It is well known that ionic solids consist of positive and negative ions arranged in a manner so as to acquire minimum potential energy. This can be achieved by decreasing the cation anion distance to a minimum and reducing anion-anion repulsions. The structures which these solids adopt can be described in terms of the large anions/cations occupying one or the other type of interstitial sites.
A large number of ionic solids exhibit one of these five structures which are discussed here: 1. The Sodium Chloride Structure 2. The Zinc Blende Structure 3. The Wurtzite Structure 4. The Fluorite (CaF2) Structure 5. The Cesium Chloride Structure.
1. The Sodium Chloride Structure:
A unit cell representation of sodium chloride is shown in Fig. 2.44.
The salient features of its structure are:
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(i) Chloride ions are in FCC type of arrangement, i.e., it contains chloride ions at the corners and at the centre of each face of the cube.
(ii) Sodium ions are so located that there are six chloride ions around it. This is equivalent to saying that sodium ions occupy all the octahedral sites.
(iii) As there is only one octahedral site for every chloride ion, the stoichiometry is 1: 1.
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(iv) For sodium ions to occupy octahedral holes and the arrangement of chloride ions to be close-packed, the radius ratio, rNa+/rCl– should be equal to 0.414. The actual radius ratio 0.525 exceeds this limit. To accommodate large sodium ions, the arrangement of chloride ions has to slightly open up.
(v) It is obvious from the diagram that each chloride ion is surrounded by six sodium ions which are disposed towards the corners of a regular octahedron. We may say that cations and anions are present in equivalent positions and the structure has 6: 6 coordination.
(vi) The structure of sodium chloride consists of eight ions in a unit cell, four are Na+ ions and the other four are Cl– ions.
In this structure each corner ion is shared between eight unit cells, each ion on a face of the cell by two cells, each ion on an edge by four cells and the ion inside the cell belongs entirely to that unit cell.
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Thus,
12 edge Na+ ions = 12/4 = 3 Na+ per unit cell
1 centre Na+ ion = 1/1 = 1 Na+ per unit cell
8 corner Cl– ions = 8/8 = 1 CI– per unit cell
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6 face Cl– ions = 6/2 = 3 Cl– per unit cell
Most of the alkali halides, alkaline earth oxides, and sulphides exhibit this type of structure. Other compounds which crystallize in sodium chloride type of structure are HN4Cl, NH4Br, NH4I, AgF, AgCl, AgBr, MgO, CaO, BaS and LiF.
2. The Zinc Blende Structure:
A unit cell representation of zinc blende is shown in Fig. 2.45.
The salient features of its structure are:
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(i) The sulphur atoms are in FCC type of arrangement, i.e., sulphur atoms are at the corners and at the centre of each face of the cube.
(ii) Zinc atoms are so located that there are four sulphur atoms around it. Or it may be said that zinc atoms occupy tetrahedral sites and their coordination number is four.
(iii) As there are two tetrahedral sites available for every sulphur atom, zinc atoms occupy only half of the tetrahedral sites. So the stoichiometry of the compound is 1:1. Only alternate tetrahedral sites are filled by zinc atoms.
(iv) Each zinc atom is surrounded by four sulphur atoms and in turn each sulphur atom is also surrounded by four zinc atoms which are also disposed towards the corners of a regular tetrahedron. We may say that cations and anions are present in equivalent positions and the coordination of zinc blende structure is 4: 4.
(v) For the arrangement of sulphur atoms to be truly close-packed and zinc atoms to occupy tetrahedral voids, the radius ratio (rZn2+/rS2-) should be 0.225. This value is greater than 0.225 and so we may say that the arrangement of sulphur atoms is not actually close-packed.
This structure is found in 1: 1 compounds in which the cation is smaller than the anion. Examples include copper halides (CuCl, CuBr, Cul), silver iodide and beryllium sulphide.
3. The Wurtzite Structure:
It is an alternative form in which ZnS occurs in nature. Its unit cell representation is shown in Fig. 2.46.
The salient features of this structure are:
(i) Sulphur atoms form the HCP type of arrangement.
(ii) Zinc atoms are so located that there are four sulphur atoms around each zinc atom. It may be said that zinc atoms occupy tetrahedral sites.
(iii) As there are two tetrahedral sites available for every sulphur atom, zinc atoms occupy only half of the tetrahedral sites and the alternate tetrahedral sites remain vacant. The stoichiometry of the compound is 1: 1.
(iv) Each zinc atom is surrounded by four sulphur atoms and in turn each sulphur atom is also surrounded by four zinc atoms (which are also disposed towards the corners of a regular tetrahedron). The coordination of the compound is 4: 4. Again we may say that cations and anions are in equivalent positions.
(v) It may be concluded that the structure of Wurtzite is very similar to the structure of zinc blende. The only difference is in the sequence of the arrangement of close-packed layers of sulphur atoms. In zinc blende, sulphur atoms follow the sequence ABCABC… etc. whereas in Wurtzite the sequence is ABABA … etc.
Other examples of this type of structure are ZnO, CdS, CdSe and BeO.
4. The Fluorite (CaF2) Structure:
A unit cell representation of fluorite structure is shown in Fig. 2.47.
Its structure can be described as:
(i) The calcium ions from the FCC arrangement, i.e., these ions occupy all the corner positions and the centre of each face of the cube.
(ii) Fluoride ions are so located that there are four calcium ions around it. It may be said that the fluoride ions occupy tetrahedral sites and the coordination number of fluoride ion is 4.
(iii) As there are two tetrahedral sites available for every calcium ion, the fluoride ions occupy all the tetrahedral sites; the stoichiometry of the compound is 1: 2.
(iv) Each fluoride ion is surrounded by four calcium ions whereas each calcium ion is surrounded by eight fluoride ions which are disposed towards the corners of a cube. The coordination of the compound is 8: 4.
Other examples of compound which show this type of structure are SrF2, BaF2, BaCl2, SrCl2, CdF2, HgF2 and PbF2.
5. The Cesium Chloride Structure:
The structure of CsCl is shown in Fig. 2.48 (a) and its unit cell representation is shown in Fig. 2.48 (b).
The structure can be described as:
(i) The chloride ions form the simple cubic arrangement.
(ii) Cesium ions occupy the cubic interstitial sites, i.e., each cesium ion has eight chloride ions as its nearest neighbours.
(iii) If we consider Fig. 2.48 (a) it can be observed that each chloride ion is also surrounded by eight cesium ions which are also disposed towards the corners of a cube.
(iv) It may be concluded that both types of ions are in equivalent positions, and the stoichiometry is 1: 1. The coordination is 8: 8.
Other examples of this type of structure are CsBr and CsI. This structure is observed only when the cations are comparable in size to the anions.