Malayaite ceramic pigments: A combined optical spectroscopy and neutron/X-ray diffraction study

摘要

Ceramic pigments based on the Cr-doped malayaite structure were synthesized by solid state reaction and characterized by optical spectroscopy and combined X-ray and neutron powder diffraction in order to elucidate the still unclear chromium substitution mechanisms. The results show that coloration is actually due to simultaneous occurrence of Cr~(4+) and Cr~(3+) ions in the crystal lattice. Spectroscopy data confirm that Cr~(4+) is replacing Sn~(4+) in the octahedral site and, in minor amount, Si~(4+) in the tetrahedral site. In addition, neutron powder diffraction data suggest that Cr~(3+) substitution for octahedral Sn~(4+) is charge balanced by the formation of oxygen vacancies with no preference over the different oxygen sites. Upon incorporation of Cr ion, the SnO_6 octahedra exhibit an off-centre displacement of central cation which in turn induces a rearrangement of both the octahedral and tetrahedral coordination shells.