Stability and coloring properties of Ni-qandilite green spinels (Ni,Mg)2TiO4: The “half color wheel” of Ni-doped magnesium titanates

• Ni-qandilite spinels (Mg2−xNixTiO4) prepared by ceramic and citrate-gel routes.
• Ni-doping favors qandilite decomposition in Ni-geikielite and periclase.
• Citrate route enables early entropy-stabilization of Ni-qandilite spinel at 1200 °C.
• Smart color tuning: orange Ni-karrooite, yellow Ni-geikielite, green Ni-qandilite.
• Ni-qandilite powders could be used as yellowish dyes for coloring ceramic glazes.

Solid solutions of Ni-doped qandilite spinel (Mg2−xNixTiO4, x = 0–0.4) prepared by solid-state and citrate-gel routes were investigated as potential ceramic pigments or dyes. Fired powders were characterized by conventional solid-state and color-measurement techniques. The increase of Ni-doping favored qandilite decomposition in Ni-geikielite (Ni,Mg)TiO3 and Ni-periclase (Ni,Mg)O. In the ceramic route, Ni-doped qandilite was stabilized at 1400 °C, but still accompanied by small amounts of MgTiO3 and MgO. Remarkably, the citrate-gel route enabled the formation of single-phase qandilite already at 1200 °C (up to 10%mol Ni), and at 1400 °C for all amounts of Ni-doping. Ni-doped qandilite developed green colors due to Ni2+ ions allocated exclusively in octahedral sites of qandilite spinel, being the green colors of higher chroma in citrate-gel powders. Ni-qandilite powders were chemically instable in low-temperature (950–1050 °C) ceramic glazes, acting only as yellow ceramic dyes. Optical properties of Ni-doped karrooite, geikielite and qandilite pigments were compared.

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