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

Pyrochlore-type [A2VIIIB2VIO6O] 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 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 colour of the powders is due to the overlapping peaks of Cr4+ in the octahedral site of the pyrochlore and of Cr3+ in the ternary perovskite. The oxygen occupancy refinement showed that no anion vacancies are formed upon Cr substitution for Sn4+ or Ti4+, 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.