solid state

• A coordination number can be defined as the number of neighbouring atoms or ions that closely surrounding a central atom or ion in a complex or crystal
• the coordination number of atoms in a cubic close pack structure is 12
• the coordination number of atoms in a body-centered cubic pack structure is 8
• regular arrangement of constituent particles systematically in 3-d space is called as crystal lattice.
• the smallest 3d portion of a crystal lattice is called as unit cell
• the solids having moderate conductivity between insulators and conductors are called as semi conductors.

  these have the conductivity range from 10^-6 to 10^4 ohm^-1 m^-1

  by doping process the conductivity of semiconductors increases.

  example: si, ge crystal
• the point/stoichiometric defect where equal no. of anions and cations leave the crystal lattice to maintain electrical neutrality is called as schottky's defect

  it decreases the density of the crystal

  generally observed in those ionic compounds where the difference between cation and anion is not matched.

  example: nacl, kcl, agbr
• frenkel's defect occurs only in ionic crystals with high melting points
• • frankel defect is a point or stoichiometric defect observed in ionic compounds where cation and anion size is more
• • frankel defect is a point/stoichiometric defect where the cation leaves its lattice point and occupies interstitial space.
  • it dosent effect the the density of the crystal.
  • example : AgBr, AgCl,ZnS
• • f centres are the anionic site occupied by unpaired electrons
• in f-centre the colour of the compound changes due to absorption of light energy
• f centre is formed when electron gets trapped at anion vacancy.
• colour change in f-centre depends on the nature of the impurity atom present in it.
• F-centre is also known as electron hole centre
• Schottky defect is a point/stoichiometric defect which occurs in ionic crystals where both cations and anions leave their respective sites simultaneously.
• In Schottky defect, there will be equal number of positive and negative charges left behind so no net charge remains.
• Frenkel defect is a type of point defect that arises in ionic compounds when some of the cation or anion leaves its regular place and occupies the interstitial space between two adjacent ions.
• f centres impart colour to crystals which occurs due to excitation of electrons when they absorb energy from the visible light.
• f centres are formed by heating alkyl hallide with excess of alkali metal
• NaCl crystals heated in the presence of Na- vapour yellow colour is produced due to f centres
• n-type conductors are obtained by adding a trace of the 5th group (P,As,Ge) element to pure silicon or germanium by doping.
• In n-type conductors when P,As or Bi is added to Si or Ge some of the Si or Ge in the crystal are replaced by P or As atoms and 4 out of 5 electrons of P or As atom will be used for bonding with Si or Ge atoms while the fifth electron serve to conduct electricity,
• p-type semiconductors are prepared by adding small amount of pentavalent impurities like antimony(Sb), bismuth(Bi) etc., to the tetravalent elements like Germanium(Ge) or Silicon(Si).
• n type conductors have electron rich impurities which belong to higher group
• type conductors have electron deficient impurities which belong to lower group
• p type conductors are obtained by doping with impurity atopms containing less electrons i.e. 3rd group elements