Structural, Optical, and Magnetic Properties of Ultramarine Pigments: A DFT Insight
2018
Rejmak, Paweł
The ultramarine pigments are among the most widely used coloring materials since the antiquity till present times. Despite many experimental studies, the characterization of ultramarines is still incomplete. In this work, we reported for the first time the density functional theory results obtained for realistic periodic and large cluster models of ultramarines with blue S₃– and yellow S₂– chromophores. Periodic calculations provided insight into Sₙ– siting inside aluminosilicate cages, normally not resolved well in experimental structural data. All electron calculations performed on large cluster models showed that the optical properties of S₃– ions depend little on their orientation within cavities, unless strong distortion from free S₃– ion C₂ᵥ symmetry is enforced by the lattice. No magnetic coupling between S₃– species occupying adjacent cages was found. Upon the present results, observed differences in the averaging of electron resonance signals should be rather ascribed to different S₃– dynamical effects. Though the quantitative computational treatment of S₂– systems is more challenging because of near orbital degeneracy, the qualitative results show that the electronic structure and spectroscopic properties of embedded S₂– radicals are more sensitive to the environment than in the case of S₃– species.
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