Copper three-fold coordinated

Many models have been proposed to explain oxygen and vacancy orderings in YBa2Cu307 g, but most of them do not take into account the copper coordination, for which three-fold coordination is not likely. The general model which is proposed here is based on the... 

The existence of new periodicities setting up along directions different from [001] implies a change of the copper coordination in one row, parallel to b. Such a change can be ensured, if we except three-fold coordination, either by a copper vacancy which can be occasionally encountered, but not in a systematic and periodic way, or by the interconnection of the rows. This interconnection can be ensured by an octahedron, a pyramid or a tetrahedron, all in agreement with the usual coordination of Cu(n) (or Cu(HI)). A model is proposed for the supercell 2a x a%/To in Figure 21a CuOB pyramids are lined up along the a-axis, with alternated positions of the vertex... 

Several models for the structure of the twin walls have been proposed (30, and ref. therein). The simplest of these is illustrated in Figure 4. The twin wall is formed by three consecutive diagonals. The copper atoms located on the central one have two-fold coordination with corresponding oxygen stoichiometry of x = 6.0. Those located on the two neighboring rows on each side of the central diagonal have three-fold coordination and oxygen stoichiometry x = 6.5. Within each domain, away from the boundary, the composition is x = 7.0 and copper has the usual four-fold planar coordination of... 

A tetragonal rutile-type structure is adopted by Ca2Cu4P2, in which each Ca atom is octahedrally coordinated by P atoms and each P atom is three-fold coordinated by Ca atoms. Edge-sharing CU4 tetrahedra are formed by the copper atoms which form continuous chains parallel to the tetragonal axis [47] (Figure 8.14). 

The best way to describe the structure of 67 is in terms of the packing of the selenium atoms. There are three distinct, parallel, and planar selenium layers the two outer layers, 1 and 3, which contain 10 selenium atoms, and the inner layer, 2, which contains 15 selenium atoms. The layers are arranged in a hexagonal close-packed manner and are spaced at a distance of about 360 pm from each other. The distribution of the selenium atoms is reminiscent of the packing in /d-Cu2Se. In this solid state compound, however, the selenium atoms are arranged in a cubic close-packed fashion, with a layer separation of approximately 339 pm. Three copper atoms lie outside the layers 1 and 3, and 32 copper atoms each lie between the layers 1 and 2, and between the layers 2 and 3. The copper atoms occupy tetrahedral sites within the selenium atom matrix, but are displaced towards one Se3 surface of the tetrahedron, giving rise to only a three-fold coordination. 

There are three well-established oxides of copper containing Cu and/or Cu" viz. CU2O, CU4O3 and CuO. Their structures are all based on a cubic-close packing (c.c.p.) of copper (somewhat distorted in C114O3 and CuO) with oxygen inserted into tetrahedral sites in such a way as to be a solution to the problem of having Cu in linear two-fold coordination and Cu in planar four-fold coordination. These have been discussed from this point of view elsewhere , and it has been speculated that the observed structures are, in fact, the only solutions to the problem. 

We might well have asked why 3d-4s hybridization on Cu1 did not lead to an egg shaped core instead of a disk shaped core. To a first approximation, these two situations would indeed to be degenerate. With hybridization to an egg shaped core, the occurrence of Cu1 in three-fold of four-fold planar coordination should be no surprise. This, of course, actually happens in the n-type copper oxides such as Nd2 xCexCu04. 

The p3-bridged carbonato complex posseses a pseudo-3-fold molecular symmetry. Each of the Cu atoms is five coordinate with the four nitrogen atoms of tren and one oxygen atom of the carbonate ligand (C). The coordination polyhedron of the Cu atom can be described as almost (TBP), the copper ions being slightly out of the plane (0.15 A) of the three primary amine groups (Fig. 5). 

The structure of YCu4.74Pbo.26 is shown as an example in Figure 19. The copper atoms on the 16c site build up a three-dimensional network of corner-sharing tetrahedra. This network leaves voids of coordination number 16 which are filled by the rare earth and Pb/Cu atoms on the four-fold sites. The large coordination number of this void readily explains the higher lead content on this site. The atoms occupying the 16c site (mainly copper in the RCus-xPb series) have coordination number 12 in the form of a distorted icosahedron. For more crystal chemical details on the Laves phases and related compounds we refer to review articles (Simon, 1983 Nesper, 1991 Johnston and Hoffmann, 1992 Nesper and Miller, 1993). 

---