Isomorphic compounds

An unusual crystal arrangement is exhibited by the isomorphous compounds CrCl and Crl. The close-packed cubic array of Cl or I atoms has two-thirds of the octahedral holes between every other pair of chlorine or iodine planes filled with chromium atoms. Alternate layers of the halogen compounds are held together by van der Waals forces (39,40). 

The crystal structures of Hf 2 (OH) 2 (S0O 3 (H2O) i, (14) and Ce2(0H)2(S0i,)3 (H20)it (14) also have been determined and found to be isomorphous to the zirconium compound. The cell constants for this series of four isomorphous compounds reflect the effect of the ionic radii on the dimensions of the unit cell. The values for these cell constants are in Table II. Thus, the cell constants for the zirconium and hafnium compounds are nearly identical and smaller than the cell constants for the cerium and plutonium compounds which are also nearly identical. This trend is exactly that followed by the ionic radii of these elements. 

The structural peculiarities of the whole class of isomorphous compounds PbsAOs (A = S, Se, Cr) are up to now investigated only by single crystal and powder X-ray methods. Thus two t)q)es of superstructures obtained are 4 3 2 from single crystal dififractometry and 2 3 2 from powder dififractometry. This contradiction between the two methods can be solved by using the SAED method as it is done here with PbsMoOs. SAED investigations undoubtedly confirmed that 4 3 2 superstructure of deformed a-PbO is more probable to this class of compounds [18]. 

Iron(III) sulfate readily forms alums with many isomorphous compounds by mixing equimolar amounts of both the salts in aqueous solutions followed by crystaUization ... 

The different structure types belonging to the same formula t3rpe Aa MeFj, are described successively, with lattice constants of isomorphous compounds given in tables. Isomorphous compounds containing the same ion A were listed according to the indicated value of their molecular volume (volume per formula unit). 

The normal sulphates show a marked tendency to the formation of double salts,8 the best known case being that of the alums, which are isomorphous compounds of the general formula M2(S04)3.X2S04. 24H20, where M and X represent a tervalent and univalent metal, respectively in aqueous solution these double salts are almost entirely resolved into the ions of their constituent salts,9 recombination taking place as the solution crystallises. Double salts are also formed by the crystallisation of fused mixtures of anhydrous sulphates, the freezing-... 

Isomorphous compounds are Rh2Se3 and Ir2S3. Attempts to replace half the cations in RI12S3 by Co, Cr or Ga failed, although these cations fit into octahedral symmetry and are not too different in size. 

In various cases not all coordinated ions are the same, but the same sized fluorine and oxygen ions occur side by side, as in the series of isomorphous compounds K3ZrF7, K3TaOF6, K3wo2F5, also nitrogen with oxygen in the osmiamates ... 

Although only the heavy atoms (Au, P, and S) have been located, the compound has a central gold atom surrounded by ten remaining gold atoms, each of which has one ligand attached to it three of these ligands are, thus, AS-thiocyanato groups. Some isomorphous compounds with other phosphines have been reported 175). 

Single JT molecules or clusters in a crystalline environment may be found distorted, when they are investigated by X-ray (or other) diffraction methods. In his monumental work Liehr extensively dealt with JT instabilities and static distortions for many types of molecular structures. We have previously seen (Sect. 2.1) that one of the equivalent distorted configurations could be stabilized by any weak low symmetry perturbation In order to be sure that a given distortion is JT induced and not due to lattice forces a comparison with isomorphous compounds containing non-JT ions or an analysis of analogous compounds are necessary ... 

Sometimes different compounds give apparently identical crystals. Isomorphism is the similarity of crystal shape, unit cell dimensions, and structure between substances of nearly, but not completely, identical chemical composition. It is derived from the Greek words - isos meaning equal and morphe for form or shape. The arrangements of atoms in the isomorphous crystals are identical, but the identity of one or more atoms in this arrangement has been changed. For example, sulfur in a sulfate may often be replaced by selenium, to give an isomorphous selenate. Ideally, isomorphous compounds are so closely similar in composition that... 

An excellent example of a series of isomorphous compounds is that of the alums, described in Chapter 2. The space group of the alums, which are cubic, is Pa3 with four 7 i/ 3(S04)2T2H20 per unit cell. The potas-... 

It resembles AlgOg in having a corundum structure and in resisting solution particularly after having been heated. The ionic radii, Al 0.50 A and Cr 0.64 A, are similar enough to allow the formation of many isomorphous compounds, exemplified in the fluorides and the alums. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

An important characteristic of perovskites, mentioned in the preceding sections, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions will be studied, where these oxides can participate as catalytic agents. 

In order to be sore that a distortion of a Cu compound, for example, is Jahn-Teller induced, a comparison with the isomorphous compounds with Zn or Ni is useful. These ions possess orbitally nondegenerate ground states in octahedral coordination and have nearly the same ionic radii as Cu. If the latter compounds contain Np- - and Zn in regular octahedral coordination, while the ligand enviromnent is strongly distorted in the case of Cu, the Jahn-Teller effect nmst be lespoimble for the symmetry reduction. 

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