A highly tetrahedrally bonded form on nonhydrogenated amorphous carbon (a-C) is produced by deposition from filtered medium-energy ion beams. A range of such films was grown and the sp3-bonded fractions, plasmon energies, compressive stresses, and resistivities were measured as a function of ion energy. These properties are found to be strongly correlated and each to pass through a maximum at an ion energy of about 140 eV. The optimum ion energy is observed to depend on the type of carbon ions deposited and, possibly, on the deposition flux rate. The data are found to support deposition models in which the sp3 bonding arises from the subplantation of incident ions, giving rise to a quenched increase in density and strain.