The story of Jacutingaite: how a wonder material went from the mine to theory, crystal growth and experiments

That’s exactly what happened, and more than once, in the years-long story of jacutingaite, a very exotic material that has kept both experimentalists and theorists busy for years, collaborating and taking turns in leading new discoveries.

The story started in 2008, when a strange tiny inclusion was noticed in minerals collected from a mine in the Brazilian state of Minas Geiras. A group of Czech mineralogists who got hold of the sample were surprised to find platinum in it, which is rare and unexpected. They extracted the sample, crashed it, solved the crystal structure, and wrote a paper in The Canadian Mineralogists reporting that it was an unexpected ternary compound containing platinum, with the chemical formula Pt₂HgSe₃. The material was called jacutingaite, from the local name of the mine where the first discovery had happened.

Not much happened for the following decade, apart from the fact that the material was added to one of the several databases that collect crystal structures of known compounds. From there, it entered the 2D materials database, created by MARVEL scientists and including materials that – according to calculations – can potentially be exfoliated from their 3D counterpart. But the potential to be turned into a two-dimensional layer would prove to be only one of the intriguing properties of this material.

More than ten years after the breakthrough by Kane and Mele, Marrazzo and the EPFL team showed that jacutingaite was the first ever material embodying strong Kane-Mele physics, with an energy scale that was several orders of magnitude larger than in graphene and that could actually be observed. In addition, monolayer jacutingaite exhibits other interesting effects: for instance, the robust topological phase could be switched on and off with relatively low electric fields due to a unique strong interplay between relativistic effects, crystal-symmetry breaking, and dielectric response. Those calculations would end up in a publication in Physical Review Letters and in Marrazzo’s PhD thesis Electronic Structure and Topology of Novel Materials. But, asked theorists, could Giannini grow crystals of it to confirm the prediction?

It was a challenging task. Jacutingaite includes both mercury, that is highly volatile and tends to evaporate when heated, and platinum, that requires very high temperatures to react with other elements. “And yet, the fact that it was found as a mineral told us that it can be stable" says Giannini. "So we asked, how can nature make it? And the answer had to be ‘by applying pressure”.

Giannini and his group used their multi-anvil cubic press in Geneva and began looking for the right combination of pressure and temperature to grow the jacutingaite crystal. “We found it around 15,000 times atmospheric pressure and 950 degrees Celsius”, explains Giannini. “The choice of the temperature is critical. You have a narrow window, because there is a secondary phase that has basically the same structure but without mercury and is stable in the same pressure range. Add more heat, and mercury will disappear from the crystal”.

What Giannini and his team managed to grow in Geneva was the bulk material, from which two-dimensional flakes could be isolated, but not the coveted 2D monolayer. “Since then, various groups have tried chemical or mechanical techniques for exfoliation, but nothing has worked. To this day, the isolated monolayer does not exist.”

Still, experiments were conducted on the 3D bulk material and on two-dimensional exfoliated flakes, using transport, electrical properties and tunnelling, angle-resolved photoemission. Not only were all data in excellent agreement with predictions, says Giannini, but they also pointed theorists towards new things.

“The experiments found things that theorists had not predicted, in particular a dual topology. The material was showing both a 2-dimensional topology creating protected edge states, and 3-dimensional topology producing surface states, that became evident in photoemission experiments”. That result went back to MARVEL, and Marrazzo, Gibertini and Marzari extended the model to also explain this dual topology, resulting in a joint publication of an experimental and a theory paper. “This back and forth between theorists and experimentalists was very exciting, and does not happen often” says Giannini.

Things have kept moving since 2020. At least a couple of groups in Geneva are continuing experiments on jacutingaite, using optics techniques (in the case of Alexei Kuzmenko’s group) – or tunnelling microscopy (in Christoph Renner’s lab), and the results may be published soon. Experimental effort is also needed to replicate the observation of a topological edge state reported by a 2020 study by a Hungarian group that used scanning tunnelling microscopy. Also on the theory side, several groups around the world have been working on jacutingaite following the prediction by the EPFL team and the first experiments done on Giannini’s samples in Geneva, and interesting theoretical results about this material continue to appear regularly in the literature.

Jacutingaite has also become a template for a whole family of materials. In 2020, a group of Brazilian theorists predicted that nine compounds must exist with the same crystal structure, replacing palladium or nickel for platinum, sulphur or telliurium for selenium, cadmium and zinc for mercury.  For all these theoretical structures the team calculated stability and predicted which phases may exist as well as their topological properties, in particular the energy gap that protects the topological state. What came out from these studies is that one of these compounds – the one with platinum, zinc and tellurium (Pt₂ZnTe₃) is predicted to have a higher topological gap than classic jacutingaite. “If it exists, it is even more interesting than jacutingaite” says Giannini, who’s been trying to synthesize this material since then but without success. “The problem is the simultaneous presence of zinc and tellurium, that tend to create very stable binary compounds.  We are trying various processing routes to achieve this goal”.

Still, experimentalists and theorists keep surprising each other with this family of materials. “There is a yet unexplored possibility to play around with chemistry and find the same structure to exist with different compositions, thus providing theorists with new input for further calculations of physical properties”, Giannini explains.

The story that started in that Brazilian mine 16 years ago may still hold surprises. “And we have to be proud of the fact that this story is entirely Swiss Made”,  says Giannini. “Everything was done between Lausanne and Geneva, where we have all the competences and equipment to go from theory to crystal growth to experiments, and back”.