Crystal structural and optical properties of Cr-doped Y2Ti2O7 and Y2Sn2O7 pyrochlores

Pyrochlore-type [AVIII2BVIO6O] rare earth stannates and titanates, doped with increasing amts. of Cr, develop colors ranging from magenta to brown. Combined x-ray and neutron diffraction data revealed that Cr soly. in both pyrochlores is limited, i.e., 0.07 atoms/formula unit in the stannates and 0.06 atoms/formula unit in the titanates. A further increase in the Cr amt. dets. the formation of YCrO3 perovskite. Diffuse reflectance spectroscopy data showed that the optical properties of Cr-doped yttrium titanates and stannates are rather similar, and that the final color of the powders is due to the overlapping peaks of Cr4+ in the octahedral site of the pyrochlore and Cr3+ in the ternary perovskite. The oxygen occupancy refinement showed that no anion vacancies are formed upon Cr substitution for Sn4+ or Ti4+, imply that such a charge compensation mechanism does not occur, and confirms that Cr in the titanate and stannate pyrochlores is mainly in the tetravalent state