Inorganic pigments with transition metal chromophores at trigonal bipyramidal coordination: Y(In,Mn)O3 blues and beyond

Robust Yellow-Violet Pigments Tuned by Site-Selective Manganese Chromophores

The rational design of multifunctional inorganic pigments relies on the manipulation of ionic valence and local surroundings of a chromophore in structurally and chemically habitable hosts. To date, the development of environmentally benign and intense violet/purple pigments is still a challenge. Here we report a family of A_3-xMn_xTeO₆ and A_3-2xMn_xLi_xTeO₆ (A = Zn, Mg; x = 0.01-0.15) pigments colored by site-selective Mn²⁺O₄ yellow and Mn³⁺O_5-6 violet chromophores. Zn_2.9Mn_0.1TeO₆ is intense bright yellow, comparable with commercial BiVO₄, and has better near-infrared reflectivity (∼89%) in comparison to commercial TiO₂. The codoped Li⁺ "activator" generates holes and charge-balanced Mn³⁺ (Mn³⁺O_5-6), realizing a color transformation from yellow to the bright violet pigments of A_3-2xMn_xLi_xTeO₆. The most vivid Mg_2.8Mn_0.1Li_0.1TeO₆ is probably the best violet pigment known to date, exhibits excellent chemical and thermodynamic stability, and demonstrates pressure-dependent stability up to 5-7 GPa, before a (reversible) phase transition to pink. Theoretical calculations revealed the correlation between site-preference occupancy and chromophore motifs and predicted a wide color gamut of pigments in Zn₃TeO₆-hosted 3d transition-metal ions other than manganese.

One-Pot Hydrothermal Synthesis of Victoria Green (Ca3Cr2Si3O12) Nanoparticles in Alkaline Fluids and Its Colour Hue Characterisation

One-pot hydrothermal preparation of Ca₃Cr₂Si₃O₁₂ uvarovite nanoparticles under alkaline conditions was investigated for the first time. The experimental parameters selected for the study considered the concentration of the KOH solvent solution (0.01 to 5.0 M), the agitation of the autoclave (50 rpm), and the nominal content of Si⁴⁺ (2.2–3.0 mole). Fine uvarovite particles were synthesised at 200 °C after a 3 h interval in a highly concentrated 5.0 M KOH solution. The crystallisation of single-phase Ca₃Cr₂Si₃O₁₂ particles proceeded free of by-products via a one-pot process involving a single-step reaction. KOH solutions below 2.5 M and water hindered the crystallisation of the Ca₃Cr₂Si₃O₁₂ particles. The hydrothermal treatments carried out with stirring (50 rpm) and non-stirring triggered the crystallisation of irregular anhedral particles with average sizes of 8.05 and 12.25 nm, respectively. These particles spontaneously assembled into popcorn-shaped agglomerates with sizes varying from 66 to 156 nm. All the powders prepared by the present method exhibited CIE-L*a*b* values that correspond to the Victoria green colour spectral space and have a high near infrared reflectance property. The particle size and structural crystallinity are factors affecting the Victoria pigment optical properties, such as CIE-L*a*b* values, green tonality, and near-infrared reflectance.

Hibonite Blue: A New Class of Intense Inorganic Blue Colorants

Commercially available spinel cobalt blue (CoAl₂O₄) utilizes a significant amount of carcinogenic Co²⁺, which makes its synthesis more hazardous and environmentally harmful. Considerable effort has been put into developing more environmentally benign and robust blue pigments to replace cobalt blue. A new class of blue pigments with tunable hue were prepared. The solid solution series, CaAl_12-2x Co _x Ti _x O₁₉ (0 < x ≤ 1), crystallizes in a hexagonal mineral hibonite (CaM₁₂O₁₉) structure with five distinct crystallographic sites for M cations (M = Al, Co, and Ti). The origin of intense blue color is attributed to a synergistic effect of allowed d-d transitions involving the chromophore Co²⁺ in both tetrahedral and trigonal bipyramidal crystal fields. Compared with commercial cobalt blue, these tunable hibonite blues possess a reddish hue that intensifies the blue color as observed in Y(In,Mn)O₃ (YInMn) blues, with a significant reduction of Co²⁺ concentration from 33% to as low as 4% by mass. A significant advantage of hibonite blues over cobalt blue is the substantial reduction in carcinogenic cobalt content while enhancing the color properties at a reduced cost for raw materials.