Famous for raising hopes of riches beyond imagination—and then dashing them—the mineral pyrite is better known as fool’s gold. Its metallic yellow luster has fooled many over the years, with consequences that helped shape the modern world, along with the fortunes (and misfortunes) of individuals: According to one story, a fool got what he deserved by marrying a woman for the “hills of gold” on her land that—as you might have guessed—turned out to be hills of pyrite.
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| A close-up of a small piece of fool’s gold, about 2 cm across. Image Credit: US Geological Survey, public domain. |
It may be worthless as a currency, but that doesn’t mean pyrite doesn’t have value—or at least the potential for it. In recent decades, scientists have been intrigued by the possibility of using pyrite in semiconductor technologies, especially in solar cells and other renewable energy applications. But although it has many promising features, things haven’t panned out so far. Compared to what theories predict, the material consistently underperforms in its ability to convert light into electricity, for reasons that aren’t completely understood.
But a breakthrough in unraveling this mysterious shortcoming may be on the horizon: The source of the trouble have to do with hidden hydrogen atoms lurking inside of the material, according to new research from scientists at Japan’s High Energy Accelerator Research Organization and the Graduate University for Advanced Studies.
Most people think of hydrogen, the first and simplest element of the periodic table, only as a gaseous material, occasionally relevant for its flammability or applications in fuel cells. But according to the senior scientist behind this new research, Ryosuke Kadono, that’s not the whole story. Hydrogen can also stealthily enter matter and remain hidden, causing trouble for its host. “Hydrogen is a ninja-like impurity in matter, most ubiquitous and very active, but hard to identify,” says Kadono.
Sometimes it can be useful—even preferable—to embed hydrogen in semiconductor materials. In the right quantity and at the right place, hydrogen can reduce the impact of troublesome structural defects. But too much hydrogen, especially where it’s unexpected or unidentified, can change the electrical properties of the host material in undesirable ways.
After learning that pyrite exhibits some mysterious electrical activity, Kadono wondered whether the material could be home to some ninja-like hydrogen. If so, perhaps controlling the hydrogen could finally enable pyrite to live up to its potential a semiconducting material. Hydrogen is notoriously difficult to detect in materials, especially in low concentrations, so the team turned to a unique approach: muon implantation.
A muon is an elementary particle, a sort of cousin to the electron: it’s got the same negative electric charge as the electron, but about 200 times the mass. Muons are usually spotted when they’re generated by cosmic rays and other high-energy events, but they’re never really seen as a component in ordinary matter. Like most of the fundamental particles, though, muons have an antimatter counterpart, with the same mass but an opposite charge—earning them the imaginative name “positive muons“. With its relatively hefty mass and a positive charge that matches the proton’s, a positive muon impersonates a hydrogen nucleus pretty well. The resemblance gets a level deeper when the positive charge of the muon attracts a negative electron to it, forming exotic particles that Kadono and his team call “muogens”—the key to their study.
Here’s the basic idea. Like all particles, muons also have a property called spin. If you implant muons into a material with aligned spins and then measure their spins over time, the changes in spin provide insight on the electronic structure of the muons. Since these “muogens” have a lot in common with genuine hydrogen atoms, you can infer the electronic structures of hidden hydrogen atoms from the muon results.
This process doesn’t tell you whether the sample contains hydrogen, but it does tell you the electronic properties of the hydrogen atoms if they are there. By comparing these results to theoretical predictions for a material with and without hydrogen, you can determine if there are “ninjas” in hiding.
In this experiment, the researchers started with a block of natural pyrite. They cut it into slabs and characterized the general properties of the material. Then, they exposed a slab to a beam of muons, implanting them within the pyrite. To see how their implanted muons were behaving, the researchers monitored their properties under different experimental conditions using instruments at the Japan Proton Accelerator Research Complex and TRIUMF, a particle accelerator lab in Canada.
According to the data, the implanted muons occupied two different electronically active states. This means that if there are hidden hydrogen atoms, they are indeed changing the electrical properties of the pyrite. Their complicated combined influence could very well be the source of some or all of the “mysterious electrical activity” observed in pyrite, and a reason why it’s not living up to its full potential.
The next step in this quest is an in-depth theoretical analysis of what hidden hydrogen in pyrite would look like. Comparing predictions from such an analysis with the results of this experiment could help us uncover the true potential of fool’s gold.