From Serendipity To Rational Design: Tuning The Blue Trigonal Bipyramidal Mn3+ Chromophore To Violet And Purple Through Application Of Chemical Pressure

We recently reported that an allowed dd transition of trigonal bipyramidal (TBP) Mn3+ is responsible for the bright blue color in the YIn1xMnxO3 solid solution. The crystal field splitting between a'(d(z)(2)) and e'(dx(2)y(2), d(xy)) energy levels is very sensitive to the apical MnO distance. We therefore applied chemical pressure to compress the apical MnO distance in YIn1xMnxO3, move the allowed dd transition to higher energy, and thereby tune the color from blue to violet/purple. This was accomplished by substituting smaller cations such as Ti4+/Zn2+ and Al3+ onto the TBP In/Mn site, which yielded novel violet/purple phases. The general formula is YIn1x2yzMnxTiyZnyAlzO3 (x = 0.0050.2, y = 0.10.4, 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 MnO distance compared with that in the blue YIn1xMnxO3. Magnetic susceptibility measurements verify the 3+ oxidation state for Mn. Diffuse reflection spectra were obtained over the wavelength region 2002500 nm. All samples show excellent near-infrared reflectance comparable to that of commercial TiO2, 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-3(PO4)(2), our pigments are much more thermally stable and chemically inert, and are neither toxic nor carcinogenic.