From Serendipity to Rational Design: Tuning the Blue Trigonal Bipyramidal Mn Chromophore to Violet and Purple through Application of Chemical Pressure.

We recently reported that an allowed d-d transition of trigonal bipyramidal (TBP) Mn is responsible for the bright blue color in the YInMnO solid solution. The crystal field splitting between a'(d) and e'(d, d) energy levels is very sensitive to the apical Mn-O distance. We therefore applied chemical pressure to compress the apical Mn-O distance in YInMnO, move the allowed d-d transition to higher energy, and thereby tune the color from blue to violet/purple. This was accomplished by substituting smaller cations such as Ti/Zn and Al onto the TBP In/Mn site, which yielded novel violet/purple phases. The general formula is YInMnTiZnAlO (x = 0.005-0.2, y = 0.1-0.4, and z ≤ 0.1), where the color darkens with the increasing amount of Mn. Higher y or small additions of Al provide a more reddish hue to the resulting purple colors. Substituting other rare earth cations for Y has little impact on color. Crystal structure analysis by neutron powder diffraction confirms a shorter apical Mn-O distance compared with that in the blue YInMnO. Magnetic susceptibility measurements verify the 3+ oxidation state for Mn. Diffuse reflection spectra were obtained over the wavelength region 200-2500 nm. All samples show excellent near-infrared reflectance comparable to that of commercial TiO, making them ideal for cool pigment applications such as energy efficient roofs of buildings and cars where reducing solar heat to save energy is desired. In a comparison with commercial purple pigments, such as Co(PO), our pigments are much more thermally stable and chemically inert, and are neither toxic nor carcinogenic.