Halooxocuprate Superconductors and Related Compounds with the 02(n − 1)n and 0222 Structures

Abstract

Halooxocuprates with the so-called 02(n − 1)n (n = 1−5) and the 0222 structures, containing F, Cl, and Br, are reviewed. These compounds are prepared by conventional solid-state reaction: low-temperature fluorination or high-pressure synthesis. Their structural properties and superconductivity are focused on in this review. Bulk superconductivity has been observed in various samples prepared using high-pressure techniques. On the other hand, the reported halooxocuprate superconductors prepared at ambient pressure need more data for verification of superconducting behavior. The carrier-doping routes for the halooxocuprates are discussed. It is confirmed that p-type superconductivity occurs in oxychlorides in which the apical sites are completely occupied by chlorine. This indicates that the existence of the apical oxygen is not a must for high-T c superconductivity with p-type carriers.

Calcium-free double-layered cuprate superconductors with critical temperature above 100 K

“…For the Ca-free 0212-F compound, T c of the fluorinated sample is plotted. Part of data were acquired from the literature 10,18,24,34,39,42,44,45 30 . This result suggests the existence of cation disorder near the apical site and/or between CuO 2 planes, as reported in the literature 31 .…”

“…In the case of hole-doped single-layered systems (n = 1), the O atoms in octahedral coordination surround the Cu atom, whereas in the double-layered systems (n = 2), Cu is located at the base of the pyramidal coordination formed by O atoms 9 . On the other hand, for oxyhalide cuprates, the apical O site is occupied by a halogen atom 10 . As can be seen from the chemical formula, the composition and cation framework in the blocking layer are almost unaffected by an increase in n. Consequently, with increasing n, the structure of the single-layered phase (0201-X or M-1201) pulls in infinite-layered Ae 0 CuO 2 units one by one between the blocking layers.…”

Core-hole screening response in two-dimensional cuprates: A high-resolution x-ray photoemission study

“…We suggest this to be a possible explanation for the observed splitting of 45 In this context we mention that Ca 2 CuO 2 Cl 2 is much more easy to dope chemically than Sr 2 CuO 2 Cl 2 . 28,29 It is natural to assume that the component due to the disordered Ca gives rise to the smaller doublet, thus meaning that these Ca ions appear at lower BE values close, in fact, to those of metallic calcium. 30 If the BE of the disordered component were to be taken at face value in a simple initial state charging model, it could point to an alteration of the overall charge balance in the system, leading to a p-type doping of the CuO 2 planes.…”

Structure and physical properties of the layered iron oxychalcogenide BaFe2Se2O

“…Halooxocuprates are example where copper ions are coordinated with four oxygen ions, forming the CuO 2 sheet, whereas the halogen ion usually occupies a so-called apical site. 3 With proper electron or hole doping, halooxocuprates will become superconductors. 3 Besides superconductivity, mixed-anion materials also exhibit diverse physical properties.…”