Structural vacancies

Anti-Frenkel Anti-Structure Vacancy- AntiStructure Xi Vx + Mx Vm -h Mx... 

Anti-Structure Interstitial Anti-Structure Vacancy + Ml Xm + Vx... 

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

A closer examination of Table 6-3 reveals a rather startling conclusion. It can be seen that most of these compounds are subject to structure-vacancy defects. Certainly all of the pnictides fall into this class. We are even given the defect concentration. Thus in GaP, we have ... 

Host cations are easily oxidized and reduced [transition metals (Fe203, Mn203, etc.)]. In both donor (higher valence) and acceptor (lower valence) doping electronic compensation (donors by electrons and acceptors by holes) will be preferred to structural (vacancies and interstitials) compensation. 

As a final example, we may mention the NaCl-type phases formed in the V-0 systems. The V01 x phase is homogeneous in the composition range 42-57 at.% O. Lattice parameter determination in combination with density measurements evidenced that, in the structure, vacancies occur in both V and O sublattices through the entire range of composition. At the stoichiometric composition VO there are = 15% of sites vacant in each sublattice. 

Fig.6. Relationship between structure vacancies and Si(3Si, 10H) sites (O) Si, ( ) trivalent atom, ( ) vacancy...

Fe203 species are believed to preferably to deposit on these structural vacancy sites and, therefore, the Fe-0 bond polarization was induced, giving rise to the increase in the ionicity and binding energy of Fe-0 bond. 

The main result of the experimental part of the study, which is shown in Fig. 4, confirms the supposition, which was made in the Introduction above, that one cannot expect formation of structural vacancies in the oxygen sublattice of strontium-alloyed Lai-ySryMnC -s perovskites and, correspondingly, a considerable increase... 

Eyring, H and T. Bee Significant liquid structures. Vacancy theory of liquids. Proc. Natl. Acad. Sci. U.S. 47, 526 (1961). 

This atomic arrangement results in a hexagonal skeleton of space group Ric with large structural vacancy sites per unit cell of octahedral symmetry (Ml) occupied by cations with the coordination number [6] such as titanium or zirconium and trigonal-prismatic symmetry (M2) occupied by vacancies as in Nb2(P04)3, by two-valent ions as in Ca05Ti2(PO4)3 or three-valent ions as in Lao, 33Ti2(P04)3. These vacancies account for the structural variability of the NASICON family as well as their ionic conductivity. 

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. 

Figure 2 shows that the framework Si02/Al203 ratio increased with increasing hydrothermal aging temperature, while structure collapse temperature decreased. This was attributed to the imbalance between dealumination and Si migration into the A1 defects. The structural vacancy resulting from rapid dealumination decreased the zeolite structural stability. 

Samples of zeolite Beta with various degrees of dealumination were prepared via various treatments by Yang and Xu [263]. These authors provided framework spectra of their samples, obtained in the region 1400-400 cm through the conventional FTIR/KBr pellet technique. A band at about 950 cm was attributed to Si-O vibrations in structural vacancies (cf. also [264]) and monitored in dependence on the treatment. 

Extrapolation of the diffusivity to the melting point (3225C) revealed a very low value of about 1 m /s which reflected the covalent bonds present in the material. A possible explanation for the low values obtained for the Arrhenius parameters was diffusion via vacancies where, in addition to thermal vacancies, a substantial concentration of structural vacancies was present. 

Balankin et al. [8] analyzed the stability of fee Th. N crystals with ordered Th and N sublattices containing structural (nonstoichiometry) and thermal vacancies. Their results give for the free energy of formation of the structural vacancies... 

DFT calculations have been performed recently, but did not solve the problem completely. Sohlberg et al. [141,142] arrived to a structure very similar to that proposed by Zhou and Snyder [138], based on spinel but with occupation of extraspinel sites. On the contrary, Digne et al. [143] and Krokidis et al. [144] proposed a stmeture based on cep oxide lattice (S.G. P2i/m Z = 8) but different from that of a spinel, with 25% of Al ions in tetrahedral interstices and no structural vacancies. According to these authors, this structure, although unstable with respect to corundum, is more stable than that of the spinel based structures. 

More detailed ARPES studies of nonpolar surfaces for some stoichiometric and nonstoichiometric 3d- and 4d-metal carbides and nitrides reveal a series of finer effects, which could be connected with the specific features of electron distributions near the crystal surface or could be induced by structural vacancies. For example, some states determined experimentally for the (100) surface of NbCo.sa by Lindberg (1987) do not seem to have counterparts in the calculated results for NbCi.o. but... 

Buckminsterfullerene (perfect structure) vacancy defect fullerenes generated from Ceo (a) monovacancy, (b) di-vacancy, (c) tri-vacancy, (d) three different structures of tetra-vacancy (Reprinted with permission from Hu and Ruckenstein (2004). Copyright 2004, American Institute of Physics)... 

Structural vacancies in non-stoichiometric intermetallic compounds have been detected by coupled density and lattice-parameter measurements. The measured density is compared to calculated densities in order to choose between several models antistructure defeas, vacancies within a sublattice, interstitials, etc. The application of this method to CoAl alloys is shown in Figure 2 (FIdscher, 1993). Care must be taken to check the absence of porosities and to determine precisely the chemical content of the sample. 

The progress of X-ray and especially neutron diffraction techniques now allows one to obtain information on structural vacancies and antisite defects (in the percent concentration range) from the precise determination of structure factors (see for example Kogachi et ai., 1992) neutron measurements are possible up to very high temperatures. 

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