Synthesis of diphosphate Mn2−xMgxP2O7 solid solutions with thortveitite structure: New pink ceramic dyes for the colouration of ceramic glazes

Solid solutions of Mn and Mg mixed diphosphates Mn2-xMgxP2O7 (0.2 < x < 1.5) have been prepared by calcination (1000 °C/4 h) of coprecipitate precursors and characterized for the first time (through thermal analysis, XRD, SEM/EDX and UV–vis-NIR techniques). Its colouring performance (CIE-L*a*b* parameters) within a ceramic glaze has been also analyzed. XRD and SEM/EDX results confirm the formation of a homogeneous and continuous β-Mn2-xMgxP2O7 solid solution having the thortveitite structure, isomorphous to β-Mn2P2O7 and β-Mg2P2O7. The diphosphates exhibit light brown or pinkish-white colours associated to Mn2+ ions in low-symmetry octahedral coordination. Interestingly, they develop intense purple (x < 1) or pink (1.0 ≤ x ≤ 1.5) colours once enamelled (5 wt.%) within a single-firing ceramic glaze, which are mainly associated to the presence of Mn3+ ions according to UV–vis-NIR spectra. Therefore, the Mn–Mg binary diphosphates could be used as new pink ceramic dyes for the colouration of conventional ceramic glazes.