Li et al. (1) report on superconductivity (Tc ∼73 K) that emerged in Ba2CuO4-y (Ba214) synthesized at extremely high pressures (∼18 GPa). They claim that one of the unique features of this compound is an exceptionally compressed local octahedron. I read this article with great interest. However, I am afraid they stretch things a bit. While they claim the crystal structure of the compound is of the K2NiF4 (coined as T) type, as is the case with (La, Ba)2CuO4, their X-ray powder diffraction data resemble more the Nd2CuO4 (coined as T′)-type structure (2): In the latter, no local octahedron exists. When lattice parameters are fixed to the reported values (a = 4.003 Å and c = 12.942 Å), a distinction between T- and T′-Ba214 may not be simple using X-ray diffraction. Nevertheless, the large a/c ratio (∼0.309) suggests fluorite-structure Ba2O2 layers, equivalently T′-Ba214: At the least, Rietveld refinements assuming the T′ structure may be worth being considered.
Superconducting T-Sr2-xBaxCuO3+δ (3) (or Sr2-xBaxCuO4-v) (4), where a substantial number of O vacancies exist in the CuO2 planes, is challenging the notion that the CuO2 planes are the playground of high-Tc superconductivity. Out-of-plane lattice constants of these materials are smaller than that for T-La2CuO4 (5) despite the larger ionic radii of Sr2+ and Ba2+ compared to La3+. This reduced lattice length stems from the reduced Jahn–Teller distortions due to the O vacancies in the CuO2 planes. The material (Ba214) reported by Li et al. (1) may be related to those largely O-deficit T-Sr2-xBaxCuO4-v cuprates as their Rietveld refinement results (table S1 in supporting information of ref. 1) indicate (∼40% of the oxygen vacancies in the CuO2 planes). However, the c value (12.942 Å) of the Ba2CuO4-v presented in ref. 1 is far off the extrapolation to x = 2.0 in T-Sr2-xBaxCuO4-v with various x (3): c would be ∼13.4 Å. This may indicate that the Ba2O2 layer does not crystallize in the rock-salt structure as should be in the T structure. Altogether, a formation of a local octahedron in their Ba2CuO4-v remains elusive and the concept of an exceptionally compressed local octahedron may lose its vindicability.
Finally it is worth noting that we have synthesized Sr2CuO4±δ (c = 13.55 Å) (6) and Ba2CuO4±δ (c = 14.6 Å) (7) whose c-axis lattice constants agree well with those expected from the variation of ionic radius [ri(La3+) < ri(Sr2+) < ri(Ba2+)] and also from the trends derived from the n ≥ 2 members of 0(Sr)2(n-1)n and 0(Ba)2(n-1)n homologous series materials in the Sr-Ca-Cu-O (8) and Ba-Ca-Cu-O (9) systems. Also, Tc of ∼73 K reported by Li et al. (1) roughly coincides with the onset Tc in the resistivity-temperature curves for our T-Ba2CuO4±δ specimens (7).
Footnotes
The author declares no conflict of interest.
References
- 1.Li W. M., et al. , Superconductivity in a unique type of copper oxide. Proc. Natl. Acad. Sci. U.S.A. 116, 12156–12160 (2019). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Tokura Y., Takagi H., Uchida S., A superconducting copper oxide compound with electrons as the charge carriers. Nature 337, 345–347 (1989). [Google Scholar]
- 3.Gao W. B., et al. , Out-of-plane effect on the superconductivity of Sr2-xBaxCuO3+δ with Tc up to 98 K. Phys. Rev. B Condens. Matter Mater. Phys. 80, 094523/1–6 (2009). [Google Scholar]
- 4.Geballe T. H., Marezio M., Comment on “Out-of-plane effect on the superconductivity of Sr2-xBaxCuO3+δ with Tc up to 98 K.” Phys. Rev. B Condens. Matter Mater. Phys. 82, 216501 (2010). [Google Scholar]
- 5.Chou F. C., Cho J. H., Johnston D. C., Synthesis, characterization, and superconducting and magnetic properties of electrochemically oxidized La2CuO4+δ and La2-xSrxCuO4+δ (0.01 ≤ x ≤ 0.33, 0.01 ≤ δ ≤ 0.36). Physica C 197, 303–314 (1992). [Google Scholar]
- 6.Karimoto S., Yamamoto H., Greibe T., Naito M., New superconducting Sr2CuO4-δ thin films prepared by molecular beam epitaxy. Jpn. J. Appl. Phys. 40, L127–L130 (2001). [Google Scholar]
- 7.Yamamoto H., Naito M., Sato H., A new superconducting barium cuprate prepared by molecular beam epitaxy. Physica C 338, 29–37 (2000). [Google Scholar]
- 8.Kawashima T., Takayama-Muromachi E., Superconductivity in the series of compounds Sr2Can-1CunOy (n = 1 ∼4) prepared under high pressure. Physica C 267, 106–112 (1996). [Google Scholar]
- 9.Yamauchi H., Karppinen M., Hosomi T., Fjellvåg H., Water-containing phases derived from “02(n-1)n” superconductors I: Novel 02(n-1)n homologous series in the Ba-Ca-Cu-O system and the derivative series. Physica C 338, 38–45 (2000). [Google Scholar]