Superconducting cuprates, which exhibit a critical temperature (T_c) above 100 K, crystallize in a layered structure with double or more CuO₂ planes in the unit cell [1]. Such materials are called multilayered cuprates, and their chemical formula are expressed, for instance, as 02(n–1)n-F, Hg-12(n–1)n, and Bi-22(n–1)n, where n stands for the number of the CuO₂ planes. The multilayered structure is characterized as a superlattice consisting of conducting and blocking layers [2]. In particular, the former layer is formed by repeating CuO₂-Ca-CuO₂ slabs where (n–1) unit cells of infinite-layered CaCuO₂ are stacked along the c-axis. From the viewpoint of materials development, although there is a rich variety of elements that construct the blocking layer, the CaCuO₂ unit forming the conducting layer has been believed as an essential component. For this reason, the previous multilayered cuprates always contain Ca in their composition [3]. Another well-known infinite-layered compound is SrCuO₂; however, no cuprate material with pure Sr between the CuO₂ plane has been reported so far.

In this talk, we present the successful synthesis of the double-layered cuprates without containing Ca: Sr₂SrCu₂O₄(F, O)₂ (0212-F) and MSr₂SrCu₂O_y (M-1212 with M = Hg/Re, Tl, and B/C) [4]. Novel structural features of these compounds are that they contain an SrCuO₂ unit (CuO₂-Sr-CuO₂ slab) as the conducting layer and that the cations between the CuO₂ planes and in the adjacent blocking layer are of the same kind, i.e., Sr²⁺. Hence, the Ca-free cuprates are expected to be one of the 100 K-class superconducting materials with fewer constituents and better chemical homogeneity than the conventional homologous series. The polycrystalline samples are obtained by solid-state reaction at 900 ^oC and a high-pressure (HP) condition (~3.4 GPa).

Except for the (B, C)-1212 oxycarbonate with T_c^max ~ 80 K, whose carrier amount is controlled by the interchange of (CO₃)^2– and (BO₃)^3–, all the as-synthesized samples show a relatively low T_c (~ 20–60 K) compared to that of the conventional 0212 and 1212 systems with the CaCuO₂ unit. This result is due to overdoping attributed to the HP synthesis in a strongly oxidizing atmosphere. Indeed, the post-annealing in a vacuum-sealed quartz tube for the reduction results in a significant increase in T_c of (Hg, Re)-1212 and Tl-1212 to 110 K and 75 K, respectively. Interestingly, 0212-F exhibits a drastic T_c enhancement from ~50 K to 107 K via the low-temperature (< 200 ^oC) annealing with a fluorinating agent, CuF₂.

Correspondingly, the crystal symmetry decreases from tetragonal to orthorhombic. This kind of soft-chemical process is the so-called topochemical reaction, involving the removal and/or insertion of anions while maintaining the surrounding cation framework [5]. In the present case, we believe that the exchange between excess O^2– and CuF₂-derived F^– occurred in 0212-F, which regulates the doping level from overdoped to nearly optimally doped [6]. To discuss the structural stability and a possibility of further multilayering of the Ca-free cuprates, their structural parameters were carefully examined. Owing to the replacement of the CaCuO₂ unit by SrCuO₂ one, the out-of-plane Cu-Cu distance extends by a few percent. Nevertheless, we confirm that the Ca-free double-layered phases are structurally stable as a hole-doped cuprate because they have a similar in-plane lattice constant to the Ca-containing systems. Moreover, we find that to obtain a Ca-free triple-layered phase with a potentially higher T_c, the in-plane lattice parameter needs to extend to approximately that of the infinite-layered SrCuO₂ (~3.93 Å).

References
[1] For example, H. Maeda, Y, Tanaka, M. Fukutomi, and T. Asano, Jpn. J. Appl. Phys. 27, L209 (1988), A. Schilling, M. Cantoni, J. D. Guo, and H. R. Ott, Nature 363, 56 (1993).
[2] H. Yamauchi, M. Karppinen, and S. Tanaka, Physica C 263, 146 (1996).
[3] E. Takayama-Muromachi, Chem. Mater. 10, 2686 (1998).
[4] H. Ninomiya, K. Kawashima, A. Iyo, H. Fujihisa, S. Ishida, H. Ogino, Y. Yoshida, Y. Gotoh, and H. Eisaki, Commun. Mater. 2, 13 (2021).
[5] For example, H. Kageyama, K. Hayashi, K. Maeda, J. P. Attfield, Z. Hiroi, J. M. Rondinelli, and K. R. Poeppelmeier, Nat. Commun. 9, 772 (2018).
[6] H. Ninomiya, K. Kawashima, H. Fujihisa, S. Ishida, H. Ogino, Y. Yoshida, H. Eisaki, Y. Gotoh, and A. Iyo, to be submitted.

Keywords: New superconductors, High-Tc cuprates, High-pressure synthesis, Topochemical reaction