Number of cation vacancies in lattice of NaCl = Number of divalent Sr 2 + ions added.
Concentration of cation vacancy on doping with 10-3 mol % of SrCl2. = 10-
3 mol % = 10-3 / 100 = 10-5 mol. Number of Sr 2 + ion in 10-5 mol = 10-5 x 6.023 x 10 23 = 6.023 x 10 18 Hence number of cationic vacancies = 6.023 x 10 18
Electron – rich impurities
In a periodic table Silicon and germanium belongs to group 14 with four valence electrons each. In their crystals each atom forms four covalent bonds with surrounding atom.
When doped with a group 15 element like P or As, which contains five valence electrons,
They occupy some of the lattice sites in silicon or germanium crystal.
Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron in P is extra and becomes delocalized.