Crystal-chemical isotypic

The crystal structures of two compounds are isotypic if their atoms are distributed in a like manner and if they have the same symmetry. One of them can be generated from the other if atoms of an element are substituted by atoms of another element without changing their positions in the crystal structure. The absolute values of the lattice dimensions and the interatomic distances may differ, and small variations are permitted for the atomic coordinates. The angles between the crystallographic axes and the relative lattice dimensions (axes ratios) must be similar. Two isotypic structures exhibit a one-to-one relation for all atomic positions and have coincident geometric conditions. If, in addition, the chemical bonding conditions are also similar, then the structures also are crystal-chemical isotypic. 

As a conclusion to these comments, we may mention that two structures are defined as crystal-chemically isotypic if they are isoconfigurational and the corresponding atoms (and bonds) have similar chemical/physical characteristics. 

In a report presented by Uma-de-Faria, Hellner, Liebau, Makovicky, Parth6 (1990) (henceforth denoted 1990 Report) a detailed analysis of this problem is made, and the following hierarchy of terms is proposed, based on the degree of stmctural similarity Isopointal, Isoconfigurational, crystal chemically Isotypic, and homeotypic structures ... 

ABSTRACT. Based on the nomenclature report of the lUCr (1990) the concepts of isopointal, isoconfigurational, Isotypic and homeotypic structures are described and illustrated by several examples. The notation proposed In this report for the crystal - chemical formulae is also developed, including the symbologies for close packings which are not closest, and for the condensation process of the structural units. 

Some mineral structures allow many chemical substitutions, or cation/anion exchanges, such as Mg replaced by Fe, Mn, Fe, or Af and replaced by OH", F , or Cl". Where the valency is the same (homovalent or isovalent exchange, as opposed to heterovalent), this is known as isomorphic substitution isomorphism for the phenomenon, but this term is also used in a different way concerning crystal faces isotypism or isostructuralism are sometimes used and the last is, by far, the most appropriate). 

With the limited space available it is impossible to consider all these crystal-chemical, view points for all structures in detail. The different crystal structures are presented in the order of their composition code. For compounds where the T and M atoms occupy certain structure sites at random the code represents one composition for one compound on which a structure determination has been made. Other isotypic phases may exist with different numerical values within the parentheses of the composition code. 

GdNi and NiB represent another couple of isotypic compounds. The role (the position in the crystal structure), however, of the same atom, Ni, in the two compounds is exchanged. In NiB, the Ni atoms are those centring the trigonal prism formed by Gd atoms. A reason for the existence of this structure type could possibly be related to the atomic size difference of the elements involved or, perhaps, to their relative position in the Pettifor chemical scale. 

Many compounds of technetium and rhenium are of analogous composition and of corresponding physical and chemical properties. Because of the very similar ionic radii, isotypic crystal structure formation of analogous compounds could often be observed. Technetium remarkably differs from manganese by the high stability of pertechnetate compared with permanganate. Moreover, divalent technetium does not exist as a hydrated ion but only as a stabilized complex. 

Q -SeBr4 crystallizes in the trigonal system and contains cubane-like [SeBr4]4 units. Monoclinic /3-SeBr4 also consists of tetrameric cubane-Uke [SeBr4]4 molecules. /3-SeBr4 is isotypic withTeCU and with jS-SeCU its chemical behavior and reactions are similar to SeCU-... 

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 . 

Minor chemical variations can lead to entirely different physical properties. The corresponding isotypic Tb compounds TbBr(H,D), do not exhiUt electron localization and a transition to the semiconducting state at small values of x and low temperature. Obviously, crystal field effects which are absent with the S = 0 ion Gd play an important role with the Tb " ion. The investigation on the magnetism of TbBr(H,D)jt as a function of x has provided clear hints as to the importance of these effects. 

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