Tetrahedrality parameter

Froyen and Harrison (1979) also used this procedure to obtain for a nearest-neighbor fit to the bands for a face-centered cubic structure. The four values (in comparison to the tetrahedral values) were —0.62 (— 1.39), 2.33 (1.88), 2.47 (3.24), and 0 (—0.93). These are major differences, but the nearest-neighbor LCAO theory of close-packed structures is of little use in any case. The tetrahedral parameters seem to be adequate for covalent and ionic systems but not to be relevant to the simple metals. 

Here is the separation where g(r) reaches the value unity after the peak, that is before any other modulation due to higher separation. In most liquids, the tetrahedral parameter increases with lowering of temperature but in water it reaches a maximum and then drops, as discussed in the text. 

Is there any evidence of large fluetuations in the tetrahedral order parameter th in supercooled water While experiments and simulations show an increase in isobaric specific heat and compressibility on lowering the temperature, there is hardly any convincing evidence for an increase in the fluctuation of the tetrahedrality parameter. This is paradoxical because, as mentioned above, the two-state model in some form or other has been used for a long time to explain the properties of water. 

Figure 22.5. Temperature dependence of distributions of (a) local volume, (b) the tetrahedrality parameter, and (e) the static structure factor. The figure has been reprodueed from private eommunieation with Prof Shinji Saito.

The Steinhardt parameter has been introduced in Sect. 3.2.2.I. For water the sum of the spherical harmonics qim runs over all oxygen-oxygen pairs and was smoothed with a radial cutoff function f rij). The tetrahedral parameter is defined as... 

FIGURE 6.2 The amide or peptide bond planes are joined by the tetrahedral bonds of the ff-carbon. The rotation parameters are p and Ip. The conformation shown corresponds to clockwise rotation as viewed from Starting from 0°, a rotation of 180° in the clockwise direction ( + 180°) is equivalent to a rotation of 180° in the counterclockwise direction (—180°). (truing G s)... 

For electron diffraction, the nonbonded distances of the tetrahedral sulfur bond configurations in sulfonyl molecules are closely spaced and cause strong correlation among the relevant parameters. Assuming one of them (O O) to be known can greatly facilitate the determination of the rest of the parameters (see more details in Reference 5). 

The structure found for binnite is shown in Figure 1, the interatomic distances corresponding to the finally accepted parameter values being given in Table V. Each Si atom is surrounded tetrahedrally by four other atoms, to which it is presumably attached by bonds which are largely covalent in character. The interatomic distances are in approximate... 

The compounds formed were studied by esr spectroscopy, with the magnetic parameters being used to determine the geometries. Mn02 is linear, whereas MnOa is trigonal planar (Dsh), and Mn04 is distorted tetrahedral with Csv symmetry. 

The results of the XRD measurement showed that the Fe jAl, jPO catalyst was almost in amorphous state. Only a very broad peak at 29 of ca. 23 degree was observed. The Mossbauer spectroscopic study on this catalyst showed one doublet of iron with the isomeric shift of 0.31 mm s (a-Fe was used as the reference) and the quadrupole splitting of 0.62 mm s. These parameters are very close to those observed for FePO [13, 14], suggesting that the iron cation in the catalyst is tetrahedrally coordinated with oxygen and isolated by four PO tetrahedral units. Such coordination circumstance was suggested to be a key factor for the iron site effective for the oxidation of CH to CHjOH by H -Oj gas mixture [15]. 

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