LK-99

LK-99

3D structure
Identifiers
InChI InChI=1S/Cu.6H3O4P.O.9Pb/c;6*1-5(2,3)4;;;;;;;;;;/h;6*(H3,1,2,3,4);;;;;;;;;;/q+2;;;;;;;-2;9*+2/p-18 Key: KZSIWLDFTIMUEG-UHFFFAOYSA-A
SMILES [Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Cu+2].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].[O-2]
Properties
Chemical formula	CuO25P6Pb9
Molar mass	2514.2 g·mol−1
Appearance	grey–black solid
Density	≈6.699 g/cm3
Structure
Crystal structure	hexagonal
Space group	P63/m, No. 176
Lattice constant	a = 9.843 Å, c = 7.428 Å
Lattice volume (V)	623.2 Å3
Formula units (Z)	1
Related compounds
Related compounds	Oxypyromorphite (lead apatite)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

LK-99 (from the Lee-Kim 1999 research) is a gray–black, polycrystalline compound, identified as a copper-doped lead‒oxyapatite. A team from Korea University led by Lee Sukbae (이석배) and Kim Ji-Hoon (김지훈) began studying this material as a potential superconductor starting in 1999. In 2023, they published preprints claiming that it acts as a room-temperature superconductor at temperatures of up to 400 K (127 °C; 260 °F) at ambient pressure.

Many different labs attempted to replicate the work, and were able to reach initial results within weeks, as the process of producing the material is relatively straightforward. As of 14 August 2023, the consensus is that it is an insulator in pure form, and not a superconductor at any temperature, but that copper-deficient copper(I) sulfide formed as an impurity in the proposed synthesis can produce resistance drops, lambda transition in heat capacity, and magnetic response in small samples, which mimic transitions in superconductors.

While no replications have gone through the peer-review process of a journal, the first attempted replications were reviewed by a materials science lab. A number of replication attempts identified ferromagnetic and diamagnetic causes for partial levitation of small fragments, the one feature of the original paper that was widely replicated.

The initial studies announcing the discovery of LK-99 were uploaded to arXiv, an open-access preprint archive. Lee later claimed the uploaded preprints were incomplete, and coauthor Kim Hyun-Tak (김현탁) stated that one of the papers contained flaws.

Chemical properties and structure

The chemical composition of LK-99 is approximately Pb₉Cu(PO₄)₆O, in which— compared to pure lead-apatite (Pb₁₀(PO₄)₆O)— approximately one quarter of Pb(II) ions in position 2 of the apatite structure are replaced by Cu(II) ions.

The structure is similar to that of apatite, space group P6₃/m (No. 176).

Synthesis

Lee et al. provide a method for chemical synthesis of LK-99 in three steps. First they produce lanarkite from a 1:1 molar mixing of lead(II) oxide (PbO) and lead(II) sulfate (Pb(SO₄)) powders, and heating at 725 °C (1,000 K; 1,340 °F) for 24 hours:

PbO + Pb(SO₄) → Pb₂(SO₄)O.

Then, copper(I) phosphide (Cu₃P) is produced by mixing copper (Cu) and phosphorus (P) powders in a 3:1 molar ratio in a sealed tube under a vacuum and heated to 550 °C (820 K; 1,000 °F) for 48 hours:

3 Cu + P → Cu₃P.

Then, lanarkite and copper phosphide crystals are ground into a powder, placed in a sealed tube under a vacuum, and heated to 925 °C (1,200 K; 1,700 °F) for between 5‒20 hours:

Pb₂(SO₄)O + Cu₃P → Pb_10-xCu_x(PO₄)₆O + S (g), where 0.9 < x < 1.1.

There were a number of problems with the above synthesis from the initial paper. The reaction is not balanced, and others reported the presence of copper(I) sulfide (Cu₂S) as well. For $x=1$ a balanced reaction might be:

5 Pb₂SO₄O + 6 Cu₃P → Pb₉Cu(PO₄)₆O + 5 Cu₂S + Pb + 7 Cu.

Many syntheses produced fragmentary results in different phases, where some of the resulting fragments were responsive to magnetic fields, other fragments were not. The first synthesis to produce pure crystals found them to be diamagnetic insulators.

Physical properties

(a) Diamagnetic susceptibility measurements of LK-99, (b) sample of LK-99 partially levitating over large magnet

Some LK-99 samples were reported to show strong diamagnetic properties, including partial levitation over a magnet. This was widely interpreted as a sign of superconductivity, although it is more commonly a sign of regular diamagnetism or ferromagnetism.

While the initial preprints claimed the material is a room-temperature superconductor, they did not claim to have seen any definitive features of superconductivity, including zero resistance, the Meissner effect, flux pinning, AC magnetic susceptibility, the Josephson effect, a temperature-dependent critical field and current, or a sudden jump in specific heat around the critical temperature.

As it is common for a new material to spuriously seem like a potential candidate for high-temperature superconductivity, thorough experimental reports would normally demonstrate a number of these expected properties. As of 15 August 2023, none of these properties have been observed by the original experiment or by attempted replications.

Proposed mechanism for superconductivity

Partial replacement of Pb²⁺ ions with smaller Cu²⁺ ions is said to cause a 0.48% reduction in volume, creating internal stress inside the material. The internal stress is proposed to cause a heterojunction quantum well between the Pb(1) and oxygen within the phosphate ([PO₄]³⁻), generating a superconducting quantum well (SQW).

This proposed mechanism is based on a 2021 paper by Kim Hyun-Tak describing a complicated theory of superconductivity combining ideas from a classical theory of metal-insulator transitions, the standard Bardeen–Cooper–Schrieffer theory of superconductivity, and a controversial theory of hole superconductivity by J.E.Hirsch.

Response

On 31 July 2023, Sinéad Griffin of Lawrence Berkeley National Laboratory analyzed LK-99 with density functional theory (DFT), showing that its structure would have correlated isolated flat bands, and suggesting this might contribute to superconductivity. However while other researchers agreed with the DFT analysis, a number suggested that this was not compatible with superconductivity, and that a structure different from what was described in Lee, et al. would be necessary.

Analyses by industrial and experimental physicists noted experimental and theoretical shortcomings of the published works. Shortcomings included the lack of phase diagrams spanning temperature, stoichiometry and stress; the lack of pathways for the very high T_c of LK-99 compared to prior heavy fermion superconductors; the absence of flux pinning in any observations; the possibility of stochastic conductive artifacts in conductivity measurements; the high resistance and low current capacity of the alleged superconducting state; and the lack of direct transmission electron microscopy (TEM) of the materials.

Compound name

The name LK-99 comes from the initials of discoverers Lee and Kim, and the year of discovery (1999). The pair had worked with Tong-Seek Chair (최동식) at Korea University in the 1990s.

In 2008, they founded the Quantum Energy Research Centre (퀀텀 에너지연구소; also known as Q-Centre) with other researchers from Korea University . Lee would later become CEO of Q-Centre, and Kim would become director of research and development.

Publication history

Lee has stated that in 2020, an initial paper was submitted to Nature, but was rejected. Similarly-presented research on room-temperature superconductors (but a completely different chemical system) by Ranga P. Dias had been published in Nature earlier that year, and received with skepticism—Dias's paper would subsequently be retracted in 2022 after its data was questioned as having been falsified.

In 2020, Lee and Kim Ji-Hoon filed a patent application. A second patent application (additionally listing Young-Wan Kwon), was filed in 2021, which was published on 3 March 2023. A World Intellectual Property Organization (WIPO) patent was also published on 2 March 2023. On 4 April 2023, a Korean trademark application for "LK-99" was filed by the Q-Centre.

Scholarly articles and preprints

A series of academic publications summarizing initial findings came out in 2023, with a total of seven authors across four publications.

On 31 March 2023, a Korean-language paper, "Consideration for the development of room-temperature ambient-pressure superconductor (LK-99)", was submitted to the Korean Journal of Crystal Growth and Crystal Technology. It was accepted on 18 April, but was not widely read until three months later.

On 22 July 2023, two preprints appeared on arXiv. The first was submitted by Young-Wan Kwon, and listed Kwon, former Q-Centre CTO, as third author. The second preprint was submitted only 2 hours later by Kim Hyun-Tak, former principal researcher at the Electronics & Telecommunications Research Institute and professor at the College of William & Mary, listing himself as third author, as well as three new authors.

On 23 July, the findings were also submitted by Lee to APL Materials for peer review. On 3 August 2023, a newly-formed Korean LK-99 Verification Committee requested a high-quality sample from the original research team. The team responded that they would only provide the sample once the review process of their APL paper was completed, expected to take several weeks or months.

On 31 July 2023, a group led by Kapil Kumar published a preprint on arXiv documenting their replication attempts, which confirmed the structure using X-ray crystallography (XRD) but failed to find diamagnetism or levitation.

Other discussion by authors

On 26 July 2023, Kim Hyun-Tak stated in an interview with the New Scientist that the first paper submitted by Kwon contained "many defects" and was submitted without his permission.

On 28 July 2023, Kwon presented the findings at a symposium held at Korea University. That same day, Yonhap News Agency published an article quoting an official from Korea University as saying that Kwon was no longer in contact with the University. The article also quoted Lee saying that Kwon had left the Q-Centre Research Institute four months previously.

On the same day, Kim Hyun-Tak provided the New York Times with a new video presumably showing a sample displaying strong signs of diamagnetism. The video appears to show a sample different to the one in the original preprint. On 4 August 2023, he informed SBS News that high-quality LK-99 samples may exhibit diamagnetism over 5,000 times greater than graphite, which he claimed would be inexplicable unless the substance is a superconductor.

Authors

Author credit and affiliation matrix:

Author Affiliation	Lee, Sukbae (이석배)	Kim, Ji-Hoon (김지훈)	Kim, Hyun-Tak (김현탁)	Im, Sungyeon (임성연)	An, SooMin (안수민)	Kwon, Young-Wan (권영완)	Auh, Keun Ho (오근호)
HYU							Professor Emeritus
KU‒KIST						'former Professor†
W&M			Professor
Q-Centre (주)퀀텀에너지연구소	CEO	R&D Director		Yes	Yes	'former CTO	CTO
Patent (2020)	1	2
Patent Application (2021)	1	2				3
Lee & Kim+ (18 April 2023)	1	2		3	4	5	6
Lee & Kim+ (22 July 2023.a)	1	2		Acknowledged	Acknowledged	3	Acknowledged
Lee & Kim+ (22 July 2023.b)	1	2	3	4	5	Acknowledged	6

Response

Materials scientists and superconductor researchers responded with skepticism. The highest-temperature superconductors known at the time of publication had a critical temperature of 250 K (−23 °C; −10 °F) at pressures of over 170 gigapascals (1,680,000 atm; 24,700,000 psi). The highest-temperature superconductors at atmospheric pressure (1 atm) had a critical temperature of at most 150 K (−123 °C; −190 °F).

On 2 August 2023, the The Korean Society of Superconductivity and Cryogenics established a verification committee as a response to the controversy and unverified claims of LK-99, in order to arrive at conclusions over these claims. The verification committee is headed by Kim Chang-Young of Seoul National University and consists of members of the university, Sungkyunkwan University and Pohang University of Science and Technology. Upon formation, the verification committee did not agree that the two 22 July arXiv papers by Lee et al. or the publicly available videos at the time supported the claim of LK-99 being a superconductor.

As of 10 August 2023, the measured properties do not prove that LK-99 is a superconductor. The published material does not explain how the LK-99's magnetisation can change, demonstrate its specific heat capacity, or demonstrate it crossing its transition temperature. An alternative explanation for LK-99's stated partial magnetic levitation could be from a mix of ferromagnetism and non-superconductive diamagnetism. A number of studies found that copper(I) sulfide contamination common to the synthesis process could explain the observations that inspired the initial preprints.

Public response

The claims in the 22 July papers by Lee et al. went viral on social media platforms the following week. The viral nature of the claim resulted in posts from users using pseudonyms from Russia and China claiming to have replicated LK-99 on both Twitter and Zhihu. Other viral videos described themselves as having replicated samples of LK-99 levitating, most of which were found to be fake.

Scientists interviewed by the press remained skeptical, of both the original preprints, the lack of purity in the sample they reported, and the failure of previous claims of room temperature superconductivity to show legitimacy. The Korean Society of Superconductivity and Cryogenics expressed concern on the social and economic impacts of the preliminary and unverified LK-99 research.

A video from Huazhong University of Science and Technology uploaded on 1 August 2023 by a postdoctoral researcher on the team of Chang Haixin, apparently showed a micrometre-sized sample of LK-99 levitating. This went viral on Chinese social media, becoming the most viewed video on Bilibili by the next day, and a prediction market briefly put the chance of successful replication at 60%. A researcher from the Chinese Academy of Sciences refused to comment on the video for the press, dismissing the claim as "ridiculous".

In early August, people began to create memes about "floating rocks", meme coins based on the substance were created, and there was a brief surge in Korean and Chinese technology stocks, despite warnings from the Korean stock exchange against speculative bets in light of the excitement around LK-99, which fell again by On August 8.

Replication attempts

After the July 2023 publication's release, independent groups reported that they had begun attempting to reproduce the synthesis, with initial results expected within weeks. However while positive results could potentially come quickly, negative results are slow, as "falsification needs to verify all possibilities, and it will take a lot of time".

As of 15 August 2023, no replication attempts had yet been peer-reviewed by a journal. Of the non-peer-reviewed attempts, over 15 notable labs have published results that failed to observe any superconductivity, and a few have observed magnetic response in small fragments that could be explained by normal diamagnetism or ferromagnetism. Some demonstrated and replicated alternate causes of the observations in the original papers: Copper-deficient copper (I) sulfide has a known phase transition at 377 K (104 °C; 219 °F) from a low-temperature phase to a high-temperature superionic phase, with an sharp rise in resistivity and a λ-like-feature in the heat capacity. Furthermore, Cu₂S is diamagnetic.

Only one attempt observed any sign of superconductivity: Southeast University claimed to measure very low resistance in a flake of LK-99, in one of four synthesis attempts, below a temperature of 110 K (−163 °C; −262 °F). Doubts were expressed by experts in the field, as they saw no dropoff to zero resistance, and used crude instruments that could not measure resistance below 10 μΩ ( too high to distinguish superconductivity from less exotic low-temperature conductivity), and had large measurement artifacts.

Some replication efforts gained global visibility, with the aid of online replication trackers that catalogued new announcements and status updates.

Experimental studies

Results Key:   Success   Partial success   Partial failure   Failure

Group	Country/region	Status	Results	Publication notes
Huazhong University of Science and Technology	China	Preliminary	Measured diamagnetism of small micrometre sized flakes. Observed resistance cannot drop to zero. Suggests purity of sample is important.	arXiv: Hao Wu, et al. Video posted to bilibili. Press coverage:
Beihang University		Preliminary	No diamagnetism observed. High resistivity not consistent with superconductivity.	arXiv: Li Liu, et al. Press coverage:
Southeast University		Preliminary	LK-99 structure confirmed by x-ray diffraction. Resistance of mm-sized sample gradually reduced from 0.1Ω at room temperature to noise level (10-5Ω) at 110 K and below. No observed Meissner effect.	arXiv: Qiang Hou, et al. Video announcement: Critical responses: Press coverage:
Peking University		Preliminary	No Meissner effect nor zero resistivity observed in reproduced sample.	arXiv: Kaizhen Guo, et al.
Chinese Academy of Sciences (Condensed Matter)		Preliminary	No superconductivity observed. Proposed explanation of resistivity reduction by phase change of Cu2S impurities, which others have predicted to be present when partial levitation is observed.	arXiv: Shilin Zhu, et al.
Shanghai University		Unpublished	Synthesized LK-99 powder, saw no diamagnetism.	News report: on-site interview by reporter
Qufu Normal University		Unpublished	LK-99 sample had non-zero resistance.	Press coverage:
DIPC, Princeton, Max Planck Institut	Spain, USA, Germany	Preliminary	Synthesized LK-99, found to be multiphase material. Performed single-crystal analysis with X-ray diffraction, finding no superconductivity. Tested 4 different Cu dopings, finding samples to be magnetic rather than superconducting.	arXiv: Jiang, et al.
University of Manchester	United Kingdom	Preliminary	Synthesized and characterized samples of LK-99, no superconductivity detected.	arXiv: Ivan Timokhin et al.
National Physical Laboratory of India (CSIR-NPLI)	India	Preliminary	Initial attempt: Structure confirmed by x-ray diffraction. No diamagnetism observed, no features of superconductivity. Later attempt recorded sample standing vertically locked in magnetic field.	Video by V.P.S Awana: arXiv: Kapil Kumar, et al. Press coverage:
Lebedev Physics Institute	Russia	Unpublished	Structure confirmed with X-ray diffraction. No diamagnetism was observed. High resistance observed with 4-point probes method. Resistance increases when temperature decreases.	Statement by Lebedev Physics Institute: Press coverage:
National Taiwan University	Taiwan	Unpublished	Diamagnetism observed. Non-zero resistance, no superconductivity.	Li-min, et al. Live stream: Press coverage:
Varda Space Industries & University of Southern California	United States	Unpublished	Synthesis of LK-99. Only a few fragments responded to magnetic field. Analysis showed impurities of Iron and Cu2S, suggested this explained observed phenomenon rather than superconductivity.	Videos shared on Twitter: Team headed by Andrew McCalip. Livestreaming on Twitch and posting on Twitter. Samples analysed by University of Southern California. Press coverage:
University of Colorado Boulder		Unpublished	Samples have failed tests for superconductivity.	Daniel Dessau, et al. Press Coverage:
Argonne National Laboratory		Un­known	Not reported	Press coverage:
Sungkyunkwan University	South Korea	Un­known	Not reported	Supporting the committee reportedly evaluating the claims about LK-99. Press coverage:
Korea University		Un­known	Not reported
Seoul National University		Un­known	Not reported

Theoretical studies

In the initial papers, the theoretical explanations for potential mechanisms of superconductivity in LK-99 were incomplete. Later analyses by other labs added simulations and theoretical evaluations of the material's electronic properties from first principles.

Group	Country	Result	Publication notes
Chinese Academy of Sciences (SYNL)	China	First-principles study of the electronic structure of LK-99 and other variants. Expresses no opinion on room-temp superconductivity.	arXiv: Junwen Lai, et al. Media mentions:
Lawrence Berkeley National Laboratory	United States	DFT analysis on a simplified 3D structure explores possible electronic structure that could be favorable for superconductivity, suggests slightly decreased lattice constant. Similar work published the next day by Si & Held and Kurleto, et al.	arXiv: Sinéad Griffin Analysis: Media mentions:
Universidad de Chile	Chile	DFT analysis, finding large electron-phonon coupling in the flat bands.	arXiv: J. Cabezas-Escares, et al.
CIEMAT	Spain, Armenia	Concludes the original synthesis for LK-99 likely produces a heterogenous material, making it hard for others to reproduce the same results	arXiv: P. Abramian, et al.
Northwest University (China) and TU Wien	China, Austria	Concludes Pb9Cu(PO4)6O, without further doping, it is an insulator. Analyzes possible effects of doping.	arXiv: Liang Si & Karsten Held

See also

• Bismuth strontium calcium copper oxide: Superconductivity at T_c ≈ 33 K (−240.2 °C) to 104 K (−169 °C)
• Carbonaceous sulfur hydride: Superconductivity at T_c ≈ 288 K (15 °C) at 267 GPa
• Lanthanum decahydride: Superconductivity at T_c = 250 K (−23 °C) at 150 Gpa
• Unconventional superconductor

Further reading

• 최동식 (17 May 1994). 초전도 혁명의 이론적 체계 [Theoretical Framework of the Superconducting Revolution] (in Korean). 고려대학교출판부. ISBN 89-7641-276-1. Archived from the original on 27 July 2023. Retrieved 27 July 2023.
• "최동식". Donga Science (Interview with T.S. Chair) (in Korean): 106‒107. September 1993. Archived from the original on 20 March 2023. Retrieved 3 August 2023.
• Krivovichev, Sergey V.; Burns, Peter C. (17 October 2002). "Crystal chemistry of lead oxide phosphates: crystal structures of Pb₄O(PO₄)₂, Pb₈O₅(PO₄)₂ and Pb₁₀(PO₄)₆O". Zeitschrift für Kristallographie – Crystalline Materials. 218 (5). Munich: Oldenbourg Wissenschaftsverlag (published 1 May 2003): 357–365. doi:10.1524/zkri.218.5.357.20732. S2CID 101229927. The compound Pb₁₀(PO₄)₆O has been designated 'oxypyromorphite' ... Pb₁₀(PO₄)₆O crystallizes with an apatite-type structure. The structure contains a single O atom that is not part of a PO₄ tetrahedron; it has a site occupancy factor of 0.25 and is located on the 6₃ axis.
• Lowe, Derek (26 July 2023). "Breaking Superconductor News". Chemical News. In the Pipeline (blog). American Association for the Advancement of Science. Archived from the original on 26 July 2023. Retrieved 26 July 2023 – via Science.org.
• Orf, Darren (27 July 2023). "Scientists Claim They Found the Holy Grail of Superconductors". Energy. Popular Mechanics. Retrieved 28 July 2023.
• Harris, Margaret (27 July 2023). "Have scientists in Korea discovered the first room-temperature, ambient-pressure superconductor?". Superconductivity blog. Physics World. Institute of Physics. Archived from the original on 28 July 2023. Retrieved 29 July 2023.
• Wu, Yuewei; Sun, Wenya; Li, Ruiyang (28 July 2023). 南大教授谈韩国室温超导：不像超导，正重复实验—新闻 [NU professor talks about room temperature superconductivity found in South Korea: Unlike superconductivity, experiments are being repeated]. ScienceNet.cn (broad overview) (in Chinese). Archived from the original on 31 July 2023. Retrieved 31 July 2023.
• 안경애 (8 August 2023). "'LK-99 샘플' 미세 결정구조 논문과 같다"...에너지공대 확인 [The 'LK-99' sample crystal structure matches published paper — [Korea University] College of Energy Engineering]. Digital Times (in Korean). Archived from the original on 10 August 2023. Retrieved 9 August 2023. LK-99 연구에 참여하고 있는 한국에너지공대의 한 연구자는 8일 디지털타임스와의 통화에서 "샘플에 대한 X선 회절구조 분석 결과, 논문에 제시된 것과 샘플의 미세 결정구조가 같다는 사실을 확인했다"고 밝혔다. 이 연구자는 "미세 결정구조를 보면 물질의 물성을 확인하지 못해도 가능성을 추정할 수 있다. LK-99 분석을 위해 X레이로 미세 결정구조를 확인한 결과, 퀀텀에너지연구소 측이 논문에서 밝힌 결정구조와 샘플에서 확인되는 결정구조가 같았다"고 말했다.
• Shilin Zhu; Wei Wu; Zheng Li; Jianlin Luo (8 August 2023). "First order transition in Pb_10−xCu_x(PO₄)₆O (0.9<x<1.1) containing Cu₂S". arXiv:2308.04353 [cond-mat.supr-con].
• Puphal, P.; Akbar, M. Y. P.; Hepting, M.; Goering, E.; Isobe, M.; Nugroho, A. A.; Keimer, B. (11 August 2023). "Single crystal synthesis, structure, and magnetism of Pb_10−xCu_x(PO₄)₆O". arXiv:2308.06256 [cond-mat.supr-con].

External links

• Magnetic Property Test of LK-99 Film (video). Quantum Energy Research Centre. 26 January 2023. Retrieved 25 July 2023 – via Youtube. This effect is a conductive copper plate induced by a magnetic.
• Kim, Hyun-Tak (25 July 2023). Superconductor Pb_10-xCu_x(PO₄)₆O showing levitation at room temperature and atmospheric pressure and mechanism (video). Retrieved 25 July 2023 – via ScienceCast.
  • A more archivable YouTube version: REV LUTIONS. "LK-99 Superconductor showing levitation at room temperature & atmospheric pressure OFFICIAL". YouTube. Archived from the original on 31 July 2023. Retrieved 31 July 2023.
• List of replication claims, regularly updated:
  • Compilation of Known Replication Attempt Claims. Guderian2nd, Space Battles forum. Retrieved 2 August 2023.
  • LK-99#Online Claims. Eiri Sanada. Retrieved 2 August 2023.