Structure Of Ionic Compounds

Structure Of Ionic Compounds

Simple ionic compounds are of two types i.e. AB and AB2 type. From the knowledge of close packed structures and the voids developed there in, we can have an idea about the structures of simple ionic compounds.

Among the two ions, constituting the binary compounds, the anions usually constitute the space lattice with hcp or ccp type of arrangements whereas the cations, occupy the interstitial voids

(a) If the anions (B-) constitute the crystal lattice and all octahedral voids are occupied by cations (A+), then the formula of the ionic solid is AB.

(b) Similarly, if half of the tetrahedral voids are occupied by cations, then the formula of the solid crystal becomes A+B-.

(c) When the anions (B^-2) are constituting space lattice and all the tetrahedral voids are occupied by the cations (A+), then the formula of the solid crystal will be A2B.

Ionic compounds of the type AB

Ionic compounds of the type AB have three types of crystalline structures. (A) ZnS type (B) NaCl types (C) CsCl types

1. Sodium chloride (Rock salt) type structure

The sodium chloride structure is composed of Na+ and Cl- ions. The number of sodium ions is equal to that of Cl- ions. The radii of Na+ and Cl- ions 95 pm and 181 pm giving the radius ratio of 0.524

The radius ratio of 0.524 for NaCl suggest an octahedral void. Thus the salient features of this structure are as follows:

(iv) Since there are six octahedral holes around each chloride ions, each Cl- ion is surrounded by 6 Na+ ions. Similarly each Na+ ion is surrounded by 6 Cl- ions. Therefore, the coordination number of Cl- as well as of Na+ ions is six. This is called 6:6 coordination.

(v) It should be noted that Na+ ions to exactly fit the octahedral holes, the radius ratio should be equal to 0.414. However, the actual radius ratio exceeds this value. Therefore to accommodate large Na+ ions, the Cl- ions move apart slightly i.e. they do not touch each other and form an expanded face centred lattice.

(vi) The unit cell of sodium chloride has 4 sodium and 4 chloride ions as calculated below

No of sodium ions = 12 (at edge centres) ×1/4 + 1 (at body centre)×1= 4

No of chloride ions = 8(at corner)×1/8+6(at face centres)×1/2 = 4

Thus, the number of NaCl units per unit cell is 4.

(vii) The edge length of the unit cell of NaCl type of crystal is 2(r+R) (r = radii of Na+ ion)

i.e.,

a = 2(r_Na+ + r_cl^-)(R = radii of Cl- ion)

Thus, the distance between Na+ and Cl- ions = a/2

Most of the halides of alkali metals, oxides and sulphides of alkaline earth metals have this type of structures. Some of the common examples are NaI, KCl, RbI, RbF, NH₄Cl, NH₄Br, AgCl, AgBr and AgI.

Ferrous oxide also has sodium chloride, types structure in which O^-2 ions are arranged in ccp and Fe⁺² ions occupy octahedral ions. However, this oxide is always non stoichiometric and has the composition Fe_0.95O. It can be explained on the assumption that some of the Fe⁺² ion are replaced by 2/3rd as many Fe⁺³ ions in the octahedral voids.

2. Zinc blende (ZnS) type structures (sphelerite)

The zinc sulphide crystals are composed of equal number of Zn⁺² and S₂− ions. The radii of the two ions (Zn⁺² = 74 pm and S^-2 = 184 pm) led to the radius  (r+/r-) as 0.40 which suggests a tetrahedral arrangement.

The salient features of this structure are as follows

(iii) Since the void is tetrahedral, each zinc ion is surrounded by four sulphide ions and each sulphide ion is surrounded tetrahedrally by four zinc ions. Thus zinc sulphide has
4:4 Co – ordination.

(iv) For exact fitting of Zn⁺² in the tetrahedral holes, formed by close packing of S-2 ions, the ratio Zn⁺²/S-2 should be 0.225. Actually this ratio is slightly large (0.40)

(v) There are four Zn+2 ions and four S-2 ions per unit cell as calculated below:

No. of S-2 ions = 8(at corners)×1/8 + 6(at face centres)×1/2 = 4

No. of Zn⁺² ions = 4(within the body)×1 = 4

Thus, the number of ZnS units per unit cell is equal to 4. Some more examples of ionic solids having Zinc blende structures are CuC, CuBr, CuI, AgI, beryllium sulphide.

No. of S−2 ions per unit cell

= 12(at corners) ×= 6

Thus, there are 6 formula units per unit cell.

3. Caesium chloride (CsCl) structure

The caesium chloride crystal is composed of equal number of caesium (Cs+) and Chloride Cl− ions. The radii of two ions (Cs+ = 169 pm and Cl− = 181 pm) led to radius ratio of r_Cs+ to r_cl- as 0.93 which suggest a body centred cubic structure having a cubic hole

r_Cs+ / r_cl- = 169/181 = 0.93

The salient features of this structure are as follows:

(iii) For exact fitting of Cs+ ions in the cubic voids the ratio r_Cs+/ r_cl- should be equal to 0.732. However, actually the ratio is slightly larger (0.93). Therefore packing of Cl- ions slightly open up to accommodate Cs+ ions.

(iv) The unit cell of caesium chloride has one Cs+ ion and one Cl- ion as calculated below

No. of Cl- ion 8(at corners) ×1/8 = 1

No. of Cs+ ion = 1(at body centre)×1=1

Thus, number of CsCl units per unit cell is 1

(v) Relation between radius of cation and anion and edge length of the cube,

Other common examples of this type of structure are CsBr, CsI, TlCl, TlBr

Higher coordination number in CsCl(8:8) suggest that the caesium chloride lattice is more stable than the sodium chloride lattice in which Co – ordination number is 6:6. Actually the caesium chloride lattice is found to be 1% more stable than the sodium chloride lattice. Then the question arises why NaCl and other similar compounds do not have CsCl type
lattice – This is due to their smaller radius ratio. Any attempt to pack 8 anions around the relatively small cation (Li+, Na+, K+, Rb+) will produce a state in which negative ions will touch each other, sooner they approach a positive ion. This causes unstability to the lattice.

Effect of Temperature on crystal structure

Increase of temperature decreases the coordination of number, e.g. upon heating to
760 K, the CsCl type crystal structure having coordination 8:8 changed to NaCl type crystal structures having coordination 6:6.

Effect of Pressure on crystal structure

Increase of pressure increases the Coordination number during crystallization e.g. by applying pressure, the NaCl type crystal structure having 6:6 coordination number changes to CsCl type crystal having coordination number 8:8