Abstract
The structure of the compound Ca .85CuO 2, which was recently determined by neutron and X-ray powder diffractometry, was reexamined making use of electron diffraction and electron microscopy. The general features of the previously proposed model are confirmed. However, the modulated structure, for which only a tentative proposal was made in previous work, was found to be rather different. The calcium distribution along the “tunnels” formed within the CuO 2 framework is responsible for the modulation. The average CaCa separation along these tunnels is found to be different from the CuCu separation along the same direction in the surrounding framework, five calcium spacings corresponding with six copper spacings. This relation explains the complicated stoichiometry. The non-uniform distribution of the calcium along the tunnels causes modulation waves along (011) o planes, or along the equivalent (01 1) o planes. The presence of these two symmetry related variants leads to modulation twins.
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