Designing Novel Tunable Mn-based Inorganic Oxyfluoride Pigments

A high tunability of green-blue colors in the manganese-doped oxyfluoride Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga) and anion-deficient Sr2.5A0.5Mn0.1MO4-alpha F1-delta (A = Ca, Sr, Ba; M = Al, Ga) is reported, and the chromophores responsible for this intense pigmentation are investigated. The hues exhibited by these materials are quantified via diffuse reflectance UV/Vis spectroscopy and measurement of their direct band gaps via Tauc plot. It is shown that choice of A cation (A = Ca, Sr, Ba) and M cation (M = Al, Ga) for as-synthesized phases Sr2.5A0.5Mn0.1MO4F yield a wide range of green colors (band gap range 2.70-2.96 eV). Treatment of these phases under reducing conditions according to Sr2.5A0.5Mn0.1MO4-alpha F1-delta (A = Ca, Sr, Ba; M = Al, Ga) induces anion nonstoichiometry, shifting the observed colors to a wide range of blue/blue-purple hues (band gaps from 3.31 to 3.66 eV), showing potential as tunable inorganic blue pigments. Density field theory (DFT) calculations support the preferential occupation of the smaller 8-coordinate Sr(2) site by the substituted Mn2+ cation. X-ray absorption near-edge structure (XANES) data reveal more subtle nuances in the interplay between formal manganese oxidation state, crystallographic site and observed hue. In general, for as synthesized (green) Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga), the edge position in Mn K-edge XANES is consistent with mixed Mn3+-Mn4+ oxidation state whilst a clear pre-edge structure suggesting that Mn is present on a tetrahedral site. This would suggest that during the reduction step, Mn3+/Mn4+ is reduced to entirely Mn2+ and migrates from the tetrahedral to the Sr(2) lattice site.