BaCuQF (Q = S, Se, Te) materials exhibit band gaps that allow transmission of much of the visible spectrum. BaCuSF is transparent in thin-film form with a band gap of 3.1 eV Band gap estimates for powders of the solid solution series BaCuS1-xSexF were obtained from wavelength-dependent diffuse-reflectance measurements using an integrating sphere. The band gap can be tuned by the substitution of Se for S to 2.9 eV for BaCuSeF. The decrease scales almost linearly with the increase in the volume of the tetragonal unit cell, which is determined primarily by the expansion of the a lattice parameter; the overall volume increase is 7.0% from x = 0 to 1. Further reduction of the band gap is observed in BaCuSe1-xTexF solid solutions, where a unit cell volume increase of 5.5% produces a band gap of 2.7 eV in BaCuSe0.5Te0.5F. Powders and films of BaCuSF exhibit strong red luminescence under ultraviolet excitation, which is suppressed by K doping. Additional tuning of band gap and electrical properties (the materials are p-type conductors) can be achieved by replacing Ba with Sr. (C) 2003 Elsevier B.V. All rights reserved.