Solid State #1 by Sanjay Sharma
(a) Amorphous solids (b) Crystalline solids
4. Solid angle
5. Interfacial angle
6. Zone and Zone-axis
(i) AB type structures
The AB2 or A2B type of ionic crystals contain the ions in the ratio 1:2 or 2 : 1 respectively. For Example : CaF2 populary called to have fluorite structure with other examples like SrF2, BaF2, PbF2and BaCl2 etc. The coordination number of Ca++ in CaF2 is 8 while that of F- is 4. On the contrary Na2O have antifluorite structure i.e., here the place of cations in occupied by anions and vice versa.
The size of the unit cell and arrangement of atoms in a crystal is determined with the help of measurement of differaction of x-rays by the crystal. When a beam of monochromatic x-ray strikes two planes of atoms in a crystal at a certain angle Q, It is reflected. The intensity of reflected beam will be maximum if -
In actual practice it is difficult to grow a perfect crystal. Even single crystals grown with-all care are found to contain
In an ionic crystal, the electrons are mostly concentrated around the electronegative component. Some of these electrons have the tendency of thermal release i.e., the property of loosing its position on increase in temperature. These thermally released electrous become mobile resulting to increase in conductivity of solid
When an electron is thermally removed from its position the electron deficient site thus formed is called a HOLE. Holes also impart electrical conductivity but their movement is opposite in direction to which the electrons move. The electrons and holes in solids give rise to electronic imperfections.
The defect discussed above is/are called point defects and can be categorised to following 3 types -
(A) Stoichiometric defects
(B) Non - stoichiometric defects
(C) Impurity defects.
If imperfections in crystals are such that the ratio between the cations and the anions remains the same as described in its molecular formula, the defect will be called STOICHIOMETRIC DEFECT. These can be further categorised to-
(a) Schottky defect (b) Frenkel defect.
In an ionic crystal of A+B- type, if equal no. of cations and anions are missing from their lattice sites, the defect is called SCHOTTKY DEFECT. In this defect electrical neutrality is maintained due to disappearence of similar no. of cations and anions (Lattice Vacancy).
The schottky defect is shown by highly ionic compounds having -
(i) High co-ordination number
(ii) Small difference in the size of cations and anions
For Example : NaCl, KCl, KBr, CsCl etc.
This type of defect is seen in those crystals where the difference in the size of cations and anions is very large and their coordination no. is low for example AgCl, AgBr, ZnS etc. Due to such a defect the density of the solid remains unchanged.
When the ratio of cations and anions due to imperfection differ from that indicated by their molecular formula, the defects are called Non-stoichiometric defects. These defects results in either excess of metal atom of excess of non metal atom.
The metal excess may occur in either of the following two ways -
(i) Due to missing of a negative ion from its lattice site,
thus leaving a hole which is occupied by an electron.
The electrons thus trapped in the anion vacancies
are called F.Centres (F=Farbe=German word for color) as these are responsible for imparting color to the crystals.
This defect is similar to frenkel defect
e.g. FeO, FeS, NiO etc.
Another common method of introducing defects in ionic solids is by adding impurity ions having different change than host ion. These foreign atoms are present at lattice site in substitutional solids and at vacant interstitial sites in interstitial solids.
f + C = e + 2
where, f = Number of faces
e = Number of edges
c = Number of interfacial angles.
2. Coordination Number
3. Crystallographic axes
4. Standard or unit plane
5. Axial ratio.