Linked Octahedra Chain Structures

2) 1-dimensionally Linked Octahedra (Chain Structures) a) Structural Element MeFs = [MeF2/2F4] 

Whereas no structure analyses of pentafluorometallates of the formula type AMeFs have been performed yet, the chain linking of octahedra in the compounds TI2AIF5 and (NH4)2MnFs (page 30) is well established. In contrast to the pentafiuorides, the octahedra of which are linked by 

Similar chain structures are well known of other compounds and seem to be preferred by ternary scdts, the A-ions of which are between the chains and hold them together. Binary fluorides on the other hand seem to prefer the forming of closed, finite groups, i. e. low rather than high polymer molecules. In the a-modification of UFj, however, such a chain stmcture is also found (343). But by the arrangement of chains further ligands in a second sphere are added to those of the primary octahedral coordination of the uranium. 

The lattice constants in the direction of 1-dimensional linking (about 3.3 A) are determined by the length of the edges of the MeFe-octahedra in both, Na2CuF4- and ratile-type. 

The hexagonal lattice constant c in the direction of finite or infinite 1-dimensional linking in the compounds mentioned is an integer multiple of the distance between opposite octahedral faces (about 2.5 A). 

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Lundberg and Savborg [222] reported that the crystal structure of CuNb03F is composed of Nb(0, F)6 octahedrons linked via two shared comers. The linked octahedrons form zig-zag chains along the c axis. The chains are connected to one another via copper atoms, each of which is surrounded by four anions that form a four-sided structure and two additional anions positioned at a greater distance from the copper atom. Fig. 35 shows the structural elements of CuNb03F. 

In the structure of the mineral haggite the chains which by corner linking form the layers are double octahedron chains as visualized in Figure 99c. The additional corner sharing reduces the mean valence of the cation to +3.5. The displacement of the cation out of the center of the oxygen octahedron is much less pronounced V—0=1.82 and 2.06 A. The distances to the O atoms at the base of the pyramid are similar to those in VO(OH)2, namely 1.97 and 2.01 A. 

We recently reported the existence of the largest structural family of borates discovered to date. The family has the nominal composition AgMM (B03)6, where A = Sr, Ba, or large lanthanide and the elements M and M are any of those cations having a +2, + 3, or +4 formal oxidation state and a preference for octahedral coordination. The structure of the materials contains chains (Fig. 1) consisting of octahedra alternately occupied by atoms M or M and separated by triangular planar BO3 groups. The distorted octahedron occupied by atom M is smaller than that occupied by atom M. The sizes of the octahedra are dictated primarily by the interactions of the chain with the A atoms that link the chains into the three-dimensional structure. 

The structure can be viewed as chains of linked TiOe octahedra, where each octahedron shares a pair of opposite edges, and the chains are linked by sharing vertices this is shown in Figure 1.41(b). Figure 1.41(c) shows a plan of the unit cell looking down the chains of octahedra so that they are seen in projection. 

Typical examples of compounds that form chain-type structures are simple pentafluorides of tantalum and niobium, TaFs and Nbfs, which were investigated by Edwards [199]. The cry stal structure of NbF< consists of rings made up of four NbF6 octahedrons that are linked by fluorine ligands that are each shared by two neighboring octahedrons, as shown in Pig. 29. The main interatomic distances (in A) are as follows Nb-Nb 5.80, 3.90 and 4.13 Nh-P,(Ki -. 1.75 and 1.78 Nb-Ftal% - 2.06 and 2.07 angle Nl>F[jf d,e-Nh - 182.5°. 

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