Researchers discover state of matter which is simultaneously solid and liquid

A person clambers along mounds of potassium waste from a local salt mine in the town of Soligorsk, Belarus, in this file image from 2014. The metal was used by researchers to theoretically model a substance which is part solid, part liquid.

(CNN)When is a solid not a solid?

Scientists have discovered that, under certain conditions, some elements could exhibit properties of both solids and liquids.
A team of physicists based at the University of Edinburgh in Scotland used artificial intelligence to simulate what would happen if potassium was subject to huge pressure -- between about 200,000 and 400,000 times atmospheric pressure -- and heat -- between 400 to 800 Kelvin (260 to 980 degrees Fahrenheit).
    While potassium atoms, like most metals, behave in an orderly way under ordinary circumstances, the researchers noted that under extreme conditions they line up in complex arrangements -- a core of atoms lines up in a cylindrical tube, arranged in an "X" and four long chains sitting alongside this.
    The two distinct arrangements are referred to as "host-guest structures," the study's co-author, Andreas Hermann told CNN.
    The resulting substance is, on an atomic level, "two intertwined, interlinked crystal lattices -- that in itself is a strange thing," he said.

    Chain-melted state

    The study, which is published this week in journal PNAS, found that under these conditions potassium atoms entered what is known as a chain-melted state, where one of the two lattices -- the "guest" part -- dissolved into liquid, while the "host" part remained solid.
    "Upon heating, the guest atoms can 'melt,' while the host atoms remain crystalline," the paper explains.
    "The 'host' lattice is much stronger bound and so that remains a solid -- it needs more energy to melt it," he says. "The 'guest' is weaker bound which is why it liquifies."
    The two structures -- 80% host and 20% guest -- are identical on an atomic level, and only differ in how they line up.
    "You can't distinguish the (atoms), when they forming you can't tell which one will go where," he says.
    "If you could hold it, the liquid part could leak out, but because they're the same atoms, the solid will partially liquify to replace the liquid, and the liquid part will reconstitute itself (into the solid)."

    Constantly reforming block of solid-liquid

    The resulting substance -- if it could be observed by humans -- would look like a solid block of potassium which is simultaneously leaking liquid and reforming as a solid.
    Hermann describes it as "a sponge material that can soak up a liquid -- the sponge is solid, but in this case they are the same material. The sponge that soaks itself up. Then it leaks out it reforms the sponge around itself.
    As the structure is 80% "host" structure and 20% "guest," the substance is "always in a state of 80% solid, 20% liquid," he says.

    Machine learning advances

    Artificial intelligence models were developed to extrapolate what could be seen at the atomic level and apply these findings to much larger theoretical samples.
    Hermann says that the machine-learning modeling has implications for how other elements behave under similarly extreme conditions.
    "We can replicate this modeling with other metals," he says. "This machine learning technique is really taking off in the (physics) community right now -- it seems to be accurate enough to trust what comes up and we can upscale calculations and model entirely different processes.
      "It's all driven by quantum mechanics calculations, which is restricted to hundreds of atoms, with machine learning we can upscale by a factor of a hundred.
      "One can use machine learning to build a potential for how the atoms behave over a large temperature and pressure range."