Vacancy rows

The formation of vacancy rows in the layers of ionic compounds must result in the occurrence of strong electrical fields, especially at low temperatures when their electrical conductivity is insufficiently high. These fields will undoubtly affect the process of layer growth and may even lead to its arrest. The value of this effect may be estimated following N.F. Mott and R.W. Gurney.180... 

Fig. 27.10. Image of a thin edge of a crystal of Pri2022. The calculated image based upon the proposed structure is shown as an insert at the bottom right. The white dots correlate with the projected vacancy rows which exist in the proposed structure.

STM has not as yet proved to be easily applicable to the area of ultrafast surface phenomena. Nevertheless, some success has been achieved in the direct observation of dynamic processes with a larger timescale. Kitamura et al [23], using a high-temperature STM to scan single lines repeatedly and to display the results as a time-ver.sn.s-position pseudoimage, were able to follow the difflision of atomic-scale vacancies on a heated Si(OOl) surface in real time. They were able to show that vacancy diffusion proceeds exclusively in one dimension, along the dimer row. 

Additionally, we have Illustrated another type of defect that can arise within the homogeneous lattice of 3.1.2. (in addition to the vacancy and substitutional impurities that are bound to arise). This is called the "selfinterstitial". Note that it has a decisive effect on the structure at the defect. Since the atoms are all the same size, the self-interstitial introduces a line-defect in the overall structure. It should be evident that the line-defect introduces a difference in packing order since the close packing at the arrows has changed to cubic and then reverts to hexagonal in both lower and upper rows of atoms. 

On the right are the t5rpes of point defects that could occur for the same sized atoms in the lattice. That is, given an array of atoms in a three dimensional lattice, only these two types of lattice point defects could occur where the size of the atoms are the same. The term vacancy is self-explanatory but self-interstitial means that one atom has slipped into a space between the rows of atoms (ions). In a lattice where the atoms are all of the same size, such behavior is energetically very difficult unless a severe disruption of the lattice occurs (usually a "line-defect" results. This behavior is quite common in certain types of homogeneous solids. In a like manner, if the metal-atom were to have become misplaced in the lattice cuid were to have occupied one of the interstitial... 

The structure of growing crystal faces is inhomogeneous (Fig. 14.11a). In addition to the lattice planes (1), it featnres steps (2) of a growing new two-dimensional metal layer (of atomic thickness), as well as kinks (3) formed by the one-dimensional row of metal atoms growing along the step. Lattice plane holes (4) and edge vacancies (5) can develop when nniform nucleus growth is disrupted. 

The existence of new periodicities setting up along directions different from [001] implies a change of the copper coordination in one row, parallel to b. Such a change can be ensured, if we except three-fold coordination, either by a copper vacancy which can be occasionally encountered, but not in a systematic and periodic way, or by the interconnection of the rows. This interconnection can be ensured by an octahedron, a pyramid or a tetrahedron, all in agreement with the usual coordination of Cu(n) (or Cu(HI)). A model is proposed for the supercell 2a x a%/To in Figure 21a CuOB pyramids are lined up along the a-axis, with alternated positions of the vertex... 

Going back now to the non-stoichiometric oxides, in the excess metal monoxides of type A and type B, we saw that extra electrons have to compensate for the excess metal in the structure. Figure 5.41 illustrates that these could be associated either with an anion vacancy or alternatively they could be associated with metal cations within the structure. Although we have described this association as reducing neighbouring cations, this association can be quite weak, and these electrons can be free TABLE 5.8 Properties of the first-row transition element monoxides... 

The assignment of these main bands to n - n transitions was confirmed by MO calculations and MCD studies.179,598 For the first row transition metals with vacancies in dxz and dyi orbitals, a ligand to metal charge transfer band is observed at 380-500 nm.200 A weaker metal to ligand charge transfer band is also expected to occur in this region. 

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