From Serendipity to Rational Design: Tuning the Blue Trigonal Bipyramidal Mn 3+ 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 YIn1-x Mnx O3 solid solution. The crystal field splitting between a'(dz2 ) and e'(dx2 -y2 , dxy ) energy levels is very sensitive to the apical Mn-O distance. We therefore applied chemical pressure to compress the apical Mn-O distance in YIn1-x Mnx O3 , 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 YIn1-x-2y-z Mnx Tiy Zny Alz O3 (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 YIn1-x Mnx O3 . 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.