These crystals repeat their patterns at regular time intervals, not distances

It was only a matter of time.

A weird form of matter called a time crystal has made an appearance in two more types of materials, doubling the number of known time crystal habitats. In a typical crystal, its arrangement of atoms regularly repeats in space, such as the alternating sodium and chloride ions that make up a salt crystal. But time crystals’ patterns repeat themselves at regular time intervals.

A team of scientists created a time crystal in a solid material called monoammonium phosphate, the researchers report in both the May 4 Physical Review Letters and the May 1 Physical Review B. Another team made its time crystal in a type of liquid containing star-shaped molecules, according to a study also published in the May 4 Physical Review Letters.

Both time crystals rely on a quantum property called spin, which makes some atomic nuclei seem to whirl like a top. In the time crystals, the direction of that spin flipped at regular intervals.

Repeated radio wave pulses were used to trigger the spins’ flip-flopping. But even when the spins weren’t flipped perfectly, both materials kept up a regular pattern of flipping, revealing that they had a preferred time structure.

Scientists created the only other two known time crystals for the first time in 2016. One was made with defects in diamond; the other using a chain of ions, or electrically charged atoms, of the chemical element ytterbium

Unlike the other known examples, the monoammonium phosphate time crystal was created in a solid material with an orderly physical structure — a traditional crystal. The other materials were disordered.

These curiosities are so new that scientists still aren’t sure what materials time crystals are found in, or if they have any practical uses. Now, the clock is ticking on the next time crystal discovery.