Synthesis and characterisation of Ba(Zn_1- Co )₂Si₂O₇ (0 ≤ x ≤ 0.50) for blue-violet inorganic pigments

Ba(Zn_1- Co )₂Si₂O₇ (0 ≤ x ≤ 0.50) solid solutions were synthesized as novel blue-violet inorganic pigments by a conventional solid-state reaction method. The crystal structure, optical properties, and colour of the pigments were characterized. All the pigments were obtained in a single-phase form. The pigments strongly absorbed visible light at wavelengths from 550 to 650 nm, corresponding to the range of green to orange light. This optical absorption was caused by the d-d transition of the tetrahedrally coordinated Co²⁺ (⁴A₂(F) → ⁴T₁(P)), which was the origin of the blue-violet colour of the pigments. The most intense colour was obtained for Ba(Zn_0.85Co_0.15)₂Si₂O₇, where a* = +52.2 and b* = -65.5 in the CIE (Commission Internationale de l'Éclairage) L*a*b* system. These absolute values were significantly larger than those of commercial violet pigments such as Co₃(PO₄)₂ (a* = +33 and b* = -32) and NH₄MnP₂O₇ (a* = +39 and b* = -21). Therefore, the Ba(Zn_0.85Co_0.15)₂Si₂O₇ pigment could be a novel blue-violet inorganic pigment.