${\mathrm{LiGa}}_5{\mathrm{O}}_8$ in a spinel-type structure has recently been claimed to be an unintentional $p$-type ultra-wide-band-gap oxide semiconductor. While previous computational work did not yet identify the origin of $p$-type doping and in fact predicted insulating behavior by compensation of deep acceptors by shallow donors, defect characterization in terms of its optical signatures remains important. Rather than focusing on thermodynamic transition levels, as in earlier work, this present paper focuses on the vertical transitions in a defect configuration diagram of defects in different charge states, representing absorption and emission processes involving carrier capture/emission from/to band edges. In addition, the structural relaxation of several native defects is revisited by allowing for more complex symmetry breaking distortions in an effort to reconcile conflicting results in the previous literature. Special attention is given to the Li vacancy because it is the shallowest native acceptor. For this defect, the previously reported transition levels are revised on the basis of symmetry breaking relaxations. The calculated optical transition energies are also compared with a representative depth-resolved cathodoluminescence spectrum of a mist-CVD-grown ${\mathrm{LiGa}}_5{\mathrm{O}}_8$ film. Finally, we also study carbon impurities, which may become relevant for identifying carbon-related defects in MOCVD-grown ${\mathrm{LiGa}}_5{\mathrm{O}}_8$ materials.

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• First-principles calculations