Quinary oxynitride BaNb0.5Ta0.5O2N crystals were fabricated through the partial nitridation and acidification of the KCl flux grown Ba5Nb2Ta2O15 crystals.The parameters of both the solute concentrations and cooling rates are optimized for the KCl flux growth of the larger Ba5Nb2Ta2O15 crystals with clearer crystal habits.Here,the optimal Ba5Nb2Ta2O15 crystals mainly have a hexagonal plate-like shape.After the partial nitridation and acidification,the porous BaNb0.5Ta0.5O2N crystals maintained the crystal shape of the oxide precursor and had a single-crystalline nature.For the Ba5Nb2Ta2O15 crystals,the wavelength of the absorption edge was about 707 nm.Especially,the CoOx-loaded BaNb0.5Ta0.5O2N photocatalyst demonstrated the comparatively high amount of O2 gas (150.7 μmol) during the 5 h visible-light-induced sacrificial water oxidation half-reaction,which might be achieved due to the high crystallinity and visible-light absorption property.