Crystal structure of YBa₂Cu₃O_6.5

The model of alternating empty and filled chains of compositions $CuO$₂ and $CuO$₃ in $YBa$₂Cu₃O_6.5 was proposed on the basis of electron-diffraction studies and was corroborated by neutron and X-ray diffraction. But only a small number of superstructure reflections could be observed which did not allow a more detailed structural analysis concerning the oxygen ordering. We refined the crystal structure of $YBa$₂Cu₃O_6.5 from X-ray data of a twinned crystal. On the basis of 244 independent superstructure reflections a structure analysis was performed revealing systematic shifts of the oxygen atoms but also of the strongly scattering atoms Y, Ba and Cu.

The crystal structure of $YBa$₂Cu₃O_6.5 is shown in Fig. 1. The arrows indicate extent and direction of the parameter shifts compared to the structure of $YBa$₂Cu₃O_6.93. The model with alternating empty and filled chains in $YBa$₂Cu₃O_6.5 is verified. The oxygen position O(4) between Cu(1a) type atoms is fully occupied and the position between Cu(1b) type atoms is empty on the significance level of one standard deviation. The observed position of the O(1) type atoms, although less certain due to the larger standard deviations, are in line with the chemical argument that O(1a) coordinates Cu$III$ and O(1b) coordinates Cu$I$. The Cu-O distances of 1.95(2)Å and 1.78(2)Å compare with those observed in $YBa$₂Cu₃O_6.93 (1.85Å) and $YBa$₂Cu₃O_6.0 (1.79Å), respectively.

Figure 1: Unit cell of $YBa$₂Cu₃O_6.5 with alternating empty and filled chains. The shifts of the atoms relative to $YBa$₂Cu₃O_6.93 are marked by arrows, the lengths of which indicate the amount of the shift.

Our investigation reveals somewhat coupled shifts of the heavy atoms when compared with their positions in $YBa$₂Cu₃O_6.93. The Ba atom shifts by nearly 0.07Å towards the filled chain. The shift of the Y atom in opposite direction is less pronounced but still significant (approx. 0.03Å). The observed shifts of the Cu(2) type atoms should be of some relevance for the electronic balance in the CuO$$₂ planes.

Due to the displacements in c direction the buckling of the planes is slightly reduced for Cu(2a) and increased for Cu(2b) leading to closest distances between Cu atoms in adjacent planes of 3.30 and 3.48Å, respectively.

The slight differences in the coordinations of Cu(2a) and Cu(2b) might indicate marginally varied doping levels for these two kinds of atoms. However, simple electrostatics seem to offer a more evident explanation. Coulombic attraction will shift the Ba cation into a region with increased coordination by oxygen anions (although there also will be a stronger repulsion by Cu(1a) than by Cu(1b)). In turn, Cu(2a) moves away from Ba and exerts a stronger repulsion on Y. The circle is closed with the downward shift of Cu(2b) atoms.

(A. Simon, K. Trübenbach and H.Borrmann)

From the yearbook of the institute ("Wissenschaftlicher Tätigkeitsbericht") 1992

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