Tuning the Cr^IV/Cr^III Valence States in Purple Cr-Doped SnO₂ Nanopowders: The Key Role of Cr^IV Centers and Defects

A low content of chromium (≤5 mol %) has been incorporated into a SnO₂ cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O₂ flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr⁴⁺ and Cr³⁺ ions as well as ferromagnetic Cr⁴⁺-Cr³⁺ and antiferromagnetic Cr³⁺-Cr³⁺ interactions. The strong purple hue is related to the isolated Cr⁴⁺ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10Dq value around 2.4 eV, whereas the 10Dq value is around 2 eV for isotropic Cr³⁺ ions, partially substituted for Sn⁴⁺ ions in cassiterite. Just after the coprecipitation process, only Cr³⁺ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr⁴⁺ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr³⁺ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature (T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr⁴⁺ species and limiting the Cr⁴⁺-Cr³⁺ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr⁴⁺ rate and the Cr⁴⁺-Cr³⁺ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.