Abstract
Pyrochlore-type [ A 2 VIII B 2 VI O 6 O ] rare earth stannates (PS) and titanates (PT), doped with increasing amounts of Cr, develop colours ranging from magenta to brown. Combined X-ray and neutron diffraction data revealed that Cr solubility in both pyrochlores is very limited, i.e. 0.07 atoms/formula unit in PS and 0.06 atoms/formula unit in PT. A further increase in the Cr amount determines the formation of the YCrO 3 perovskite. Diffuse reflectance spectroscopy data showed that the optical properties of Cr-doped yttrium titanates and stannates are rather similar, and that the final colour of the powders is due to the overlapping peaks of Cr 4+ in the octahedral site of the pyrochlore and of Cr 3+ in the ternary perovskite. The oxygen occupancy refinement showed that no anion vacancies are formed upon Cr substitution for Sn 4+ or Ti 4+, implying that such a charge compensation mechanism does not occur, confirming that the Cr in titanate and stannate pyrochlores is mainly in a tetravalent state.
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