Cobalt--zinc molybdates as new blue pigments involving Co²⁺ in distorted trigonal bipyramids and octahedra

Lionel Robertson (1) , Mathieu Duttine (1) , Manuel Gaudon (1) , Alain Demourgues (1)

1 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

Zn_1-xCo_xMoO₄ (x < 0.3) compounds prepared by the solid-state route crystallize in a triclinic unit cell with the Pt space group. X-ray diffraction and Rietveld analyses reveal the occurrence of three highly distorted sites with equal occupancies corresponding to one trigonal bipyramidal CoO₅ group and two octahedral CoO₆ groups occupied by either Zn²⁺ or Co²⁺. Mo⁶⁺ 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 Co²⁺ d-d transitions associated with CoO₅ and CoO₆ 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 MoO₄ entities and polarizable Zn²⁺ cations creating highly distorted noncentrosymmetric sites partially occupied by Co²⁺ allows stabilization of the first blue pigment without Co²⁺ 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.

Mots clés: Cobalt; Molybdates; Structure; Local environments; d−d transitions; Blue pigment

Domaines: Matériaux

Lionel Robertson, Mathieu Duttine, Manuel Gaudon, Alain Demourgues. Cobalt--zinc molybdates as new blue pigments involving Co²⁺ in distorted trigonal bipyramids and octahedra. Chemistry of Materials, 2011, 23 (9), pp.2419-2427. ⟨10.1021/cm200795p⟩. ⟨hal-00593858⟩