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. 2001 Dec 1;106(6):921–952. doi: 10.6028/jres.106.046

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Fig. 22 — Unit cell of the crystal structure of the 94 K superconductor Hg Ba₂CuO₄₊_x. In this cell, oxygen sites O(3) are only partially occupied. The copper atoms are in a bi-pyramidal coordination similar to that of Fig. 17. The Ba atoms are in eight-fold coordination in those unit cells in which O(3) is empty and in nine-fold coordination in the others. Similarly, Hg may have two-, three-, four-, five-, and six-fold coordination, depending on how the O(3) sites occupied by oxygen are clustered together.