Rocksalt cluster

Fig. 8 Binding energy per ZnS pair as a function of size of the clusters for five different shapes namely (a) ring-like (b) hollow (c) zinc-blende (d) wurtzite and (e) rocksalt clusters. Reproduced with permissions from the American Institute of Physics [Ref. 63].

Figure 5.13 The 4 1 cluster of one tetrahedral cation and four vacant octahedral sites in one octant of the rocksalt structure.

The authors found that for clusters with q = 1 the calculated structures for small values of n agree with those of more accurate calculations and that for larger values of n clusters that look like smaller pieces of the rocksalt structure are found. On the other hand, for q, = 2 many structures were cage-like, in contrast to other theoretical or experimental findings for these systems. This... 

The last class of clusters in this list consists of ionic species, most clearly illustrated by the alkali halides. That alkali halide vapors contain many clusters was proved in the 1950s.(See especially Chap. 1 of Ref. 105.) These tend to have the same structures as the corresponding bulk solids, the rocksalt or NaCl structure in most cases, d06-i09 gj.y... 

It has been mentioned in the introduction that the condensed cluster halides of the rare earth metals based on the MgXi2-type cluster with an interstitial atom (or molecular unit) generally exhibit a defect rocksalt structure. Figure 10 provides clear evidence for this remark. The NaCl subcell in the structure of GdijInCg, marked by strong streaks is only weakly distorted (a = 6.07, b = 6.10, c = 5.92 A, a = y = 90°, p = 91°) by the ordering of I atoms and Cj units and occupation of all voids around the C2 units by Gd atoms. 

There is an interesting relation between TiO and compounds composed of discrete electron deficient clusters which are stabilized by interstitial atoms. TiO is an obvious borderline case. Having only a few electrons which can be used for M-M bonding, the arrangement with only empty clusters is easily transformed into a (statistically disordered) rocksalt structure at elevated temperature. Now, a portion of the ligand atoms plays the role of the interstitials. 

In order to rule out the possibility that the wurtzite clusters were an artifact of the employed classical force fields, the authors performed an ab initio structural optimization of bulk NaCl in the wurtzite configuration and found that this is indeed a metastable phase. The energy of the structure was found to be only 11.3 kJmol higher than that of the rocksalt arrangement. 

Actually, the defect structure in some wiistite compositions is even more complicated. In Feo 9O the vacancies and Fe ions cluster together in a specific arrangement, to be described later, giving a volumetric defect. The clusters themselves at low temperatures are spaced in a periodic array to form a supercell. In addition, within the vacancy clusters in highly nonstoichiometric wiistite some cations occupy tetrahedral sites rather than octahedrally coordinated sites as in the perfect rocksalt structure. It is probably mostly Fe ions which occupy tetrahedral sites the Fe ions are smaller than the Fe ions and the tetrahedral sites are smaller than the octahedral sites in wiistite. 

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