The materials science behind the reported room temperature superconductor

by | Aug 1, 2023

There's a fascinating link between the structure of bone and LK-99

Global electricity production is around 30,000 Terawatt hours. Its transmission and distribution in aluminium and copper cables to the consumer incurs inevitable energy losses due to resistive Joule heating.

Imagine, if a superconducting material could be discovered and fabricated in the form of cables able to transport an electrical current under ambient conditions without resistive energy losses, it would save around 7% of the global power usage in electricity transmission and distribution systems.

This dream has proven an elusive one despite intense materials research since the discovery by Heike Kamerlingh Onnes more than a century ago of resistance-free electrical conductivity in mercury that occurs when cooled to liquid helium temperatures. Above this frigid temperature mercury returns to the normal resistive conducting state of a metal. Onnes called this phenomenon superconductivity for which he was awarded the Nobel prize in physics.

In 1964, William Little predicted the possibility that superconductivity in a material could exist at room temperature. A milestone in this century-long quest was achieved recently in metal super hydride materials, exemplified by yttrium decahydride, YH10. Unfortunately, ultrahigh external pressures are required to observe superconductivity in this material at room temperature.

This shortcoming has impeded the development of superconducting technologies, such as high-speed magnetic levitation trains, magnetic resonance imaging, sensitive magnetic field detection devices, quantum computing devices, and electricity transmission cables with no thermal energy losses.

An interesting way to circumvent the need for these very high external pressures to maintain superconductivity would be to discover a material which exhibits ‘internal stress’ manifest as an extreme pressure within the material by a strain induced structural change.

This interesting idea has now been reduced to practice by a creative modification of the element composition of a lead oxy phosphate Pb10(PO4)6O material, which happens to have a crystal structure related to the inorganic biomineral found in bone.

The report appeared on the 22nd July 2023 on the preprint server published by a team of researchers from South Korea. As well as generating huge excitement, the report has engendered guarded scepticism about its authenticity and the burden-of-proof of superconductivity.

It is to be expected that with earth-shattering claims of this potential importance, replication of the observations and expansion of confirmatory testing by independent expert groups around the world will follow shortly to see if the results are authentic or artefactual.

Hoping the results stand up to analytical scrutiny, even at this early stage following the recent report it is worth trying to understand the complex physicochemical phenomenon underpinning this breakthrough in its simplest of terms. Let’s first compare the structure of apatite, a calcium hydroxy phosphate, Ca10(PO4)6(OH)2 to see how it relates to the lead oxy phosphate analogue doped with a little copper, formulated Pb10-xCux(PO4)6O, which is purported to be the first room temperature ambient pressure superconductor.

Notably, pure lead oxy phosphate Pb10(PO4)6O behaves as an electrical insulator, remarkably however on doping with copper, it displays some, but not all, of the expected diagnostic physics expected for a superconducting material but functioning at room temperature and ambient pressure. To amplify, the doped superconducting phase, Pb10-xCux(PO4)6O, has copper Cu2+ cations replacing around 10% of the lead Pb2+ cations. They form lines of lead running along the hexagonal axis of the crystallographic unit cell. The line of Pb(2) sites reside in a channel of tetrahedral phosphate PO43- anions. The oxide O2- anions occupying a quarter of the available sites in a channel comprised of Pb(1) sites.

The substitution of the Pb2+ with the much smaller Cu2+ cations causes contraction induced strain in the crystal lattice of Pb10-xCux(PO4)6O and this effect is envisioned to be the source of internal pressure exerted on the columns of Pb2+ cations from which the room temperature ambient pressure superconductivity emanates.

The solid-state mathematical physics underpinning the mechanism of this exciting discovery is complicated and not yet completely resolved. Briefly, the origin of the superconducting current in this material does not seem to involve resistance-free transport of pairs of electrons held together by vibrations of the atoms in the material, namely electron-phonon-electron coupled Cooper pairs as described by the Bardeen-Cooper-Schrieffer BCS theory. Instead, the superconductivity appears better described by a Brinkman-Rice modified version of BSC theory involving resistance-free transport of pairs of oppositely charged holes and their surrounding lattice polarization distortion called bi-polarons.   

To expand in the language of materials chemistry, the Pb10(PO4)6O phase is an electrical insulator arising from a closed shell Pb2+(6s2) electronic configuration. Devoid of empty electronic states for electrical charges to move under the application of an external potential, the transport of an electrical current in Pb10(PO4)6O is not possible, and the material behaves as an insulator. By replacing some of the Pb2+(6s2) with open shell Cu2+(3d9) the resulting electronic configuration of Pb10-xCux(PO4)6O provides empty electronic states allowing the movement of charges and the material becomes an electrical conductor.

In the absence of internal pressure, the electrically conducting Pb10-xCux(PO4)6O material would be expected to undergo a structural distortion to a more stable electrically insulating state, known in physics as a metal-insulator transition, where the delocalized electrons of an electrical conductor become localized in the insulating state.

Here, the ‘eureka’ moment is that because of the aforementioned internal pressure, this structural distortion is prevented, and the material remains electrically conducting. Incredibly not only does this electrically conducting state seem to display some of the physical characteristics of a superconductor but it is reported to retain that superconductivity above room temperature without any applied pressure, exhibiting a critical temperature Tc = 127°C, above which it becomes a normal electrical conductor.

If this breakthrough stands the test of scientific replication, survives the concern about certain measurements, and shows consistency with key additional diagnostics to meet all not just some of the benchmarks of superconductivity, and provided the copper-doped lead apatite material can be manufactured in the form of room temperature ambient pressure superconducting wires, let’s make no bones about the energy-saving ramifications of this once in a lifetime game-changing discovery for humanity, as we attempt to survive the existential threat of climate change and global warming from the continued use of fossil resources.

Feature image by Mai-Linh Doan, published under the CC BY SA 3.0 license