Color of Mn(V)-Substituted Apatites A10((B, Mn)O4)6F2, A=Ba, Sr, Ca; B=P, V

The relationship between color and chemical composition in a series of Mn(V)-substituted fluoroapatites A10(B1-xMnxO4)6F2 is studied as a function of the nature of A=Ca, Sr, Ba; B=P, V, and the Mn substitution, x. These apatites exhibit deep blue to green colors. Saturation effects occur for some tetrahedral Mn(V) absorption bands, which are correlated to the large cross section of the corresponding transitions. These account for the blue to green color change with increasing the Mn content, x. The crystal field experienced by the Mn(V) ion is independent of x and of the nature of the B ion it replaces. This is because Mn (V) imposes its own size to the host lattice (mean Mn–O DISTANCE=1.72 ? in all cases). The displacement of the Mn(V) absorption bands for A over the series Ca, Sr, to Ba is mainly due to an increasing Mn–O bond covalency.