Silicon is like a universal building block: it goes into the chips in your phone, solar panels, and lasers. But if you squeeze it with immense force, the atoms rearrange into new patterns: these forms are called allotropes. One of them, Si-XIII, was first spotted more than twenty years ago, but its exact pattern remained a mystery. Scientists knew it existed but couldn't figure out how the atoms were arranged.
Now, researchers have combined computer modeling with experiments. They built a model of the crystal and tested it against all known data — distances between layers, vibration frequencies of atoms, and structural stability. The model fit perfectly. It turned out that Si-XIII is a special packing of atoms that arises under specific conditions of pressure and temperature.
This discovery fills a long-standing gap in silicon's phase diagram — a map showing which forms of matter exist at different pressures and temperatures. Understanding such forms helps create materials with tailored properties, like more efficient solar cells or new types of memory. And it's another example of how theory and experiment together solve problems that neither could crack alone.
At pressures of millions of atmospheres, silicon can become a metal and conduct electricity, even though it's a semiconductor under normal conditions.