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 Mn_2-xMg_xP₂O₇ (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-Mn_2-xMg_xP₂O₇ 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.