Synthesis, properties and application of intense blue/green pigments based on Co/Cr doped Mg2TiO4 with excess MgO

Mg2.2-xCoxTiO4 (0 ≤ x ≤ 0.6) and Mg2.2-xCrxTiO4 (0 ≤ x ≤ 0.6) solid solutions with spinel structure were successfully synthesized by solid-phase reaction. Excess MgO was added to achieve the single phase of the compounds. The transition metal ions Co/Cr tune the colors of Mg2TiO4 to intense blue and green, respectively. The root of the observed intense blue color of Mg2.2-xCoxTiO4 is attributed to the 4A2 (F) → 4T1 (P) transition of Co2+ in tetrahedral coordination. In Cr doped Mg2TiO4, Cr exists in a mixed oxidation states of +3 and + 6. The green color of the as-synthesized Mg2.2-xCrxTiO4 is attributed to 4A2 (F) → 4T1 (F) at 410 nm–500 nm and 4A2 (F) → 4T2 (F) at 610 nm transitions of Cr3+. Besides, the Cr3+ - Ti4+ charge-transfer transition at 410 nm–500 nm is also the reason why it shows green. The as-synthesized pigments were dispersed into PMMA to prepare blue/green plastics.