Feeling blue: utilising cation ordering to engineer the colour of apatite pigments

Manganese doped chlorapatite compounds have attracted interest as inorganic pigments due to their vivid colours. In this paper, we investigate the effect of Sr incorporation in the turquoise pigment Ba10(MnO4)(PO4)5Cl2. on the structure and optical properties. The results show a turquoise colour throughout the series studied, except for a single compound, Ba6Sr4(MnO4)(PO4)5Cl2 which exhibits an intense blue hue. The colour change is explained by Ba/Sr cation ordering in the apatite structure, with this composition corresponding to complete occupation of the A1 site by Sr and the A2 site by Ba, and similar results are also observed for the related manganese doped bromoapatite system. The unique blue colour for these cation ordered compositions illustrates the potential to control the colour of apatite pigments through exploiting cation site preferences, while its’ low cost represents a key advantage compared to recently reported blue YIn1-xMnxO3 pigments.