Superconductivity comes with many unfulfilled promises. Power without loss? Sign me up. Superconducting Magnetic Resonance Magnets? They are, well, they are cool. And CERN couldn't work without liquid helium buckets to keep its magnets superconductive.
But these examples highlight the problem: almost all practical applications for superconductivity require temperatures of liquid helium. The search for high-temperature superconductors has led us to many strange places, including strange substances that are formed only at high pressure. We can now add another of these substances to the list: hydride, which is formed only under protest. However, once formed, it may be a superconductor above room temperature.
Why are we still looking for superconductors?
The search for superconductors continues because current superconductors face a number of challenges. If the magnetic field is too strong, the superconductivity disappears. Similarly, if the current density exceeds a certain limit, the resistance that warms the conductor appears, resulting in a fast and rapidly expanding failure. And the liquid helium needed to keep things first is expensive.
Low-temperature metal semiconductors are not the only game in town. There are certain ceramics that become superconducting at much higher temperatures. You only need liquid nitrogen for these materials, which is much cheaper than liquid helium. There are even industrial processes for creating bending superconducting conductors from this ceramic.
But you won't find power lines made of superconducting ceramics, since maintaining a steady supply of liquid nitrogen is a real challenge. High-temperature superconductors have also not captured the MRI magnet market ̵
Hydrogen is a very simple element. But it's also weird. It must be metal, but once hardened, it is not. It is intended to become metallic at high pressure and may even become a superconductor. However, observations for these countries are still controversial. The hydrides, on the other hand, combine the best of the two worlds. Metal atoms often bond quite well with hydrogen. Depending on the materials and the state of hydrogenation, the hydride can switch between conductive and non-conductive. If enough hydrogen is added, the hydride can even become superconducting.
The trick is to get the metal bonded to all those extra hydrogen – for example, let's talk about a lanthanum atom with 10 hydrogen atoms attached. This requires high pressure; otherwise, all of the hydrogen leaks out. Making a hydride superconductor is usually the worst of the two worlds – to create your superconducting material, you need pressure in the area of hundreds of GigaPascals, and you also need temperatures anywhere between liquid nitrogen and liquid helium.
On the plus side, there are so many hydride combinations that we haven't exhausted all the possibilities. To explore this space, a group of researchers took a lithium / magnesium alloy and began to calculate how its properties change with the addition of hydrogen. They found several stable phases from LiMgH2 to LiMgH16. After the researchers discovered all the potentially stable hydrides, they began to investigate their electronic properties.
Researchers were looking for the connection between electrons and a type of sound wave called the phonon. Superconductivity requires electrons to pair, since the pairs then form a kind of extended object that can move without resistance. Pairing is communicated through phonons.
In order to achieve superconductivity, electrons must easily generate and absorb phonons. In LiMgH 16 the researchers found a hydride with extremely strong binding. When the researchers calculated the temperature at which LiMgH 16 would be superconducting, they found it to be a remarkable 350K (77 ° C). Unfortunately, LiMgH 16 is stable only at pressures of about 250GPa.
Still a really interesting find as there are so many different hydride options. Although 250GPa is too high for practical use – it may even be too high to allow experimental demonstration of superconductivity – it does target the types of structures required to achieve superconductivity in hydrides. Other hydrides, perhaps those involving three metals, may require lower pressure to be stable and may even superconduct at reasonable temperatures.
Physical Review Letters, 2019, DOI: 10.1103 / PhysRevLett.123.097001 (About DOIs)