Pigments Based on Perovskite

Despite the wide compositional range and the flexibility of the perovskite lattice, there are just a few successful examples of colored phases based on this crystal structure. The systems proposed in the literature as pigments encompass alkaline earth titanates-stannates-zirconates and yttrium or rare earth orthoaluminates (doped with transition metals such as Cr and V) and orthoferrites/orthocobaltites as well as alkaline earth or lanthanide Ta-Nb-Ti oxynitrides. The coloration mechanism and technological prospect of these perovskites are outlined, focusing on the only system manufactured at the industrial scale and currently used as ceramic red pigment: the Cr-doped (Y,REE)AlO3. The color of these perovskites stems from intense absorption of the yellow-to-violet wavelengths by a band occurring at low Cr concentration when the pigment is synthesized with mineralizers. As the origin of such a band is still under debate, new experimental data are here provided. X-ray powder diffraction (associated with structural refinements performed by the Rietveld method) and diffuse reflectance spectroscopy experimental methods are employed to disclose the mechanism of incorporation of Cr and the role of different mineralizers. The effect of increasing chromium doping on the relaxation of the perovskite structure was investigated through the substitution of isovalent cations with different ionic radii (Cr3+ after Al3+ in yttrium orthoaluminates) as well as with almost equivalent ionic radii (Cr3+ after Ga3+ in La-Nd orthogallates). The effect of different mineralizers was appraised by adding a salt (chloride, fluoride, or carbonate of Ca, Mg, Na, K, Ba, or La) to the precursor batch of the Y(Al,Cr)O3 pigment. Although perovskites exhibit the weakest structural relaxation around Cr3+ ion, meaning that the chromophore is highly "compressed" once in small concentration (i.e., particularly in orthoaluminates), the incorporation of Cr ions occurs with increasing covalency and polarizability of the Cr-O bond. This mechanism is not replicated in orthogallates, where the tiny difference of ionic radii between octahedrally coordinated Cr3+ (0.615Å) and Ga3+ (0.62Å) induces subtle changes in crystal structural and optical properties. Thus, the response to Cr incorporation in terms of both cell and octahedral site distortion is different in Y, La, and Nd perovskites. Final remarks highlight that the red color of Y(Al,Cr)O3 is developed only in the presence of mineralizers, with Ca salts being the most effective. A little amount of calcium has to be incorporated at the cubic site and the charge mismatch between Y3+ and Ca2+ is likely compensated by oxygen vacancies. The intense absorption band responsible for the red color is thought to be originated by the occurrence of Cr3+-Ca2+pairs with a possible involvement of intervalence Cr3+-Cr4+ charge transfer. This chapter discusses the composition, crystal structure, and coloring performance of perovskites proposed as pigments in the literature. The focus is on crystal chemistry and structural principles of ideal and distorted ternary ABO3 perovskites, with special emphasis on the only perovskite pigment currently manufactured at industrial scale, that is, Cr-doped YAlO3. For this purpose, lattice parameters, A site coordination, bond valence (BV) analysis, tilting of octahedral framework, and tolerance factor in yttrium and rare earth orthoaluminates are outlined. The way by which chromophores are accommodated in the perovskite structure is explained by discussing basic concepts - local bond distances, structural relaxation coefficient, and polyhedral bond valence - and comparing chromium incorporation into YAlO3 with other Al-Cr solid solutions and with Ga-Cr substitution in La-Nd orthogallate perovskites. Finally, the chapter discusses the effects of mineralizers on the optical properties of Cr-doped YAlO3 are examined to explain the origin of red color.