Intense Turquoise Colors of Apatite-Type Compounds with Mn 5+ in Tetrahedral Coordination

The solid solutions of chlorapatite compounds Ba5Mn3−xVxO12Cl (x = 0–3.0) and Ba5Mn3−xPxO12Cl (x = 0–3.0) have been synthesized through solid state reactions and Pechini or sol–gel method using citric acid. The colors of the samples change from white (x = 3.0) through turquoise (x = 1.5) to dark green (x = 0) with increasing amount of manganese. Optical measurements reveal that the origin of the color is presumably a combination of d–d transitions of Mn5+ and cation-anion charge transfer from transition metals to oxygens. Near IR reflectance measurements indicate that synthesized compounds are promising materials for “cool pigments” applications. Magnetic measurements verify that manganese has two unpaired electrons and exhibits 5 + oxidation state. The IR spectra change systematically with sample compositions and the fingerprint region (700 cm−1 to 1100 cm−1) indicates characteristic bands belonging to (MnO4)3−, (VO4)3− and (PO4)3− functional groups. Structure refinements using neutron data confirm that Mn5+, V5+ and P5+ cations occupy the tetrahedral sites in the apatite structure.