The high-temperature superconductors are complex oxides, generally containing two-dimensional CuO 2 sheets. Various families of the cuprate superconductors are described, paying special attention to aspects related to oxygen stoichiometry, phase stability, synthesis and chemical manipulation of charge carriers. Other aspects discussed are chemical applications of cuprates, possibly as gas sensors and copper-free oxide superconductors. All but the substituted Nd and Pr cuprates are hole-superconductors. Several families of cuprates show a nearly constant n h at maximum T e . Besides this universality, the cuprates exhibit a number of striking common features. Based on Cu(2p) photoemission studies, it is found that the Cu-O charge-transfer energy, Δ, and the Cu(3d)-O(2p) hybridization strength t pd , are key factors in the superconductivity of cuprates. The relative intensity of the satellite in the Cu(2p) core-level spectra, the polarizability of the CuO 2 sheets as well as the hole concentration are related to Δ / t pd . These chemical bonding factors have to be explicitly taken into account in any model for superconductivity of the cuprates.
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