Superstructures interstitial

Cu-derivative, substitutional and interstitial superstructures. As discussed in 3.8.1 ff, the Cu-type structure is also an important reference structure because it may be considered the ancestor of several derivative structures. 

As a conclusion to this section, notice that a systematic description of ordering processes in alloys and of the superstructures which can be generated has been presented, for instance, by Khachaturyan (1983) in the framework of a theoretical treatment of structural transformation in solids. Two groups of superstructures have been specially considered substitutional and interstitial. 

This structure can be considered a superstructure of the AuCu(I) type with 1N atom inserted in an octahedral interstice. This structure, as the previously described cP5-Fe4N type, can be considered an interstitial ordered phase. 

This is, however, not the whole of the matter. The superstructure ordering of point defects the collection of interstitial ions along certain fines or sheets, as in Magneli s model for the precursor of his shear structures the temperature-dependent adjustment of composition of a nonstoichiometric phase at the boundary of the bivariant range the nucleation of a new phase of different stoichiometry—these depend on accumulating vacancies or interstitials in some regions of the crystal lattice at the expense of others. 

Fig. 9 Final structure model of the rutile (100)-(lx3) surface projected along [001], An arrow indicates the origin of the superstructure cell. The titanium atoms labelled Til-Ti5 and A, B are co-ordinated by oxygen as follows Til threefold Ti2 fivefold Ti3 bridge site Ti4 sixfold Ti5 sixfold A fivefold (interstitial site) and B sixfold (trigonal prismatic). (Reprinted from [83]).

Substitutional solid solutions can have any composition within the range of miscibility of the metals concerned, and there is random arrangement of the atoms over the sites of the structure of the solvent metal. At particular ratios of the numbers of atoms superstructures may be formed, and an alloy with either of the two extreme structures, the ordered and disordered, but with the same composition in each case, can possess markedly different physical properties. Composition therefore does not completely specify such an alloy. Interstitial solid solutions also have compositions variable within certain ranges. The upper limit to the number of interstitial atoms is set by the number of holes of suitable size, but this limit is not necessarily reached, as we shall see later. When a symmetrical arrangement is possible for a particular ratio of interstitial to parent lattice atoms this is adopted. In intermediate cases the arrangement of the interstitial atoms is random. 

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

Fig. 155, p. 334, shows the density of TmxSe at room temperature as a function of x, determined with the buoyancy method, together with theoretical values and calculated for the following defect models 1) vacancies, 2) interstitial defects, 3) antisite defects (Tm occupies both cationic and anionic Schottky vacancies), Kaldis, Fritzler [1, p. 125], [2, p. 83], Fritzler, Kaldis [3], Fritzler et al. [4]. The discontinuity at Tmo.sySe is attributed to the formation of the TmsSe superstructure. The difference between experimental and calculated densities for the compositions Tmo.sySe to Tmi oSe is explained by the increasing number of Schottky vacancy pairs. The existence of both iSchottky pairs and antisite defects is assumed between Tm oSe and Tmi oeSe. Selected numerical values of the experimental density as a function of composition ... 

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