Defect tetrahedral structure

Structure types have been established. Similar to Al, the M2X3 crystals (M = Ga, In, Tl X = S, Se, Te) are mostly based on M-defect tetrahedral structures, namely W (Ga2S3, In2Se3) and ZB (Ga2Se3, Ga2Te3, In2Te3). At atmospheric pressure, 283 can be present in three modifications. The low-temperature a-form is a cubic close-packed structure of S atoms, where 70% of the In atoms are randomly distributed on octahedral sites and the rest remain on tetrahedral sites. The P-form is related to the spinel structure, and the y-modification is hexagonal. 

In a defect tetrahedral structure not all the four vertices are occupied by an atom. A filled tetrahedral structure contains extra-atoms inserted in a normal tetrahedral structure. The bonding mechanism, however, may be different from that of the other tetrahedral structures and, generally, less simple relations are observed between the number of valence electrons and structure. 

The aforementioned rule may be extended to include the defect tetrahedral structures where some atoms have less than four neighbours ... 

By adding the symbol 0 (zero) to the described notation, vacant tetrahedral sites can be represented. Examples of formulae of defect tetrahedral structures are ... 

VEC > 4 a compound with a defect tetrahedral structure may be obtained with... 

TABLE 9. C omparisonof AverageHeatsof Atomization(Hs, in kcal/g-atom), Average Heats of Formation (Estimated from Electronegativity Differences, Ax 1% = 23(Ax), in kcal/g-atom) of Defect Tetrahedral Structures (Related to Wurtzite or Zinc Blende Type) and Phenacite-Type... 

Calculation of VEC allows one to find out If a compound might have a tetrahedral structure and what kind it should be. One distinguishes between normal tetrahedral structures, where each atom in the structure heis four tetrahedral neighbours, and defect tetrahedral structures where some atoms have less than four neighbour atoms. 

Examples for normal valence compounds (VEC/ = 8) with normal and detect tetrahedral structures. There are known many nonnal valence compounds with normal eind defect tetrahedral structure. A subgroup of this family is formed by the adamantane... 

Examples for polyanionic valence compounds (VEC/ < 8) with normal and defect tetrahedral structures. We shall present red ZnP2 as an example for a normal and CU2P7 as an example for a defect tetrahedral structure. 

Examples for polycatlonic valence compounds (VEC/ > 8) with defect tetrahedral structures. Except for the high pressure form of B2O with diamond-like stmcture (Endo, Sato Shimada, 1987), no polycatlonic valence compounds with normal tetrahedral structure are known. As examples for compounds with defect tetrahedral structure we discuss here GaSe and "InsS/. The latter compound served as a test case for the validity of these valence electron rules. 

IngS/ The original crystol structure determination (Wadsten, Amberg Berg, 1980) revealed a defect tetrahedral structure which, based on the reported atom coordinations and atom contacts, can be described by the following crystal chemical formula ... 

The mixed oxide mineral spinel MgAl204 gives its name to an important structure type. One-half of the octahedral holes and one-eighth of the tetrahedral holes are filled in a cubic close-packed array of oxide ions. In the normal spinel form adopted by MgAl204 the divalent Mg2+ ion is in tetrahedral sites the trivalent Al3+ is octahedral. In the defect spinel structure of g-Al203 a fraction of the cation sites are occupied at random. 

The y-Al203 crystal structure is best described as a defect spinel structure an FCC array of 0 ions, with Al + ions in 21 of the 16 octahedral and eight tetrahedral sites. By contrast, a-Al203 is an HCP array of with Al + in 2/3 of the octahedral sites. 

In a-Al203 the oxide ions form a hexagonal close-packed array, and the aluminum ions occupy octahedral holes. y-Al203 is a defect spinel structure in which there are not enough cations to occupy the usual fraction of tetrahedral and octahedral holes. (See Section 7.6, pp. 188-189 for more on spinel structures.) The AI2O3 formed on the surface of aluminum metal is a third polymorph based on the rock salt (NaCl) structure, with every third aluminum ion missing in the cubic close-packed array of oxides. (See Chapter 7 for more details on these crystal structures.)... 

---