Cobalt–Zinc Molybdates as New Blue Pigments Involving Co²⁺ in Distorted Trigonal Bipyramids and Octahedra

Zn 1– x Co x MoO 4 ( x < 0.3) compounds prepared by the solid-state route crystallize in a triclinic unit cell with the P 1̅ space group. X-ray diffraction and Rietveld analyses reveal the occurrence of three highly distorted sites with equal occupancies corresponding to one trigonal bipyramidal CoO 5 group and two octahedral CoO 6 groups occupied by either Zn 2+ or Co 2+ . Mo 6+ ions are located in distorted tetrahedral sites. Electron paramagnetic resonance analysis allows characterization of the two kinds of environments with orthorhombic distortion related to two signals with different g values. The UV–vis spectra exhibit intense Co 2+ d–d transitions associated with CoO 5 and CoO 6 chromophores between 1.9 and 2.6 eV in the visible domain and O–Mo charge transfer at the frontier between the UV and visible ranges. A blue-transmitted window can then be displayed. The formation of covalent MoO 4 entities and polarizable Zn 2+ cations creating highly distorted noncentrosymmetric sites partially occupied by Co 2+ allows stabilization of the first blue pigment without Co 2+ in the tetrahedral environment. Moreover, only 5–10% cobalt is necessary to get a strong blue hue because of the highly distorted sites leading to very important oscillator strengths.