Isomorphs and polymorphs

Two or more substances that crystallize in almost identical forms are said to be isomorphous (Greek of equal form ). This is not a contradiction of Hauy s law, because these crystals do show small, but quite definite, differences in their respective interfacial angles. Isomorphs are often chemically similar and can then be represented by similar chemical formulae this statement is one form of Mitscherlich s Law of Isomorphism, which is now recognized only as a broad generalization. One group of compounds which obey and illustrate Mitscherlich s law is represented by the formula M2SO4 Mj (804)3 24H20 (the alums), where M represents a univalent radical (e.g. K or NH4) and M represents a tervalent radical (e.g. Al, Cr or Fe). Many phosphates and arsenates, sulphates and selenates are also isomorphous. 

Another phenomenon often shown by isomorphs is the formation of overgrowth crystals. For example, if a crystal of chrome alum (octahedral) is placed in a saturated solution of potash alum, it will grow in a regular manner such that the purple core is covered with a continuous colourless overgrowth. In 

There have been many rules and tests proposed for the phenomenon of isomorphism, but in view of the large number of known exeeptions to these it is now recognized that the only general property of isomorphism is that crystals of the different substances shall show very close similarity. All the other properties, including those mentioned above, are merely confirmatory and not necessarily shown by all isomorphs. 

A substance capable of crystallizing into different, but chemically identical, crystalline forms is said to exhibit polymorphism. Different polymorphs of a given substance are chemically identical but will exhibit different physical properties. Dimorphous and trimorphous substances are commonly known, 

Calcium carbonate calcite (trigonal-rhombohedral) aragonite (orthorhombic) vaterite (hexagonal) 

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Natta, G., L. Porri, A. Carbonaro, and G. Lugli Phenomena of isomorphism and polymorphism in trans-1,4-copolymers of 1,3-butadiene with 1,3-pen-tadiene. Makromol. Chem. 53, 52 (1962). 

Types of crystalline mixed-metal pyrophosphates are numerons and they include many examples of isomorphism and polymorphism. Known series include... 

The salts FeH2P30io and CrH2P30io are formed in similar dehydration sequences, and the acid salts of all three metals exist in various isomorphous and polymorphic forms. Both CrH2P30io 2H2O and AIH2P3O10 2H2O find use in anti-corrosive pigment formulations. Both salts can form intercalation compounds (Chapter 12). 

Stephenson, G.A., Stowell, J.G., Toma, P.H., Dorman, D.E., Greene, J.R., and Byrn, S.R. (1994). Solid-state analysis of polymorphic, isomorphic, and solvated forms of dithromyfciAm. Chem. So,c116 1-9. 

The data collected unambiguously indicate the existence of a large variety of actinide orthophosphates and their analogues (orthovanadates, orthoarsenates, orthosilicates, etc.). They are characterized by iso- and heterovalent isomorphic substitutions of cations and anions, and polymorphic and morphotropic transitions. The data analysis allows to predict many new actinide compounds of the group with different expected stmctures and tailored properties, including stability in different extreme physical fields and chemical media. Certain progress in this research direction has already been achieved. 

Diphenyl-2,5-cyclohexadienone 3 exists as four polymorphs, labeled A-D, whose crystallographic details are summarized in Table 3-2 [14]. Polymorphs A-D are conformational polymorphs since they have different molecular conformers in their crystal structures. They are also concomitant polymorphs because they crystallized simultaneously from the same flask and under identical crystal nucleation and growth conditions. Forms B-D with multiple molecules in the asymmetric unit Z > ) may also be classified as conformational isomorphs. Such polymorph clusters with different molecular conformations, crystal packing and C—H - O... 

Polymorphic transformations into the structure with Ac > 8 were observed in cesium halides [71-74] atfollowing pressures CsCl 65, CsBr 53, Csl 39 GPa besides, Csl at P > 200 GPa undergoes a smooth second-order transition into a hep structure with Ac = 12. Very peculiar transitions were observed in samarium chalcogenides SmX under high pressures the electron transition Sm(II) Sm(III) takes place with a sharp isomorphic decrease in the lattice parameters. Further compression leads to the transformation of the NaCl CsCl type. Values of pressures at the isomorphic (Pitr) and polymorphic (Pptr) transitions according to [75, 76] are following ... 

For instance, also the homopolymers PVF and PVDF have been described to crystallize in separate crystals in their blends [99] (Though constituted by isomorphous monomeric units which can cocrystallize in the copolymers in the whole range of composition, as seen in Sect. 4.1). Moreover, at least for the studied conditions, the polymorphic behavior of PVDF is not altered by the presence of PVF [99]. 

In the previous paragraph, it has been stated that minerals have the same structure but different compositions (phenomenon of isomorphism of minerals) while some minerals have the same composition but different structures (phenomenon of polymorphism of minerals). Mineral composition and structure are both important in studying and classifying minerals. The major class of minerals - based on composition and structure - include elements, sulfides, halides, carbonates, sulfates, oxides, phosphates, and silicates. The silicate class is especially important, because silicon makes up 95% of the minerals, by volume, in the Earth s crust. Mineral classes are divided into families on the basis of the chemicals in each mineral. Families, in turn, are made of groups of minerals that have a similar structure. Groups are further divided into species. 

The chemical and physical stability of aqueous and nonaqueous suspensions of a number of solvatomorphs of niclosamide has been evaluated in an effort to develop pharmaceutically acceptable suspension formulations [90]. Studied in this work was the anhydrate, two polymorphic monohydrates, the 1 1, Y, A"-dimethyI I ormam ide solvatomorph, the 1 1 dimethyl sulfoxide solvatomorph, the 1 1 methanol solvato-morph, and the 2 1 tetraethylene glycol hemisolvate. All of the solvatomorphs were found to convert initially to one of the polymorphic monohydrates, and over time converted to the more stable monohydrate phase. The various solvatomorphs could be readily desolvated into isomorphic desolvates, but these were unstable and became re-hydrated or re-solvated upon exposure to the appropriate solvent. 

Isomorphous pigments compounds which, for instance, chemically differ only in a single substituent but whose spatial requirements are similar, are frequently more uniform in their physical and application characteristics than is true for polymorphous pigments. 

Crystalline form,—Lithium nitrate crystallizes in rhombohedra (trigonal system).15 P. W. Bridgman observed no new form of lithium nitrate between 20° and 200°, and press, between 1 and 12,000 kgrms. per sp. cm. The older authorities—e.g. P. Kremers—supposed this salt to be trimorphic, but the supposed polymorphism is probably due to their mistaking hydrates for polymers of the anhydrous salt. It is doubtful if lithium nitrate is isomorphous with silver or sodium nitrate, although J. W. Retgers says that sodium and lithium nitrates are isomorphous. 

Corma, A., Navarro, M. T., Rey, F., Rius, J. and Valencia, S. Pure polymorph C of zeolite heta synthesized hy using framework isomorphous substitution as a structure-directing mechanism, Angew. Chem. Int. Ed., 2001, 40, 2277-2280. 

Fig. 5.1 Schematic illustration of three possibilities for arrangement of molecules of the same chemical composition but different conformations in the crystal. For each case two conformations represent symbolically cisoid and transoid dispositions around a single bond. The rectangle defined by a broken line represents one possible choice of the unit cell, (a) and (b) conformational polymorphs (c) conformational isomorphism (d) conformational syn-morphism. (From Bernstein 1987, with permission, as adopted from Corradini 1974, with permission.)...

R = -C4H9) has been reported to crystallize in four polymorphic forms (Brandt et al. 1982). In the copper phthalocyanines Pigment Blue 15, containing no Cl, is isomorphous with Pigment Blue 15 1, which on an average contains 0.5-1.0 atoms of chlorine per molecule (Hao and Iqbal 1997). Recognition of this phenomenon can be useful, for instance, in attempting to work out the crystal structures of unknown polymorphic forms. 

Phillips, J. W. C. and Mumford, A. (1934). Dimorphism of certain aliphatic compounds. V. Primary alcohols and their acetates. J. Chem. Soc., 1657-65. [98] Pierce-Butler, M. A. (1982). Structures of the barium salt of 2,4,6-trinitro-l,3-benzenediol monohydrate and the isomorphous lead salt (/3-polymorph). Acta Crystallogr B, 38, 3100. [294]... 

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