposition (CVD). The band gaps of these materials are measured to be as low as 0.15 eV, which allows them to cover a significant portion of the coveted mid-IR optical range past 8 μm, making them a potential alternative to Hg1-xCdxTe for long-wavelength applications. The film synthesis, based on stoichiometric CVD reactions of polygermanes and stannanes, is carried out at temperatures between 240 and 290 °C, indicating that the alloys are stable at typical device-operating temperatures and at complementary metal oxide semiconductor-compatible conditions. The main difference between Ge1-ySny (y ≈ 0.33) and the more diluted alloys (y < ∼0.2) featured in prior studies is the predicted presence of a significant fraction of Sn-Sn bonds, which could have a profound impact on the structural and electronic properties in view of the large size mismatch between Ge and Sn. However, X-ray measurements of the lattice parameter over the entire 0-33% Sn compositional range show a linear dependence of the cubic lattice parameter consistent with Vegard's law. Theoretical ab initio simulations assuming random alloys show a monotonic compositional dependence of the alloy energy of formation and confirm the experimental lattice parameter. The crystalline structure and the interface defects that accommodate the large lattice mismatch with the substrate are elucidated using transmission electron microscopy studies. Optical studies indicate that the fundamental direct band gap and other critical point features in the electronic structure also display a smooth compositional dependence, which should facilitate device engineering based on these materials.