From Serendipity to Rational Design: Tuning the Blue Trigonal Bipyramidal Mn3+ Chromophore to Violet and Purple through Application of Chemical Pressure

High Near-Infrared Reflective Zn1-xAxWO4 Pigments with Various Hues Facilely Fabricated by Tuning Doped Transition Metal Ions (A = Co, Mn, and Fe)

A series of novel transition metal ion-substituted Zn_1-xA_xWO₄ (A = Co, Mn, and Fe, 0 < x ≤ 0.1) inorganic pigments with blue, yellow, brown, and pale green colors have been prepared by a solution combustion method and exhibit extremely high near-infrared reflectance (R > 85%). X-ray energy-dispersive spectroscopy analysis makes it clear that transition metal ions have already been incorporated into the host ZnWO₄ lattice and do not change the lattice's initial wolframite structure. The optical absorption spectrum in the UV region of the ZnWO₄ pigment calcined at 800 °C for 3 h is a ligand-to-metal charge transfer from O 2p nonbonding orbits to antibonding W 5d orbits. On account of the doping Co²⁺ (3d⁷), Mn²⁺ (3d⁵), and Fe³⁺ (3d⁵) transition metal ions, these chromophore ions have occupied the distorted octahedral site of Zn²⁺, leading to d-d transition and metal-to-metal charge transfer from the occupied 3d orbits of A²⁺ to unoccupied W 5d orbits in UV and visible ranges and generating some bright colors. Significantly, these inorganic pigments are also endowed with excellent thermal and chemical stability and are conducive to harsh working conditions. All of the analysis results have offered some design strategies for various colorful inorganic pigments with high near-infrared reflectance.

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.

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.