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Rare-Earth vs. Heavy Metal Pigments and their Colors from First Principles

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for non-toxic alternatives based on rare-earth elements. The computation of colors from first principles is a challenge to electronic structure methods however, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the color of the red pigment cerium fluorosulfide CeSF, as well as of mercury sulfide HgS (classic "vermilion"). Our methodology employs many-body theories to compute the optical absorption, combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration and granularity. We introduce a quantitative criterion for the performance of a pigment. While for HgS this criterion is satisfied due to large transition matrix elements between wide bands, CeSF presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi two-dimensionality and the localized nature of 4f-states. Our work demonstrates the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties.