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Three Dimensional Packing of Constituent Particles
(i) ABAB arrangement gives hexagonal close packing (hcp)
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In both these arrangements 740/0 space is occupied
Coordination number in hop and ccp arrangement is 12 while in bcc arrangement, it is 8
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All noble gases have ccp structure except He (hcp structure)
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When particles are closed packed resulting in either cpp or hcp
structure, two types of voids are generated:
Tetrahedral voids are holes or voids surrounded by four spheres Present at the corner of
a tetrahedron
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Octahedral voids are holes surrounded by six spheres located on a regular tetrahedron
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[The number of octahedral voids present in a lattice is equal to the number of close packed
particles
...
]
Density of Unit Cell (d)
Density of unit ce11 = mass of unit cell / volume of unit cell
d = Z * M / a3 = ZM / a3 * NA
(The density of the unit cell is same as the density of the substance
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of atoms per unit cell
NA = Avogadro number
a = edge length of unit cell
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Radius Ratio and Crystal Structure
Ionic crystals may be of two types
(i)AB type and
(ii) A2B or AB2
Structure of Ionic Crystals
9|Page
On applying pressure NaC} structure (6 : 6 coordination) changes into CsCI structure (8 : 8
coordination) and reverse of this occur at high temperature (760 K)
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derivations from perfect arrangement
may occur due to rapid cooling or presence of additional particles
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Point Defects
Point defects are the irregularities or deviations from ideal arrangement around a point or an
atom in a crystalline substance Point defects can be classified into three types : (1)
psychometric defects (2) impurity defects (3) non–stoichiometric defects
1
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They are also called
intrinsic or thermodynamic defects
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Frenkel defects are not found in pure alkali metal
halides because cations are of large size
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Impurity Defect
It arises when foreign atoms or ions aloe present in the lattice
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When the impurity has the same charge
as the host ion
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Impurity defects can also be introduced by adding impurity ions having different charge
than host ions
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g
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In
such cases,
Cationic vacancies produced = [number of cations of higher valence * Difference in
valence of the host cation and cation of higher valence
3
...
The
numbers of positive and negative ions present in such compounds are different from those
expected from their ideal chemical formulae
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Types of n-stoichiometric defects are as follows:
(i) Met excess defect Metal excess defect due to anionic vac