Blue-violet ceramic pigments based on Co and Mg Co2−xMgxP2O7 diphosphates

Solid solutions of Co and Mg diphosphates with compositions Co2−xMgxP2O7 (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional coprecipitation route and calcined up to 1000 °C/2 h. Samples were characterized by thermal analysis, XRD, SEM/EDX, UV–vis-NIR spectroscopy and colour measurements (CIE-L*a*b*). Isostructural Co2−xMgxP2O7 diphosphate solid solutions (monoclinic system and P21/c spatial group) formed successfully within the studied range of compositions, accompanied only by a minor quantity of residual Co or Mg orthophosphates (M3(PO4)2). Interestingly, the obtained solid solutions developed nice blue-violet colourations even with high Mg doping after enamelling within double-firing (x = 1.5–1.8) and single-firing (x = 1.0–1.5) ceramic glasses. These optimal compositions containing a minimized Co amount (measured values around 7–16 wt%) could be therefore less toxic alternatives to the conventional Co3(PO4)2 blue ceramic pigment.