Vacancy structure

These five defects are based upon an excess in the number of sites available. This excess we call "6". Note that we are not speaking of the ratio of cations to anions, i.e.- stoichiometry, but of the total number of sites. To see how this is possible, consider the Vacancy-Structure type. 

Schottky Frenkel Anti-Frenkel Anti-Structure Vacancy-Structure. 

The structure of the (2 x 2)—Na phase as determined in a combined LEED, SEXAFS, and DFT study [58], and shown in Fig. 9, contains some of the features of the models proposed previously and, in particular, confirms the qualitative conclusions of the CLS study [57]. As can be seen from the figure, the structure consists of an Na-Al-Na sandwich, on a reconstructed Al layer with a (2 X 2) vacancy structure. Na atoms in the lower layer of the sandwich are located in the substitutional sites of this vacancy layer. Na atoms in the upper layer of the sandwich and Al atoms in the middle of the sandwich are located in fee sites and hep sites, respectively, in the vacancy layer. A later photoelectron diffraction study [59] is reported to be in agreement with the structure as described here. 

Perhaps the most important outcome of the DFT studies was the explanation of why alkali metal adsorption on A1 can result in the formation of surface alloys. This can be attributed in part to the exceptionally low energy of formation of a surface vacancy on A1 surfaces, which was calculated to be E ac = 0-41 eV for an Al(l 11)—(.y/3 x 2>)R3Q° vacancy structure [78]. Thus substitutional adsorption is thermodynamically favoured if the difference in binding energy between adsorption in a vacancy and adsorption in a normal site is > E ac- The... 

Recently Simakova et al. [94] have reported a detailed study of the influence of a series of dehydration/rehydration cycles on the morphology of the solid product. Results show the stability of the "vacancy structure" produced by dehydration. 

Anti- Frenkel, Anti-Structure, Vacancy-Structure. 

Of the nine defect-pairs possible, only 5 have actually been experimentally observed in solids. These are Schottky, Frenkel, Anti- Frenkel, Anti-Structure, Vacancy-Structure. 

It was also found that the oxygen vacancy structure as well as nonstoichiometry in the mixed oxide systems plays important roles for the oxygen reduction performance. It is known that transition-metal oxides and mixed perovskite-type oxide system (Lai xA xCoi yFey03 A Sr,Ca x, y = 0.2-0.5) have high catalytic activity for the peroxide decomposition. 

In conclusion, let us consider the results of TiC and VC, electronic structure calculations performed for ordered vacancy structures by the band methods. Electronic structure calculations for the hypothetical Bl-like carbide Ti4C with a y-Fe4N structure have been carried out by Ivanovsky et al (1989b). The unit cell included two types of metallic centres those with and without nonmetal atoms in the immediate vicinity... 

Desolvation and injection of Li ions into the vacancy structure... 

Clustering into three-dimensional essentially vacancy structures firstly to create vacancy cluster germs, and then voids when the net flux of vacancies is sufficient ... 

B5 surface alloy stmcture Na-Al-Na sandwich on a reconstructed A1 layer with a (2x2) vacancy structure lower Na in substitutional site in A1 vacancy upper Na in fee sites sandwich A1 in hep sites... 

In addition to existing or potential framewoik vacancies, structural defects may also be present in as-synthesized zeolites (ex. Beta zeolite). Structural defects also lead to mesopores formation during dealumination. If such defects are characterized by specific orientations, the mesopores created can present similar orientations (this is the case for dealuminated fluoride medium synthesized Beta). 

---