Theory of Oxygen Ordering and Absorption in 1,2,3 High-Tc Superconductors
The oxygen content and the oxygen ordering in 1,2,3 high Tc compounds plays an important role in the superconducting properties of these materials. Experiments [1,2] show that the absorption and desorption of oxygen mainly occurs in the CuO planes, which are located at maximum distance from the trivalent ion, and that the ordering of the oxygen atoms in linear Cu-O-Cu-O-chains in these planes leads to the orthorhombic structure at lower temperatures. Full occupancy of the oxygen positions in these chains along the direction of the b axis leads to 7 oxygen atoms per formula unit. Complete order means that all oxygen positions in the perpendicular direction (along the a axis) are vacant. Disordering in combination with oxygen desorption causes the orthorhombic to tetragonal phase transition at high temperatures. In the tetragonal phase with about 6.5 oxygen atoms per formula unit the oxygen atoms are distributed over the ‘b’ and ‘a’ positions in equal amounts. This causes the a and b lattice parameters to be equal and leads to the tetragonal phase.
KeywordsNeutron Diffraction Oxygen Pressure Tetragonal Phase Metal Hydride Site Energy
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