  We present ab initio calculations of the electronic and optical properties of
hexagonal SiGe alloys in the lonsdaleite structure. Lattice constants and
electronic band structures in excellent agreement with experiment are obtained
using density-functional theory. Hexagonal Si has an indirect band gap, while
hexagonal Ge has a pseudo-direct gap, i.e. the optical transitions at the
minimum direct band gap are very weak. The pseudo-direct character of pure
hexagonal Ge is efficiently lifted by alloying. Already for a small admixture
of Si, symmetry reduction enhances the oscillator strength of the lowest direct
optical transitions. The band gap is direct for a Si content below 45 %. We
validate lonsdaleite group-IV alloys to be efficient optical emitters, suitable
for integrated optoelectronic applications.
