V ⁵⁺ -Induced Crystal Field Modulation in Mn ⁵⁺ -Rich Ba ₃ Mn ₂ O ₈ for High-Purity Green Pigments

Ba₃Mn_{2(1-<i>x</i>)}V_2<i>x</i>O₈ (0 ≤ <i>x</i>⁵ ≤ 0.25) pigments were synthesized by a conventional solid-state reaction method. This study shows that diluting Mn⁵⁺ in the Mn-rich host Ba₃Mn₂O₈ with V⁵⁺ provides a high-purity green hue. All samples were obtained in a single phase with the rhombohedral Ba₃Mn₂O₈ structure (space group of <i>R</i>3̅<i>m</i>). Optical absorption bands of the samples were observed around 378 and 689 nm due to d-d transitions of Mn⁵⁺. Ba₃Mn_1.6V_0.4O₈ showed the pure green hue with a greenness value (-<i>a</i>*) of -31.1 and a hue angle (<i>h</i>°) of 179.9. The values are higher than those of the commercially available Cr₂O₃ pigments (<i>a</i>* = -16.73, <i>h</i>° = 140.6). XANES analysis confirmed the persistence of Mn⁵⁺, suggesting that the enhanced color purity was caused by crystal field modulation rather than a valence change. The decrease in the average Mn-O bond length determined from the Rietveld refinement and EXAFS analysis caused a blue shift of the absorption bands, resulting in a color evolution from yellowish-green to a vivid, high-purity green. Furthermore, the optimized pigment showed excellent thermal stability, humidity, and chemical stability, indicating its significant potential for practical use as a next-generation green pigment.