Effect of [MnO6] Octahedra to the Coloring Mechanism of (Li1–xNax)2MnO3

(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.