Crystallographic study of pyrochlore ceramic pigments

Pyrochlore-type [AVIII2BVI2O6O] rare earth stannates (PS) and titanates (PT), doped with Cr, are promising ceramic pigments, whose colour is expected to depend on mechanisms of charge balance Cr3+-Ti4+ and Cr3+-Sn4+ involving the occupancy of the seventh oxygen site. Some of the interesting properties of pyrochlores depend on order-disorder and non-stoichiometry of the structure. As a general rule, disorder increases with the ionic radius of B cation r(B4+) and with decreasing r(A3+). Different competing mechanisms govern the non-stoichiometry: an excess of cation A or B affects cation/oxygen vacancies or O accommodation in interstitial position. The aim of this research has been to improve the knowledge of the relationships between structural modifications and coloration induced by the incorporation of different amounts of Cr inside PS and PT. This goal has been pursued through combining Rietveld refinements of neutron and x-ray powder diffraction with UV-Vis-NIR spectroscopy data. Two series of pyrochlores (12 samples on the whole) with general formula A2B2-xCxO7-x/2 where A=Y, B=Ti or Sn, C=Cr and x=0.02, 0.07, 0.25, 0.50, 0.75,1.00 were prepared through the conventional ceramic route. In both series, Cr substitutes Sn and Ti in the octahedral site, but the Cr solubility is larger in PS than in PT. For increasing Cr amounts, a linear decrease of the cell volume and a shortening of B-O distance is observed in the PS series, while in the PT series the trend of cell volume and B-O distance isn't regular. Moreover, no variation in the oxygen occupancy is found, so the Cr substitution for Sn4+ or Ti4+ is not charge compensated. As concerns the colour of pigments, it must be underlined that the red hue of Cr-doped pyrochlores is affected by the occurrence, when Cr content is higher than 0.07 mole per formula, of the green coloured YCrO3 perovskite.