Oxidation state and localization of chromium ions in Cr-doped cassiterite and Cr-doped malayaite

The oxidation state and the localization of the chromium ions in the ceramic matrix of Cr-doped cassiterite (SnO2) and Cr-doped malayaite (CaSnSiO5) pigments have been investigated through the use of X-ray absorption near-edge (XANES), extended X-ray absorption fine structure (EXAFS), optical absorption and electron spin resonance (ESR) spectroscopies as well as by measurements of the unit cell parameters and the magnetic susceptibility of the pigments. We have found that three types of chromium species are present in the Cr-doped cassiterite pigments; a majority phase consisting of Cr(III) oxide clusters, a small amount of CrO2 nanoparticles and Cr(IV) ions dissolved in the cassiterite lattice (∼5% of the total Cr amount), which must be responsible for the violet colour of this pigment. In the case of Cr-doped malayaite, most chromium cations are tetravalent forming a solid solution with the malayaite lattice by mainly substituting for Sn(IV) cations in isolated octahedral positions, although a very small amount of Cr(IV) also substitutes for tetrahedral Si(IV)