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 O_h-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.