Tuning the CrIV/CrIII valence states in purple Cr-doped SnO2 nanopowders : the key role of CrIV centers and defects

Béatrice Serment (1) , Manuel Gaudon (1) , Olivier Toulemonde (1) , Mathieu Duttine (1) , Cyril Brochon (2) , Alain Demourgues (1)

1 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
2 LCPO - Laboratoire de Chimie des Polymères Organiques

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.

Domaines: Matériaux

Béatrice Serment, Manuel Gaudon, Olivier Toulemonde, Mathieu Duttine, Cyril Brochon, et al.. Tuning the CrIV/CrIII valence states in purple Cr-doped SnO2 nanopowders : the key role of CrIV centers and defects. Inorganic Chemistry, 2019, 59 (1), pp.678-686. ⟨10.1021/acs.inorgchem.9b02943⟩. ⟨hal-02490016⟩