Deciphering Herapath's Crystal

When Bart Kahr opened the box he received from an eBay seller, he didn’t realize that he’d wind up trying to solve an 80-year-old puzzle by way of a 157-year-old mystery.

The puzzle is how thin single-crystal films of the crystal called herapathite are made; the mystery is herapathite’s crystal structure.

A professor of chemistry at New York University and an expert on the optical properties of crystals, Kahr came upon herapathite in an unusual way. Inspired by a 1929 volume by Ferdinand Bernauer, a former mineralogy professor at the Technical College of Charlottenburg, in Germany, titled “ ‘Gedrillte’ Kristalle,” or “ ‘Twisted’ Crystals,” Kahr posted a reference to the book on his website. Subsequently, he heard from two of Bernauer’s grandchildren, Rosalie and Walter Bright.

The two wanted to meet with Kahr to share what they knew about their grandfather, who was born in 1892 and died in Berlin in 1945. Among the materials they brought along was a vintage Carl Zeiss brochure for a “Bernotar” polarizing camera lens filter. Curious, Kahr purchased a Bernotar filter through eBay.

Sandwiched between the glass layers of the filter was hera­pathite, a polarizing crystalline material accidentally discovered by a pupil of the toxicologist William Bird Herapath. Although Herapath and his pupil initially had no idea how the “peculiarly brilliant emerald-green crystals” formed, further experimentation yielded the material when iodine was added to a solution of quinine disulfate in diluted sulfuric acid (Phil. Mag. 1852, 3, 161).

Herapath recognized the material’s light-polarizing properties, but it wasn’t until the 1920s that researchers figured out how to work with the fragile crystals. The most well-known work is that of Edwin H. Land, who ground the crystals in a ball mill until they were reduced to submicrometer dimensions, at which point he extruded them in polymers such as nitrocellulose to make polarizing films for sunglasses and photographic filters. The technology launched the company that later became Polaroid.

What Kahr found in the Bernotar filter, however, was not polymer-embedded microcrystals but what appeared to be a thin single-crystal film. “I was intrigued because growing thin single-crystal films is now a big deal for organic electronics,” Kahr says. “I couldn’t imagine how Bernauer did it. If we could rediscover his recipe, it could be generalizable.”

The first thing Kahr looked for was the crystal structure of herapathite, which didn’t exist. “I was astonished,” Kahr says, noting that an entire industry had been based on a substance that scientists still didn’t fully understand.

That’s not to say they didn’t try. Frank H. Herbstein, a professor of chemistry at Technion—Israel Institute of Technology, tackled herapathite, along with other polyiodide materials, in the 1980s. He recalls that the crystals in general were beautiful, but their diffraction patterns were poor, suggesting disorder or twinning. Advances in diffractometers and computer software have made coping with difficult-to-analyze crystals easier, Herbstein says.

Even with modern tools, Kahr says it was a struggle to solve the mystery of hera­pathite’s structure (Science 2009, 324, 1407). He and colleagues grew the crystals in a solution of quinine, concentrated sulfuric acid, and I2, roughly following a recipe published by Herapath (Phil. Mag. 1853, 6, 346).

The unit cell is, indeed, complicated, with the composition 4QH₂²⁺•3SO₄^2–•2I₃^–•6H₂O•CH₃COOH, where Q is quinine (C₂₀H₂₄N₂O₂). The I₃^– molecules are the light-absorbing entities and run in a zigzag chain along the absorption axis, and their close association allows delocalization of excited electrons.

The structure essentially confirms what researchers had suspected about herapathite, says Lars Kloo, a chemistry professor at Sweden’s KTH Royal Institute of Technology, noting that it is related to other compounds containing triiodide chains. Nevertheless, he adds, “it is an achievement to have solved the crystal structure of this complex and well-known material.”

Having solved the structure, Kahr is turning back to his original goal of figuring out how Bernauer made the thin films. Bernauer’s 1929 patent is enigmatic, and neither the Carl Zeiss archivist nor a Technical University of Berlin librarian can find any records on the Bernotar, Kahr says. He assumes the information was destroyed during World War II. But Kahr is determined. “We are going to figure it out somehow,” he says.