Abstract
(Li(1-x) Na (x) )(2)MnO(3) (0 ≤ x ≤ 0.10) solid solutions were synthesized by a conventional solid-state reaction technique to investigate the relationship between the steric structure of the [MnO(6)] octahedra and coloration mechanisms. The color, optical properties, and crystal structure of the solid solutions were characterized. The (Li(1-x) Na (x) )(2)MnO(3) (0 ≤ x ≤ 0.10) solid solutions absorbed the visible light at wavelengths shorter than 550 nm and around 680 nm. The former and latter optical absorption bands were attributed to the spin-allowed ((4)A(2g) → (4)T(1g), (4)T(2g)) and spin-forbidden ((4)A(2g) → (2)E(g), (2)T(1g)) d-d transitions of tetravalent manganese ions, respectively. The absorption band assigned to the (4)A(2g) → (4)T(2g) transition shifted toward longer wavelengths with the enlargement of the average [Mn(2)O(6)] bond distance by doping Na(+). In contrast, the latter absorption bands did not shift but the absorption intensities increased due to the distortion of the [Mn(2)O(6)] octahedra. Consequently, the red color purity of the sample gradually increased with the increase in the Na(+) concentration. Among the (Li(1-x) Na (x) )(2)MnO(3) (0 ≤ x ≤ 0.10) samples synthesized in this study, the highest red color purity was obtained in the (Li(0.93)Na(0.07))(2)MnO(3) (hue angle: h° = 39.1) sample. The results of this study provide important insights for the development of environment-friendly inorganic red pigments containing Mn(4+) ions as a coloring source.