Solid solutions of mixed metal Mn3−xMgxFe4(PO4)6 orthophosphates: Colouring performance within a double-firing ceramic glaze

Solid solutions of mixed metal Mn3−xMgxFe4(PO4)6 orthophosphates (x = 0, 0.5, 1, 1.5, 2, 2.5, and 3) were prepared for the first time (from coprecipitate powders calcined up to 1000 °C) and characterized by thermal analysis, XRD, SEM/EDX, UV–vis–NIR spectroscopy and colour measurements (CIE-L*a*b*). Orthophosphate (Mn,Mg)3−yFe4+z(PO4)6 solid solutions isotypic to Fe7(PO4)6 structure (triclinic P-1(2) spatial group) formed successfully as the major crystalline phase within the studied range of compositions, accompanied only by variable quantities of α- and/or β-Mg2P2O7 diphosphates as secondary phases. Noteworthy, the obtained solid solutions were tested as potential ceramic dyes and exhibited an interesting interaction upon enamelling within a double-firing ceramic glaze: considerable amounts of Fe segregated from the solid solutions to be stabilized as hematite particles (α-Fe2O3) in the ceramic glaze and conferring the glaze an intense dark-brown colouration, which was almost independent of the amount of Mg doping. Thus, the obtained solid solutions with a minimized Mn content (especially Mg3Fe4(PO4)6 composition, without Mn) could serve as low-toxicity Fe reservoirs to stabilize hematite in double-firing glazes and produce an interesting dark-brown colouration, being an alternative to other brown ceramic pigments containing hazardous metals (i.e. Cr, Ni, Zn, or Sb).