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 d-d transition of trigonal bipyramidal (TBP) Mn³⁺ is responsible for the bright blue color in the YIn_1-xMn_xO₃ solid solution. The crystal field splitting between a'(d_z²) and e'(d_x²-y², d_xy) energy levels is very sensitive to the apical Mn-O distance. We therefore applied chemical pressure to compress the apical Mn-O distance in YIn_1-xMn_xO₃, 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 Ti⁴⁺/Zn²⁺ and Al³⁺ onto the TBP In/Mn site, which yielded novel violet/purple phases. The general formula is YIn_1-x-2y-zMn_xTi_yZn_yAl_zO₃ (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 YIn_1-xMn_xO₃. 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 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 Co₃(PO₄)₂, our pigments are much more thermally stable and chemically inert, and are neither toxic nor carcinogenic.