Effect of [MnO6] Octahedra to the Coloring Mechanism of (Li1- x Na x )2MnO3

Ryohei Oka; Kohei Kusukami; Toshiyuki Masui

doi:10.1021/acsomega.0c01071

Effect of [MnO₆] Octahedra to the Coloring Mechanism of (Li_{1- x} Na _x )₂MnO₃

ACS Omega. 2020 May 27;5(22):13108-13114. doi: 10.1021/acsomega.0c01071. eCollection 2020 Jun 9.

Authors

Ryohei Oka¹, Kohei Kusukami¹, Toshiyuki Masui¹

Affiliation

¹ Department of Chemistry and Biotechnology, Graduate School of Engineering, Department of Engineering, Graduate School of Sustainability Science, Faculty of Engineering, and Center for Research on Green Sustainable Chemistry, Tottori University, 4-101, Koyama-cho Minami, Tottori 680-8552, Japan.

Abstract

(Li_1-x Na _x )₂MnO₃ (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 [MnO₆] octahedra and coloration mechanisms. The color, optical properties, and crystal structure of the solid solutions were characterized. The (Li_1-x Na _x )₂MnO₃ (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 (⁴A_2g → ⁴T_1g, ⁴T_2g) and spin-forbidden (⁴A_2g → ²E_g, ²T_1g) d-d transitions of tetravalent manganese ions, respectively. The absorption band assigned to the ⁴A_2g → ⁴T_2g transition shifted toward longer wavelengths with the enlargement of the average [Mn(2)O₆] 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)O₆] 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 )₂MnO₃ (0 ≤ x ≤ 0.10) samples synthesized in this study, the highest red color purity was obtained in the (Li_0.93Na_0.07)₂MnO₃ (hue angle: h° = 39.1) sample. The results of this study provide important insights for the development of environment-friendly inorganic red pigments containing Mn⁴⁺ ions as a coloring source.