Why is YBa2Cu3O7 a super conductor? Explain Chemistry and Structural bases.
YBa2Cu3O7, which stands for Yttrium-Barium-Copper Oxide, is a well-known high-temperature superconductor. It exhibits superconductivity at temperatures above the boiling point of liquid nitrogen (-196°C or 77 K). The reasons YBa2Cu3O7 is a superconductor can be explained from both a chemical and structural perspective.
From a chemical standpoint, YBa2Cu3O7 belongs to a class of materials called cuprates. These cuprate compounds are characterized by layers of CuO2 planes. The unique arrangement of copper and oxygen atoms within these planes is critical for their superconducting properties.
In YBa2Cu3O7, there are alternating layers of yttrium (Y), barium (Ba), and copper oxide (CuO2). The yttrium and barium atoms act as charge reservoirs, providing extra electrons to the CuO2 planes. The copper atoms are in a +2 oxidation state, but due to the oxygen vacancies within the CuO2 plane, they can donate one of their electrons to the plane, resulting in a net charge of +1 for each copper ion. This excess positive charge is balanced by mobile electrons that can move freely within the CuO2 plane.
Structurally, the CuO2 plane in YBa2Cu3O7 has a unique arrangement known as a "pseudotetragonal" structure. This means that even though the crystal lattice appears tetragonal (four-sided), there is a distortion due to the presence of the yttrium and barium ions. This distortion introduces anisotropy, meaning that the superconducting properties are directionally dependent within the crystal.
The key to YBa2Cu3O7's superconductivity is its ability to form Cooper pairs of electrons. These Cooper pairs arise due to the interaction of electrons with lattice vibrations, or phonons. At low temperatures, the CuO2 planes in YBa2Cu3O7 undergo a transition where they become free of any imperfections or impurities, creating what is known as a "clean" superconductor. The absence of impurities allows the Cooper pairs to travel through the material without being scattered, resulting in zero electrical resistance.
In summary, the superconductivity of YBa2Cu3O7 can be explained by its chemical composition, where the layers of yttrium, barium, and copper oxide contribute to the mobility of electrons within the CuO2 plane. Structurally, the pseudotetragonal arrangement of the crystal lattice provides the necessary conditions for the formation of Cooper pairs. The combination of these chemical and structural factors allows YBa2Cu3O7 to exhibit superconducting behavior at higher temperatures than traditional superconductors.