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Scientists Just Found an Unexpected Property in a Solid Metal: It 'Remembers' its Liquid State

Wha??

FIONA MACDONALD
14 MAR 2017
 

Researchers have probed samples of metal bismuth, and found a completely unexpected property - under certain conditions, the solid metal can retain a type of 'structural memory' of its liquid state.

The fact that scientists have found a new property of metals is exciting enough. But this also means solid bismuth can go from being repelled by a magnetic field (diamagnetic) to being attracted to a magnetic field (ferromagnetic), which could lead to a whole new way of creating materials with unique properties.

 

The phases of matter we learn about in high school, such as liquid, gas, and solid, are all defined by the way molecules in matter are arranged depending on external conditions. For example, liquid water freezes and contracts together, expanding into ice, or relaxes and boils into steam.

But bismuth is one element that's not so straightforward. It's most commonly found in solid form, but under increasing pressure and temperature conditions, it can undergo a broad array of phase transitions. For example, scientists have observed eight different types of solid phases in the metal so far.

So far, so weird.

But it gets stranger. One of the most interesting properties of bismuth is that it's usually repelled by a magnetic field when it's a solid, thanks to a phenomenon known as diamagnetism, but under certain high pressure and high temperature conditions - which usually coincide with it being in a liquid state - the metal can become ferromagnetic, or attracted to a magnetic field.

Because of this weird property, scientists have used bismuth to make a lot of important observations about the effect of magnetic fields on electrical conductivity (usually bismuth is an incredibly weak conductor).

And now they've found something else about the metal - it appears to be capable of retaining structural memory of its liquid phase, even when it's a solid.

 

Structural memory in metals isn't that odd - we already know that it's possible for a solid metal to retain a memory of a previous solid arrangement, which is what you see happening in shape-memory alloys (if you're not familiar with those whacky materials, go and watch this video immediately).

Shape-memory alloys have a distinct arrangement that they'll always bounce back to when heated or cooled to a certain temperature. But the arrangements are always solid - the metals aren't bouncing between different phases.

In this study, researchers from the Carnegie Institution for Science brought bismuth to a liquid state at pressures an incredible 14,000 to 24,000 times greater than normal atmospheric pressure, and about 1,250 Kelvin (977 degrees Celsius, or 1,800 degrees Fahrenheit).

When they slowly cooled that bismuth back down to a solid state, they found that the solid 'remembered' some of the structure of its liquid past life.

That doesn't mean the metal was morphing back into the same liquid shape it had before, but it was remembering the ferromagnetic properties of that liquid state and was being attracted to a magnetic field, rather than repelled by one.

"The high-pressure liquid becomes more structurally disordered when the heat is applied, taking on what we call a 'deep liquid' state, certain structural characteristics of which remain even when the bismuth is cooled back to solid," explained one of the researchers, Guoyin Shen.

"This is the first time such an effect has been seen in an elemental metal."

For now, it's very early days, and the team isn't entirely sure how this structural memory is stored in the metal, or how the solid bismuth is able to act as a ferromagnet, instead of being repelled by a magnet as it usually would.

"The physical origin of this surprising behaviour requires additional investigation," the team writes in the journal PNAS.

But they do think that this strange new property could also induce a similar shift in other elements, such as cerium, antimony, and plutonium. And if that's the case, it could lead to a whole new way of creating these elements with untapped properties.

The research has been published in PNAS.

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