Cyan Ni1-x Al2+2x/3□ x/3O4 Single-Phase Pigment Synthesis and Modification for Electrophoretic Ink Formulation.

Cyan Ni1-x Al2+2x/3O4 single-phase pigments with various Ni/Al atomic ratios (from 1:2 down to 1:4) have been prepared by a sol-gel route (Pechini) followed by postannealing treatments. Nickel aluminates crystallize in the well-known spinel structure (Fd3m space group), where metals are located at two different Wyckoff positions: 16d (octahedron) and 8a (tetrahedron). Based on X-ray diffraction (XRD) Rietveld refinements, Ni2+ cations are shown to be partially located in both tetrahedral and octahedral sites and, in addition, cationic vacancies occupy the Oh environment. In the pure-phase series, Ni/Al = 0.35, 0.40, 0.45, as the Al content increases, the Ni2+ rate in the Td site decreases for Ni/Al = 0.45, thus altering the cyan color; within this series, the most saturated cyan coloration is reached for the highest Al concentration. Inorganic pigment drawbacks are their high density and hydrophilic surface, which induce sedimentation and aggregation in nonpolar media used in electrophoretic inks. Hybrid core-shell particle pigments have been synthesized from cyan pigments using nitroxide-mediated radical polymerization (NMRP) with methyl methacrylate monomer in Isopar G, leading to a dispersion of electrically charged hybrids in apolar media. Surface functionalization of the pigments by n-octyltrimethoxysilane (OTS) and n-dodecyltrimethoxysilane (DTS) modifiers has been compared. The inorganic pigments are successfully encapsulated by organic shells to allow a strong decrease in their density. Cyan inks, adequate for their use in e-book readers or other electrophoretic displays, taking further advantage of the high contrast ratio and reflectivity of inorganic pigments in regard to organic dyes, have been stabilized.

Tuning the CrIV/CrIII Valence States in Purple Cr-Doped SnO2 Nanopowders: The Key Role of CrIV Centers and Defects.

A low content of chromium (≤5 mol %) has been incorporated into a SnO2 cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O2 flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr4+ and Cr3+ ions as well as ferromagnetic Cr4+-Cr3+ and antiferromagnetic Cr3+-Cr3+ interactions. The strong purple hue is related to the isolated Cr4+ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10Dq value around 2.4 eV, whereas the 10Dq value is around 2 eV for isotropic Cr3+ ions, partially substituted for Sn4+ ions in cassiterite. Just after the coprecipitation process, only Cr3+ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr4+ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr3+ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature (T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr4+ species and limiting the Cr4+-Cr3+ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr4+ rate and the Cr4+-Cr3+ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.

Tailoring the Chemical Composition of LiMPO4 (M = Mg, Co, Ni) Orthophosphates To Design New Inorganic Pigments from Magenta to Yellow Hue.

New inorganic pigments with intense and saturated coloration have been prepared by a solid-state route and exhibit a large color scale from magenta to yellow. Indeed, yellow and magenta are two of the three subtractive model's colors with wide application in printing or displays as e-book readers. To develop yellow and magenta hue, we focused on cobalt- and nickel-based orthophosphates thanks to the chemical stability, low density, low price, and easy preparation of such a pigment class. All of these orthophosphates crystallize with the well-known olivine-type structure (orthorhombic Pnma space group) where transition metals are stabilized in a distorted octahedral site. This paper deals with the optical absorption properties of various orthophosphates, the correlations with structural features, and their colorimetric parameters (in L*a*b* color space). The LiCo1- xMg xPO4 series show near-magenta color with tunable luminosity, while the LiNiPO4 compound exhibits a frank yellow coloration. Co2+ (4T1) and Ni2+ (4A2) chromophore ions occupy a more or less distorted octahedral site, leading to tuning of the intensity of the d-d electronic transitions in the visible and near-IR ranges and providing a subtractive color scale; i.e., a LiCo1- xNi xPO4 solid solution possesses a very rich panel of colors between the two yellow and magenta extremes. It is worth noting that the crystal-field splitting and B Racah parameter have been estimated in a first approximation on the basis of the Tanabe-Sugano diagram and lead to the conclusion of a slightly higher crystal-field splitting of around 0.9 eV for Ni2+ ions and similar ß covalent parameters, despite the same crystallographic sites of both of these transition metals.