Structures of Simple Ionic Compounds

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Structures of Simple Ionic Compounds

Post by Algerien1970 on Sun 17 May - 14:50



Metals have structures which may be discussed in terms of the close packing of spheres, and as a result have the high coordination numbers of the close packed systems. Ionic solids, however, have lower coordination numbers, and the discussion of the structure simply in terms of close packed species has to be adapted.

The idea of an ionic solid, though, depends on being able to define an ion. The Ionic Model treats a solid as being made up of oppositely charged spheres that interact by the coulombic forces between them, and the short range repulsive forces which occur between closed shell species at small separations.

The use of the idea of close packed spheres, and the holes within these structures is very useful in the discussion of the structures of ionic solids.

The structures of many ionic solids of the formula AB and AB2 may be visualized in terms of the close packed arrangement of the negatively charged anions, with the positively charged cations occupying the holes within the structure.

Characteristic structures of ionic solids

The simple structures come from the arrangement of the anions (though sometimes the cations) in the positions of the spheres in the fcc or hcp lattices, and the cations go into some or all of the octahedral and tetrahedral holes within the lattices.


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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Sun 17 May - 14:56

1/ Structures based on face centered cubic lattices


1.1. The Rock Salt, or NaCl, structure


The sodium cations (green) occupy the octahedral holes in the fcc lattice of chloride anions (red).


The Rock Salt structure is based on the fcc array of the large chloride anions, and the sodium cations occupy all the octahedral holes in the fcc lattice.


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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:20

1.2.The Sphalerite, or Zinc Blende, structure


The zinc cations (green) occupy half of the tetrahedral holes in the fcc lattice of sulphide anions (red).


Here there is an fcc array of sulphide anions, and the zinc cations occupy half the tetrahedral holes.


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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:22

1.3. The Fluorite, CaF2, structure



The fluoride anions (green) occupy all of the tetrahedral holes in the fcc lattice of calcium cations (red).


There is now an fcc array of calcium cations, and the fluoride anions occupy all of the tetrahedral holes.


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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:26

2/ Structures based on other cubic lattices


2.1. The Caesium Chloride Structure



The simple cubic array of chloride anions (red) has a cesium cation (green) at the center of the cube. There is an interleaved arrangement of simple cubic anionic and cationic arrays.


The Caesium Chloride structure is formed when the anion and cation have similar sizes.


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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:27

2.2. The Perovskite Structure


The simple cubic array of titanium cations (red) has a calcium cation (blue) at the center of the cube, and oxide anions (green) at the center point of each of the edges of the cube of Ti ions.
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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:29

3/ Structures based on hexagonal close packed lattices

3.1. The Wurtzite, ZnS, Structure


The zinc cations (red) occupy half of the tetrahedral holes in the hcp lattice of sulphide anions (green).
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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:30

3.2. The Nickel-Arsenide, NiAs, Structure




The hcp array of As anions (red) has Ni cations (green) in the octahedral holes.
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Re: Structures of Simple Ionic Compounds

Post by Algerien1970 on Mon 18 May - 13:32


3.3. The Rutile Structure



The Ti cation (red) is octahedrally surrounded by O anions (green): the O anion has a planar triangle of Ti cations around it.
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