Co2+-doped diopside: crystal structure and optical properties

Abstract

Synthetic clinopyroxenes along the CaMgSi2O6–CaCoSi2O6 join were investigated by a combined chemical-structural-spectroscopic approach. Single crystals were synthesized by flux growth methods, both from Ca-saturated and Ca-deficient starting compositions. Single crystal structure refinements show that the incorporation of Co2+ at the octahedrally coordinated cation sites of diopside, increases the unit-cell as well as the M1 and the M2 polyhedral volumes. Spectroscopic investigations (UV–VIS–NIR) of the Ca-rich samples reveal three main optical absorption bands, i.e. 4 T 1g → 4 T 2g(F), 4 T 1g → 4 A 2g(F) and 4 T 1g → 4 T 1g(P) as expected for Co2+ at a six-coordinated site. The bands arising from the 4 T 1g → 4 T 2g(F) and the 4 T 1g → 4 T 1g(P) electronic transitions, are each split into two components, due to the distortions of the M1 polyhedron from ideal Oh-symmetry. In spectra of both types, a band in the NIR range at ca 5000 cm−1 is caused by the 4 A 2g → 4 T 1g(F) electronic transition in Co2+ in a cubic field in the M2 site. Furthermore, an additional component to a band system at 14,000 cm−1, due to electronic transitions in Co2+ at the M2 site, is recorded in absorption spectra of Ca-deficient samples. No variations in Dq and Racah B parameters for Co2+ at the M1 site in response to compositional changes, were demonstrated, suggesting complete relaxation of the M1 polyhedron within the CaMgSi2O6–CaCoSi2O6 solid solution.