UNIT 1.3 THE SOLID STATE: DEFECT, ELECTRONIC AND MAGNETIC PROPERTIES

LEC 3   

1.Imperfection in lattice

An ionic crystal is perfect when it contains same unit cell containing same lattice point thoughout the crystal .but it is possible at absolute zero temperature [i.e 0 k]. above this deviation in ideal crystal lattice is seen called as defect or imperfection.

 Point defect :-  the irregularities or deviation from ideal arrangement around a point or atom.

 Line defect :-  the deviations from ideal arrangement around entire rows of lattice.

Defect is of 3 types:- 

A} Stoichiometric defect   _  in this type of defect equal no. of anion and cation are missing from there lattice sites. so, in defective crystal also the molecular formulae remain same and electrical neutrality remains maintained. Example of this type of defect are vacancy defect , interstitial defect ,schottky defect ,frenkel defect.........

1.1 Defect in non ionic crystal

vacancy defect - in this type of defect atoms[lattice point] are missing from their lattice site. this is very common and simplest type of stoichometric defect found in non ionic crystal.

interstitial defect- in this type of defect atoms or molecule comes into interstital space of the lattice but reamins in the crystal lattice.

1.2 Defect in ionic crystal

schottky defect-  in this type of defect equal no. of anian and cation are missing from their lattice site creating holes at the site of vacancy. this is very common and simple type of stoichiometric defect found in ionic crystal. These defect are found in compounds having high coordination number and anion and cation are of equal size. 
example are- NaCl,CsCl.

1.  Electrical neutrality is maintained becoause of the fact that equal no. of anion and cation are missing.

2.  Density decrease because holes are created.

3.  Stability of the crystal is decreased because holes are created and deviated from ideal structure.

4.  Electrical conductivity is increased because of the presence of holes . when electric field is applied than ion from nearby lattice site fills the hole and hole is created at that site .In this way holes moves from one end to other and electrical conductivities increases.

frenkel defect- in this type of defect ion is missing from its normal lattice site and occupy a interstitial  position.this defect is seen in compounds with low coordination number and anion is larger in size than cation. 
example are-all silver halide.
1.Electrical neutrality is maintained because anion and cation are  not missing from lattice.

2.Density remain the same .

3.Stability decreases.[same reason]
4.Electrical conductivity increases[same reason].

B}Non stoichiometric defect - in this type of defect the no. of anion and cation missing are not equal so chemical formulae is changed.

 This defect is of two types:-

1. Metalal excess defects -This occur due to following reason:-

1.1 Due to anion vacancy-  This type of defect is mostly seen in compounds having schttky defect[silver halides]. It involves removal of anion from lattice site leaving electron at that site so that electrical neutrality is maintained called as F- Centre or ferbenzenter centre.  In this way electrical neutrality is maintained.

When alkali metal halide are heated in vapour of alkali metal the metal atoms get deposited on the surface of alkali metal halide crystal and halide ion  get diffused into the surface and combine with metal atoms. the electron thus produced due to ionization of metal atom comes at the site from where halide ion is removed called as F-Centre giving coulered crystal. 

LiCl [pink], KCl[violet], NaCl[yellow].

1.2 Due to extra cation in interstitial position- In this cation occupy extra interstial site and to maintain electrical neutrality electron is present in other interstitial site.
Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow.
Now there is excess of zinc in the crystal and its formula becomes Zn1+xO. The excess Zn2+ ions move to interstitial sites and the electrons to neighbouring interstitial sites. In this way electrical neutrality is maintained.

2 Metal deficiency defect- this defect occur due to following two reasons:-

2.1 Due to cation vacancy- In this cation is missing from their lattice point and another cation occupy extra postive charge to balance the cation loss .  In this way electrical neutrality is maintained.this is found in those cation in which variable oxidation state is found like ferrous oxide,ferrous sulphite,nickel oxide etc....

2.1 due to extra anion occupying interstitial site-  In this type of defect extra anion is present in interstitial space and charge is balanced by oxidation ofequal no. of  adjacent cation to higher oxidation state. this defect is very common because the size of anion is large enough to fit in interstitial site. 

C}Impurity defect

If we add impurity in crystal lattice than its electrical property changes called impurity defect.these defect is easily seen in compound of 14 group mostly silicon and germanium element in which every atom is bonded with with four other atom to complete its octate. If in this compound small amount of group 13 or15 is added [process is known as doping] than its electrical conductiviy increase and compound is known as P type and N type semiconductor respectively.

P Type semiconductor- If doping is done with 13 group element[electron deficit impurity] than this type of semiconductor is formed . In this the 14 group element form 3 covalant bond with 13 group element [as its valancy is3] and one hole is created at the place of 4th covalant bond . when electric field is applied hole is filled by adjacent electron  so hole and electron moves in opposite direction .since current is carried by positive hole it is called as P- Type semiconductor.

N Type semiconductor- If doping is done by 15 group element [electron rich impurity] than this type of semiconductor is formed. In this the 14 group element form 4 covalant bond with 15 group element  and one electron is left free which is responsible for its extra conductivity.since current is carried by electron negative  it is called as N- Type semiconductor.

2. Electrical property

Solids exhibit an amazing range of electrical conductivities, extending over 27 orders of magnitude ranging from 10–20 to 107 ohm–1 m–1.Solids can be classified into three types on the basis of their
conductivities.
(i) Conductors: The solids with conductivities ranging between 104 to 107 ohm–1m–1 are called conductors. Metals have conductivities in the order of 107 ohm–1m–1 are good conductors.

(ii) Insulators : These are the solids with very low conductivities ranging between 10–20 to 10–10 ohm–1m–1.
(iii) Semiconductors : These are the solids with conductivities in the intermediate range from 10–6 to 104 ohm–1m–1.



3.Magnetic properties

Every substance has some magnetic properties associated with it.The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet. Its magnetic moment originates from two types of motions (i) its orbital motion around the nucleus and (ii) its spin around its own axis .Electron being a charged particle and undergoing these motion can be considered as a small loop of current which possesses a magnetic. Thus, each electron has a permanent spin and an orbital magnetic moment associated with it. Magnitude of this magnetic moment is very small and is measured in the unit called bohr magneton.

(i) Paramagnetism: Paramagnetic substances are weakly attracted by a magnetic field. They are magnetised in a magnetic field in the same direction. They lose their magnetism in the absence of magnetic field. Paramagnetism is due to presence of one or more unpaired electrons which are attracted by the magnetic field.

(ii) Diamagnetism: Diamagnetic substances are weakly repelled by a magnetic field. They are weakly magnetised in a magnetic field inopposite direction. Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons. Pairing of electrons cancels their magnetic moments and they lose their magnetic character.

(iii) Ferromagnetism: A few substances like iron, cobalt, nickel,gadolinium are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances.Besides strong attractions, these substances can be permanently magnetised. In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented and their magnetic moments get cancelled.When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced. This ordering of domains persist even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.

(iv) Antiferromagnetism: Substances like MnO showing antiferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other's magnetic moment.

(v) Ferrimagnetism: Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers . They are weakly attracted by magnetic field as compared to ferromagnetic substances.These substances also lose ferrimagnetism on heating and become paramagnetic.