Anomalous solid solutions

Table I. Anomalous Solid Solutions Having Fluorite Structure...

Heterogeneous exchange of radionuclides on carbonates has already been mentioned. Exchange on other sparingly soluble minerals (e.g. halides, sulfates, phosphates) may lead to rather selective separation of radionuclides. Following the exchange at the surface, ions may be incorporated into the solids in the course of recrystallization, which is a very slow but continuous process. Anomalous solid solutions with radioactive ions of different charges may also be formed. 

For convenience we distinguish (i) anomalous solid solutions in elements, (ii) anomalous mixed crystals of ionic solids (heterotype solid solutions) and (iii) controUed-valency semi-conductors. 

There are numerous examples of these anomalous solid solutions which involve the creation of either interstitial ions or vacant sites. They will be discussed in somewhat greater detail in later chapters. 

Even with such a classification, some difficulty arises in assigning certain defect solids to one class or another. There can be no dispute that defects of the Schottky and Frenkel type in stoicheiometric crystals are thermal in origin there can equally be no dispute that the stoicheiometric dual-valency compounds are biograpliical, but the situation with regard to non-stoicheiometric compounds and anomalous solid solutions of various types is by no means as clear. Various authors have stated that non-stoicheiometric compounds are biographical in type this is incorrect, as the departure from stoicheiometry (or inversely the range of existence of a non-stoicheiometric phase) is a function of temperature which tends to zero as For example, zinc oxide... 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Solid solutions can also be formed to a limited extent with substances that have a similar formula but a different crystal structure. One of the substances imposes its structure upon the other. In lattices which are stable even if some sites in the unit cell are unoccupied, substances with different formulae can form solid solutions (anomalous mixed crystals). The incorporated ions must be of similar size, but do not need to have the same valency because electroneutrality is ensured by the presence of a compensating valency elsewhere in the lattice. Changes in the relative concentrations in the solid solutions lead to continuous variation of their properties. 

Figure 7.7. The observed and calculated powder diffraction patterns of LaNi4 gsSno.is after the completion of Rietveld refinement. Notations are identical to Figure 7.2. The second set of vertical bars indicates the calculated positions of the Kai components of Bragg peaks in the impurity phase, which is a solid solution of Sn in Ni. The virtual absence of a Bragg peak at 20 s 9° and the presence of the same reflection at 20 s 20° when Cu Ka radiation was employed (see Figure 7.5) is the result of differences in the anomalous scattering (see also Eqs. 2.101 and 2.108).

Another system which illustrates the transition from the random arrangement of the solid solution to the ordered structure of the superlattice is that of iron and aluminium. Strictly speaking, this system should be considered under our second group of alloys since we have classed aluminium as a B sub-group metal. We have already emphasized, however, that the position of aluminium is somewhat anomalous in that it also behaves in many respects as a true metal, and it is therefore not out of place to consider this system here. 

Above ca. 0.03 mol 1", the first is predominant but at lower concentrations the second is more important. Freezing-point studies in benzene and carbon tetrachloride are interpreted as showing the formation of solid solutions in the latter, and when allowance is made for this, the molecular weight of pels is no longer anomalous and is in good agreement with that for the monomer. 

The intermediate-valence phase of the solid solution system Smi Y,.S with X >0.15 was the first striking example of anomalous electron-lattice interactions... 

The y-phase is a solid solution with a face-centered crystal lattice and randomly distributed different species of atoms. By contrast, the y -phase has an ordered crystalline lattice of II2 type (Figure 10.2). In pure intermetallic compound NisAl the atoms of aluminum are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at the centers of the faces and form the sublattice B. The y -phase has remarkable properties, in particular, an anomalous dependence of strength on temperature. The y -phase first hardens, up to about 1073 K, and then softens. The interatomic bondings Ni-Al are covalent. 

The consequences of these tendencies are the technological difficulties encountered in the homogenization of solid solutions in pseudobinary and binary (Ge-Si, Se-Te) systems and the anomalous behavior of some properties (mobility, thermal conductivity) when considered as functions of the composition. We shall analyze here only the ZnSb-C dSb system. 

When two couples of enantiomers have a high degree of similarity, the S enantiomers can fit in the same crystal lattice that is, a complete solid solution. This mechanism of substitution expands across the binary system (idem for the two R enantiomers). By contrast, the couples of antipodes (5 l-i 2 and S2-RI) lead to the formation of the so-called quasi-racemic compounds and , which can be considered as co-crystals (Figure 13.12). Prior to the emergence of the X-ray diffraction technique using the anomalous... 

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