The tripling of the perovskite unit cell leads to nine oxygen sites, whereas YBa 2Cu 3O 7 has seven, and is therefore referred to as an oxygen-deficient perovskite structure. And there are four possible crystallographic sites for oxygen: O(1), O(2), O(3) and O(4). All the corner sites of the unit cell are occupied by Cu, which has two different types of coordination-Cu(1) and Cu(2)-with respect to oxygen. The unit cell of YBa 2Cu 3O 7 consists of three pseudo-cubic perovskite unit cells. When the oxygen content is reduced from 7 to 6.5, all Cu will change to the +2 state and the material is a semiconductor. The composition of YBa 2Cu 3O 7 can be written as (Y 3+)(Ba 2+) 2(Cu 2+) 2(Cu 3+)(O 2−) 7, with one-third of Cu in the +3 state. Its T c reaches the maximum (92 K) when x ≈ 0.15 and the structure is orthorhombic and the superconductivity disappears at x ≈ 0.6, where the structure of YBCO changes from orthorhombic to tetragonal. The Cu oxidation state in this compound is determined by the oxygen stoichiometry. The first superconductor with critical temperature ( T c) over 77 K, yttrium barium copper oxide (YBCO, YBa 2Cu 3O 7− x ( x = 0 to 1)) has been studied extensively. Great efforts have been made for decades to attain superconductivity at a practically feasible temperature. Superconductivity is a fundamental property of certain materials. Furthermore, we explore how such condensed-matter-based studies may lead to deeper understanding and insights about fundamental chemistry problems important to materials science and life sciences. In this perspective, we suggest possible ways to study their properties and reactions in relevance to specific organizational characteristics of their surroundings in the condensed-matter states, using solid-state materials with special functions and living organisms with complex high-order structures, specifically solid-state superconductive materials, catalysts and biological condensed materials (BCMs) in the context of synthetic and biological chemistry, as illustrative examples. We have proposed, in a recent essay entitled ‘Towards a new discipline of condensed matter chemistry’, a new research field to study the functionalities and chemical reactions of condensed matter with multi-level structures, characterized by strong intermolecular forces and local organizational order.
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