Superconducting cuprates, which exhibit a critical temperature (Tc) above 100 K, crystallize in a layered structure with double or more CuO2 planes in the unit cell . 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 . 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 . 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 MSr2SrCu2Oy (M-1212 with M = Hg/Re, Tl, and B/C) . 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 Tcmax ~ 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 Tc (~ 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 Tc of (Hg, Re)-1212 and Tl-1212 to 110 K and 75 K, respectively. Interestingly, 0212-F exhibits a drastic Tc 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 . In the present case, we believe that the exchange between excess O2– and CuF2-derived F– occurred in 0212-F, which regulates the doping level from overdoped to nearly optimally doped . 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 Tc, the in-plane lattice parameter needs to extend to approximately that of the infinite-layered SrCuO2 (~3.93 Å).
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Keywords: New superconductors, High-Tc cuprates, High-pressure synthesis, Topochemical reaction