Figure shows the experimentally measured absorption coefficient for undoped and Ga-doped single crystals, as well as the PBE0 calculated value. The experimental absorption coefficient of both crystals is unusually low, with the doped crystal showing around 88 cm -1 at a wavelength of 550 nm (approximately 2.25 eV). This results from the forbidden nature of the fundamental band gap, as predicted by DFT calculations. The absorption coefficient in the near UV region is also unusually low, on the order of 300 cm -1 , but this plateau is an artefact of the thickness of our crystals rather than an intrinsic material property, an effect also observed in GeSe single crystals. Instead, we present the calculated absorption coefficient on an extended scale (Figure ), showing the predicted onset at around 4.25 eV. Both experiment and theory point towards excellent transparency in the visible region.