page_title: High-temperature superconductivity
section_title: Other cuprates
hierarchical_section_title: High-temperature superconductivity / Cuprates / Other cuprates
caption_attribution_description: English: Bi-2201, Bi-2212 and Bi-2223 lattice
context_page_description: High-temperature superconductors are operatively defined as materials that behave as superconductors at temperatures above nearly 73.15 K.
  This is in fact the lowest temperature reachable by liquid nitrogen, one of the simplest coolants in cryogenics. All superconducting materials known at ordinary pressures currently work far below ambient temperatures and therefore require cooling. The majority of high-temperature superconductors are ceramics materials. On the other hand, Metallic superconductors usually work below -200 °C: they are then called low-temperature superconductors. Metallic superconductors are also ordinary superconductors, since they were discovered and used before the high-temperature ones.
  Ceramic superconductors are now becoming suitable for some practical use, but they still have many manufacturing issues and there are very few successful practical examples of employment.
  Most ceramics are brittle which makes the fabrication of wires from them very problematic.
  The major advantage of high-temperature ceramic superconductors is that they can be cooled by using liquid nitrogen.
context_section_description: The preparation of other cuprates is more difficult than the YBCO preparation.
  They also have a different crystal structure: they are tetragonal where YBCO is orthorhombic.
  Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure. 
  Moreover, the crystal structure of other tested cuprate superconductors are very similar. Like YBCO, the perovskite-type feature and the presence of simple copper oxide (CuO₂) layers also exist in these superconductors. However, unlike YBCO, Cu–O chains are not present in these superconductors. The YBCO superconductor has an orthorhombic structure, whereas the other high-Tc superconductors have a tetragonal structure.
  There are three main classes of superconducting cuprates: bismuth-based, thallium-based and mercury-based.
  The second cuprate by practical importance is currently BSCCO, a compound of Bi–Sr–Ca-Cu-O. The content of bismuth and strontium creates some chemical issues. 
  It has three superconducting phases forming a homologous series as Bi₂Sr₂Caₙ−1CuₙO₄₊₂ₙ₊ₓ (n=1, 2 and 3). 
  These three phases are Bi-2201, Bi-2212 and Bi-2223, having transition temperatures of 20, 85 and 110 K, respectively, where the numbering system represent number of atoms for Bi, Sr, Ca and Cu respectively. The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The unit cell of these phases has double Bi–O planes which are stacked in a way that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the CuO₂ layers in both Bi-2212 and Bi-2223; there is no Ca layer in the Bi-2201 phase. The three phases differ with each other in the number of cuprate planes; Bi-2201, Bi-2212 and Bi-2223 phases have one, two and three CuO₂ planes, respectively. The c axis lattice constants of these phases increases with the number of cuprate planes (see table below). The coordination of the Cu atom is different in the three phases. The Cu atom forms an octahedral coordination with respect to oxygen atoms in the 2201 phase, whereas in 2212, the Cu atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 structure, Cu has two coordinations with respect to oxygen: one Cu atom is bonded with four oxygen atoms in square planar configuration and another Cu atom is coordinated with five oxygen atoms in a pyramidal arrangement.
  Cuprate of Tl–Ba–Ca: The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBa₂Caₙ₋₁CuₙO₂ₙ₊₃, whereas the second series containing two Tl–O layers has a formula of Tl₂Ba₂Caₙ₋₁CuₙO₂ₙ₊₄ with n =1, 2 and 3. In the structure of Tl₂Ba₂CuO₆ (Tl-2201), there is one CuO₂ layer with the stacking sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In Tl₂Ba₂CaCu₂O₈ (Tl-2212), there are two Cu–O layers with a Ca layer in between. Similar to the Tl₂Ba₂CuO₆ structure, Tl–O layers are present outside the Ba–O layers. In Tl₂Ba₂Ca₂Cu₃O₁₀ (Tl-2223), there are three CuO2 layers enclosing Ca layers between each of these. In Tl-based superconductors, Tc is found to increase with the increase in CuO₂ layers. However, the value of Tc decreases after four CuO₂ layers in TlBa₂Caₙ₋₁CuₙO₂ₙ₊₃, and in the Tl₂Ba₂Caₙ₋₁CuₙO₂ₙ₊₄ compound, it decreases after three CuO₂ layers.
  Cuprate of Hg–Ba–Ca The crystal structure of HgBa₂CuO₄ (Hg-1201), HgBa₂CaCu₂O₆ (Hg-1212) and HgBa₂Ca₂Cu₃O₈ (Hg-1223) is similar to that of Tl-1201, Tl-1212 and Tl-1223, with Hg in place of Tl. It is noteworthy that the Tc of the Hg compound (Hg-1201) containing one CuO₂ layer is much larger as compared to the one-CuO₂-layer compound of thallium (Tl-1201). In the Hg-based superconductor, Tc is also found to increase as the CuO₂ layer increases. For Hg-1201, Hg-1212 and Hg-1223, the values of Tc are 94, 128 and the record value at ambient pressure 134 K, respectively, as shown in table below. The observation that the Tc of Hg-1223 increases to 153 K under high pressure indicates that the Tc of this compound is very sen
caption: Crystal lattice of Cuprate of Bismuth and Strontium (BSCCO)
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