Non-ammonolysis synthesis and characterisation of environmentally benign yellow pigments based on calcium–tantalum perovskite oxynitrides

Abstract

Calcium–tantalum-based perovskite oxynitrides were synthesised as candidates for environmentally benign inorganic yellow pigments. The compounds were prepared via a urea-nitriding method, which does not involve a harmful ammonolysis process. Rapid single-phase synthesis was achieved under the optimum conditions, and only removable by-products were generated. Although the colours and bandgap energies of the obtained calcium–tantalum oxynitrides, CaTa(O,N)₃, fell short of those of CaTaO₂N synthesised by ammonolysis, they were made comparable by doping with La³⁺. Among the samples synthesised in this study, the most brilliant yellow hue was achieved with Ca_0.8La_0.2Ta(O,N)₃, which has lightness (L*) and yellowness (b*) values comparable to those of conventional inorganic yellow pigments. In this synthetic method, harmful substances are eliminated both from the products and the synthesis process. In addition, the short heat-treatment duration enables the products to be obtained as fine powders suitable for pigment applications. Calcium–tantalum-based perovskite oxynitrides prepared by the strategy developed in this study have potential applications as environmentally benign yellow pigments.