Abstract
Neutron powder diffraction has been used to refine the structure of the high- T c superconductor Tl 2 Ca 1 Ba 2 Cu 2 O 8 to determine the precise oxygen co-ordination and the effective valence of copper. The results are in excellent agreement with the Dupont X-ray structure, confirming the possible substitution of about 9% of the Ca-sites with Tl in our sample. The Tl-sites may be either 3% Tl-deficient or 7% Ca-substituted. The apparent valence of copper is 2.22. Oxygen in the center of the Tl square implies an average valence of less than 3 for Tl, and it is therefore not suprising that this oxygen is desordered toward the mid-point of the shortest Tl-Tl distance. High resolution electron images and diffraction reveal a quite different superstructure to the 5 x 5.4 Å superstructure seen in Bi 2 Sr 1 Ca 2 Cu 2 O 8 . Very weak and diffuse superlattice spots may be due to a charge density wave in the TlO plane with period 6 x 3.8 Å.
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