Divalent Chromophores in Trigonal Bipyramidal Coordination: YIn _{1– x} ( M _0.5 ²⁺ Ti _0.5 ) _x O ₃ (M = Co, Mn, Ni, Fe) Solid Solutions

The rational design of inorganic pigments with predetermined colors remains challenging due to the complex interplay of factors governing their optical properties, among which the oxidation state of the chromophore plays a critical role. Chromophores obeying selection rules are preferred for producing intense colors in transition-metal oxides. The intense blue color in the hexagonal YIn 1– x Mn x O 3 (YInMn blue) system originates from the allowed d–d transition of Mn 3+ in trigonal bipyramidal (TBP) coordination. Motivated by this, a series of hexagonal YIn 1– x ( M 0.5 2+ Ti 0.5 ) x O 3 (M = Co, Co/Zn, Mn, Ni, Fe) solid solutions were synthesized in evacuated quartz tubes to stabilize divalent chromophores. The Co and Co/Zn containing series exhibit intense, tunable magenta colors, the Mn-containing series shows olive green hues, the Ni-containing series displays lime green colors, and the Fe-containing series produces tunable brown shades. Neutron diffraction refinements confirmed the proposed crystal structures and compositions, while optical, magnetic, and X-ray absorption spectroscopy (XAS) verified the divalent oxidation state of the chromophore ions in the as-synthesized samples. Upon reheating in air, all compositions except the Ni-containing series exhibited a noticeable color change, which was largely reversible upon subsequent reduction, with colors returning to shades similar to the original samples. Combined optical, magnetic, and XAS analyses indicate oxidation of Co, Mn, and Fe from M 2+ toward M 3+, whereas optical and magnetic measurements suggest that Ni remains in the +2 oxidation state.