A crystal made of strontium, iodine, oxygen, and hydrogen looks like an ordinary white powder. But when you squeeze it with a force of hundreds of thousands of atmospheres, a rearrangement begins inside. Iodine atoms, which usually keep their distance, suddenly move closer and form new bonds.
At a pressure of 2.5 GPa (equivalent to a 25-ton weight on your thumbnail), individual molecules of iodic acid and iodate start to pair up. Squeeze harder — to 4.5 GPa, and these pairs merge into chains of four units. So iodine, which typically forms no more than one or two bonds, now holds several neighbors at once.
These changes show up not only in X-ray images but also in how the crystal compresses. It squeezes unevenly: easier in one direction, harder in another. Light also passes through it differently — the gap between electronic energy levels narrows from 4.1 to 3.7 electron volts. And above 6 GPa, the crystal changes its type of conductivity: whereas before an electron could jump directly, now it needs an extra push.
So simple compression turns the same substance into a material with different properties. This helps us understand how materials behave deep underground or in planetary interiors.
Iodine under normal conditions forms violet-black crystals with a sharp odor, but at high pressure it can become a metal and conduct electricity.