Chain-type structure

Crystal chemistry of Ta and Nb fluorides - Compounds with chain-type structures... 

Since the coordination number of tantalum or niobium in fluoride and oxyfluoride compounds cannot be lower than 6 due to steric limitations, further decrease of the X Me ratio (lower than 6) leads to linkage between complex ions in order to achieve coordination saturation by sharing of ligands between different central atoms of the complexes. The resulting compounds have X Me ratios between 6 and 4, and form crystals with a chain-type structure. 

Sharing of an oxygen atom by two central atoms in compounds with chain-type structures weakens the binary Nb=0 bond compared to the corresponding bond in pure isolated ions such as NbOF52 This phenomenon affects the vibration spectra and increases wave numbers of NbO vibrations in the case of isolated oxyfluoride complex ions. Table 31 displays IR absorption spectra of some chain- type compounds. Raman spectra are discussed in [212],... 

Crystal chemistry ofTa and Nb fluorides — Compounds with chain-type structures 91... 

NH4NbOF4 Infinite chains of octahedrons -Nb02F43 MNbOF4 (M=K, Rb, Cs) refers also to a chain-type structure. 

The thermal decomposition of alkali metal oxyfluoroniobates is also not a trivial process. MNbOF4 compounds (where M = alkali metal) with a chain-type structure are relatively stable up to temperatures in the range of 500-600°C. Fig. 90 presents mass loss dependences on temperature of several MNbOF4 compounds. As can be seen, among the compounds presented, only CsNbOF4 exhibits significantly different behavior, beginning its thermal decomposition at a lower temperature of about 400°C. 

Fig. 90. Mass loss temperature dependences for compounds with chain-type structure - MNbOF4, where M — Li (curve 1) Na (curve 2) K (curve 3) Rb (curve 4) Cs (curve 5) (after Agulyansky et al. [379]).

It seems that structural irregularities that cause spontaneous polarization are a relatively common property of niobium and tantalum oxyfluoride crystals. Fig. 100 shows the temperature dependence of SHG signals for several compounds that form island-type and chain-type structures. 

The electrochemical intercalation/insertion is not a special property of graphite. It is apparent also with many other host/guest pairs, provided that the host lattice is a thermodynamically or kinetically stable system of interconnected vacant lattice sites for transport and location of guest species. Particularly useful are host lattices of inorganic oxides and sulphides with layer or chain-type structures. Figure 5.30 presents an example of the cathodic insertion of Li+ into the TiS2 host lattice, which is practically important in lithium batteries. 

Glucose may form a chain type structure such as the one pictured below. All of the carbon atoms except the one at the top and the one at the bottom are chiral. The four different groups making the fourth carbon chiral are outlined. 

In the halides bridging anions lead to distorted CrXe octahedra sharing corners in which there are four short approximately equal Cr—X bonds and two long trans Cr—X bonds. There is also angular distortion of the octahedra. The fluoride has the tetragonal (distorted) rutile structure and the other halides chain-type structures. The mixed crystal (Mn, Cr)I2 is isotypic with monoclinic Crl2 (Table 34). 

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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