Optical properties and application of blue and purple inorganic pigment powders based on cobalt/nickel doped Sr2MgSi2O7

Abstract

In the present work, solid solutions of Sr₂Mg_1-xCo_xSi₂O₇ (0.2 ≤ x ≤ 1) with blue coloration and Sr₂Mg_1-xNi_xSi₂O₇ (0.1 ≤ x ≤ 0.5) with purple coloration were successfully synthesized via solid state reactions. Rietveld refinement analysis confirmed the successful substitution of Mg²⁺ by Co²⁺/Ni²⁺, as indicated by corresponding changes in cell volumes. These solid solutions display tunable blue and purple hues, influenced by the doping levels of Co²⁺/Ni²⁺. Compared to commercial cobalt blue (CoAl₂O₄), the synthesized blue oxides significantly lowered Co²⁺ levels while achieving the desired color properties. The rarity of purple coloration in nickel compounds makes this topic of particular interest. UV–Vis–NIR spectra revealed that the observed blue and purple colors arise from the ${}^4{A}_2\left({}^{4}F\right)\to {}^4{T}_1\left({}^{4}P\right)$ and ${}^3{T}_1\left({}^{3}F\right)\to {}^3{T}_1\left({}^{3}P\right)$ transitions of Co²⁺ and Ni²⁺ ions at tetrahedral sites, respectively. Furthermore, the thermal stability of the pigments was evaluated. To assess their compatibility with the polymer, 2 wt% of the as-prepared pigments were incorporated into poly(methyl methacrylate) (PMMA) to fabricate blue and purple PMMA composites, and their color properties were subsequently characterized.