The unit cell is tetragonal and a summary of the fundamental parameters is provided in Table for a range of exchange correlation functionals and experiments. The powder XRD results show an excellent fit to the P 4 2 /mnm space group, as shown in Figure , and the calculated PBE0 lattice parameters are in good agreement with room temperature XRD results. We note the PBEsol lattice parameters are slightly overestimated, which is typical of the generalised gradient approximation (GGA) implementation in DFT. All crystal structures visualised using vesta. Doping with Ga causes a systematic shift in the Bragg peaks to higher angles compared to undoped samples, illustrated in the inset of Figure , indicating a shrinkage in cell size. This observation is consistent with substituting Ga with Zn in an octahedral environment, as Ga has a smaller ionic radius (0.62 Å) compared to Zn (0.74 Å). The unit cell volumes also shift from 201.78 Å3 to 201.48 Å3 after Ga doping, extracted by Rietveld refinement using the gsas-ii software. These observations are consistent with a solid solution of gallium replacing zinc in the tri-rutile structure, with no detectable phase separation. Optical images of our transparent undoped and Ga-doped single crystals are shown in Figure , approximately 1 mm 2 in area and polished to a thickness of around 150 µm.