Optical properties of correlated pigment materials

Pigments have always been important in human history and are indispensable in our daily life: dyed textiles and clothes, colorful ceramics and glass, all the paintings of great artists would not exist without pigments. A large amount of pigments is nowadays also used for coloring plastics and lacquer (e.g. for cars). Conventional inorganic pigments, however, often contain toxic heavy metals. Thus, the search for non-toxic and environmentally-benign alternatives is of common interest. In this quest it would be most useful to simulate potential pigment materials on the computer and calculate their color and optical properties before synthesizing them in the laboratory. But unfortunately no straightforward theoretical method exists to date for assessing the optical properties of pigment materials. The proposed research project wants to close this gap. We will theoretically investigate three classes of novel, environmentally-friendly pigment materials: (i) rare-earth fluorosulfides, (ii) rare-earth cuprates and (iii) particular transition metal oxides. Our target materials are brilliant pigments with colors ranging from yellow-red (i) over green (ii) to blue (iii). In order to better understand the microscopic mechanisms behind the observed colors, we will calculate the optical properties of our target materials from first principles, i.e. by starting from their constituting atoms and electrons. This task represents a big challenge since in the chosen target materials the electrons interact strongly with each other. In other words, we are investigating materials with strong electronic correlations. Elaborate methods and approximations are needed to calculate the physical observables of these materials (e.g. their color). Among these methods are e.g. density functional theory (DFT) and dynamical mean-field theory (DMFT). We will use these and develop further methods to investigate the microscopic mechanisms behind the color of our target materials. Along this study we further plan to elaborate a general theoretical methodology, with the hope of subsequently being able to use this methodology to design new pigment materials.

Pigments have always been important in human history and are indispensable in our daily life: dyed textiles and clothes, colorful ceramics and glass, all the paintings of great artists would not exist without pigments. A large amount of pigments is nowadays used for coloring plastics and lacquer (e.g. for cars). Conventional inorganic pigments, however, often contain toxic heavy metals. Thus, the search for non-toxic and environmentally-benign alternatives is of common interest. In this research project, we have developed a theoretical approach to compute the colour and optical properties of novel pigment materials from first principles, i.e. starting from their constituting ions and electrons. This brings us a step closer to predict and design new pigment materials on the computer before synthesizing them in the laboratory. Specifically, we have theoretically investigated two classes of novel, correlated pigment materials: (i) red-yellow rare-earth fluorosulfides, and (ii) YInMn blue. We were able to predict their colours and electronic properties by starting from first principles, i.e. their crystal structure and electrons. This task represented a big challenge since in the chosen pigment materials the electrons interact strongly with each other. The methods we adapted to tackle this challenge were a combination of a semilocal exchange potential (mBJ) within density functional theory and the dynamical mean-field theory (DMFT). The developed computational approach was very successful, it did not only allow us to compute the colour and electronic properties of the rare-earth fluorosulfide and YInMn pigments, but was applicable to further materials such as the rare-earth mononitride semiconductors, which are promising materials for magnetic applications.