A New Blue

By swapping a few indium atoms with manganese, chemists have created a new blue chromophore (J. Am. Chem. Soc., DOI: 10.1021/ja9080666). The Mn-doped substance suggests a route to compounds that could replace existing blue pigments with ones that are cheaper, more stable, and environmentally benign, the researchers say.

“We were not looking for a blue pigment,” confesses Oregon State University professor Mas A. Subramanian, who led the research effort. “In fact, we were actually looking for a multiferroic material”—both ferromagnetic and ferroelectric, for instance—and toward that goal were doping YInO₃ with Mn to make YIn_{1- x}Mn_xO₃. Although Subramanian’s group expected to pull black or gray material from their furnace, they were surprised to see a bright blue powder instead. “I had never seen anything like this in all my years working with metal oxides,” he recalls.

Because YInO₃ is white and YMnO₃ is black, Subramanian wondered what made the new compound blue. In collaboration with University of California, Santa Barbara’s Nicola A. Spaldin, his group determined that the color comes from the unusual trigonal bipyramidal coordination of ^Mn3+. This gives rise to energy levels in manganese’s d orbitals that result in an intense absorption in the red/green region of the visible spectrum.

The reported new composition is unlikely to be used for large-scale pigmentation because of indium’s cost and toxicity, Subramanian notes. But his team found they were able to create other blue chromophores by doping Mn³⁺ into trigonal bipyramidal sites in other metal oxides, such as LuGaMgO₄. They are using this strategy to design cost-effective blue pigments using cheaper materials.

“Today we are still using historical blue pigments like cobalt blue and Prussian blue, despite the fact that there are issues with their toxicity,” says Patrick M. Woodward, a solid-state chemistry expert at Ohio State University. “Subramanian and coworkers have found a simple route to a family of environmentally benign, chemically and thermally stable, bright blue inorganic pigments,” he says.