Crystal Structure and Chromium Distribution in Malayaite Ceramic Pigments: a Combined Neutron Diffraction and X-ray Diffraction Approach

Chromium-doped malayaite, CaSnSiO5, is a widely used ceramic pigment for its unrivalled magenta colour, while Cr-doped titanite, CaTiSiO5, is a promising candidate as brown pigment from industrial waste recycling. Malayaite and the high-temperature form of titanite are isostructural, space group A2/a, being characterised by three cation sites: seven-coordinated site A (hosting Ca), 6-fold coordinated site B (accomodating Sn or Ti), and tetrahedrally-coordinated site C (Si). Recent advances - disclosing the possibility to have multiple valent states of chromium (i.e. III, IV, V) thus different size of Cr ions - re-open the question about chromium distribution in the different sites with its important implications on colouring performance of these pigments. In order to get an insight into Cr occupancy, metal-oxygen distances, O-M-O bond angles and oxygen vacancies, the crystal structure was investigated by combined Rietveld refinement of neutron (D2B line, ILL, Grenoble) and conventional X-ray (Bruker D8 Advance, solid state detector) diffraction data, and by the appraisal of the chromium valent states by diffuse reflectance spectroscopy (DRS). For this purpose, malayaite and titanite pigments were prepared by both ceramic route and spray drying plus liofilization, following two different strategies: 1) CaM1-xCrxSiO5 and 2) CaMSi1-xCrxO5, where M=Sn or Ti and 0.02<x<0.10, in order to promote the preferential accomodation of chromium in the octahedral site (series 1) or the tetrahedral site (series 2).