Isostructural substances

Within a group of isostructural substances, small variations are therefore generally observed in the a-value. If dmm is given in nm and V in cm3/mol (moles of atoms or moles of formulae), the following n-values may be mentioned. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

We have already defined model structures as isostructural substances in which ions occur closely equivalent in size but different in charge. In this sense Li2BeF4 is a model of the structure of willemite,... 

Methoxypurine was found to crystallize as a hemihydrate from /V,/V -dimethyl formamide, and as a trihydrate from water [63]. Thermal treatment of the trihydrate could be used to obtain the hemihydrate. Zafirlukast was obtained in the form of monohydrate, methanol, and ethanol solvatomorphs, with the drug substance adopting a similar conformation in all three structures [64], In the isostructural methanol and ethanol solvates, the solvent molecules are hydrogen-bonded to two zafirlukast molecules, while in the monohydrate, the water molecules are hydrogen-bonded to three zafirlukast molecules. The structures of the acetone and isopropanol solvatomorphs of brucine have been reported, where the solvent controlled the self-assembly of brucine on the basis of common donor-acceptor properties [65],... 

The last of these is perhaps the best criterion for metallic characteristics. Most other types of inorganic substance are effectively insulators, or semiconductors whose conductivity increases with temperature. Semiconductors constitute a borderline area some lie between metals and three-dimensional polymers (e.g. GaAs, isoelectronic and isostructural with elemental germanium), while others have structures which could be described as ionic (e.g. PbS, which has the NaCl structure). 

VB language) should help us to decide between these conflicting viewpoints but the results are inconclusive, and often provide succour for both factions. A good example is PH3F2. This gaseous molecular substance (isostructural with PCl3F2 - see Section 2.3) is unstable with respect to disproportionation ... 

As shown in Table I, lanthanum and lutetium oxides have Sq ground states and consequently their heat capacities should be attributed to lattice vibration. Data on these substances may be used to represent the lattice contribution to a first approximation for neighboring isostructural (and nearly so) sesquioxides. Cubic gadolinium oxide provides a midseries lattice heat capacity approximation at relatively high temperatures... 

Isotypy. Isotypic substances are those that have the same crystallographic space group and analogous chemical formulae and crystal structures. The terms isostructural and isomorphic are essentially synonymous with isotypic . 

One of the most interesting chapters in this book announces the synthesis of a silica that is isostructural with AlPO -5. This is the first example in which synthesis of a unique aluminum phosphate structure preceded the synthesis of other isostructural chemical analogues. Perhaps the next chapter in this story will be the synthesis of an aluminosilicate that has this structure. What will be the conservative point of view regarding the classification, zeolite or nonzeolite, of a substance whose parent is not a traditional zeolite ... 

If crystals of different species are isostructural and have the same type of bonding, they also will have very similar unit-cell dimensions and will macroscopically appear almost identical. This is known as isomorphism. Examples of isomorphic materials include ammonium and potassium sulfate and KH2PO4 and NH4H2PO4. In each of these materials, the potassium and ammonium ions can easily substitute for each other in the lattice since they are of almost the same size. This illustrates one of the properties of isomorphous materials, that is they tend to form solid solutions, or mixed crystals. Crystallization from a solution of two isomorphous materials, therefore, can result in a solid with varying composition of each species with unit-cell dimensions intermediate between the two components. The purification of isomorphous substances can, therefore, be difficult. 

The identification, structural and thermal characterization of new polymorphs is an important topic in solid-state chemistry and requires a battery of techniques that includes X-ray diffraction and spectroscopic methods, in addition to thermal analysis methods and dissolution techniques to determine solubility trends. Such studies are described by Caira in Chapter 16, as well as more recent theoretical techniques aimed at the prediction of the crystal structures of new polymorphs. Crystal polymorphism is particularly important in pharmaceutical products, so there is an emphasis on this area. Systems displaying solvatomorphism (the ability of a substance to exist in two or more crystalline phases arising from differences in their solvation states) molecular inclusion and isostructurality (the inverse of polymorphism) are also given due attention in this chapter. 

Isomorphism is similarity of crystal shape, unit-cell dimensions. and structure between substances of similar chemical composition. Ideally, the substances are so closely similar that they can generally form a continuous series of solid solutions. The degree of similarity between crystals can be calculated using Kalman s parameters the unit cell similarity index, IT. and isostructurality index /,( ) ... 

We cite a total of 226 rare earth minerals, from which 104 crystal structures are known and are introduced here. The minerals are classified on the basis of their structures, and minerals without structural data are classified on the basis of their crystallographic data. The mineral species cited in this text are selected from Glossary of Mineral Species (Fleischer and Mandarino 1991) and Mineral Reference Manual (Nickel and Nichols 1991) some are new minerals described after the publications of the Glossary and the Manual. We also included several unnamed minerals and inadequately described minerals. Some of the crystal structures reported here have been determined using artificial substances. They should be isostructural with natural minerals, but they may differ in chemical composition an example is garnet. 

Isopolymorphism is used when the various polymorphic forms of a substance are isostructural or home-ostructural to corresponding polymorphs of another substance. ... 

Since these substances are isostructural, solid solution formation is easy across the whole series, and the hardness of 50 mol % alloys, Lno.5Lno.5C2, can be considered alongside the observed depression of the cubic-to-tetragonal phase transformation on cooling. It has been shown that the depression in transformation temperature, AT), is related to cell volume difference AV and the shear modulus G for such solid solutions when the dicarbide of smaller cell volume is considered as the solvent and the dicarbide of larger cell volume is dissolved in it the relationship is... 

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