Solid State : Magnetic and Electrical Properties of Solids

 

Properties of Solids

Some of the properties of solids which are useful in electronic and magnetic devices such as, transistor, computers, and telephones etc., are summarised below :

 

(1)     Electrical properties : Solids are classified into following classes depending on the extent of conducting nature.

(i)      Conductors : The solids which allow the electric current to pass through them are called conductors. These are further of two types; Metallic conductors and electrolytic conductors. In the metallic conductors the current is carries by the mobile electrons without any chemical change occurring in the matter. In the electrolytic conductor like NaCl, KCl, etc., the current is carried only in molten state or in aqueous solution. This is because of the movement of free ions. The electrical conductivity of these solids is high in the range 104 – 106 ohm–1 cm–1. Their conductance decrease with increase in temperature.

(ii)     Insulators : The solids which do not allow the current to pass through them are called insulators. e.g., rubber, wood and plastic etc. the electrical conductivity of these solids is very low i.e., 10–12 – 10–22 ohm–1 cm–1.

(iii)    Semiconductors : The solids whose electrical conductivity lies between those of conductors and insulators are called semiconductors. The conductivity of these solid is due to the presence of impurities. e.g. Silicon and Germanium. Their conductance increase with increase in temperature. The electrical conductivity of these solids is increased by adding impurity. This is called

Doping. When silicon is doped  with P (or As, group 15 elements), we get n-type semiconductor. This is because P has five valence electrons. It forms 4 covalent bonds with silicon and the fifth electron remains free and is loosely bound. This give rise to n-type semiconductor because current is carried by electrons when silicon is doped with Ga (or in In/Al, group 13 elements) we get p-type semiconductors.       

  • Conductivity of the solids may be due to the movement of electrons, holes or ions.
  • Due to presence of vacancies and other defects, solids show slight conductivity which increases with temperature.
  • Metals show electronic conductivity.
  • The conductivity of semiconductors and insulators is mainly governed by impurities and defects.
  • Metal oxides and sulphides have metallic to insulator behavior at different temperatures.

 

Conductivity
Insulator like Insulator– to –metal Metal like
FeO, Fe2O3 Ti2O3 TiO
MnO, MnO2 V2O3 VO
Cr2O3 VO2 CrO2
CoO, NiO, CuO, V2O5 ReO3

 

(2)     Superconductivity : When any material loses its resistance for electric current, then it is called superconductor, Kammerlingh Onnes (1913) observed this phenomenon at 4K in mercury. The materials offering no resistance to the flow of current at very low temperature (2-5 K) are called superconducting materials and phenomenon is called superconductivity.  e.g., Nb3 Ge alloy (Before 1986), La1.25Ba0.15CuO4 (1986),  (1987) – super conductive at a temperature up to 92 K.

Applications

(a) Electronics,                                  (b) Building supermagnets,

(c) Aviation transportation,          (d) Power transmission

“The temperature at which a material enters the superconducting state is called the superconducting transition temperature, (Tc)”. Superconductivity was also observed in lead (Pb) at 7.2 K and in tin (Sn) at 3.7K. The phenomenon of superconductivity has also been observed in other materials such as polymers and organic crystals.  Examples are

  • (SN)x, polythiazyl, the subscript x indicates a large number of variable size.
  • (TMTSF)2 PF6, where TMTSF is tetra methyl tetra selena fulvalene.

 

(3)     Magnetic properties : Based on the behavior of substances when placed in the magnetic field, there are classified into five classes.

 

Magnetic properties of solids

Properties Description Alignment of Magnetic Dipoles Examples Applications
Diamagnetic Feebly repelled by the magnetic fields. Non-metallic elements (excepts O2, S) inert gases and species with paired electrons are diamagnetic All paired electrons

 TiO2, V2O5, NaCl, C6H6 (benzene) Insulator
Paramagnetic Attracted by the magnetic field due to the presence of permanent magnetic dipoles (unpaired electrons). In magnetic field, these tend to orient themselves parallel to the direction of the field and thus, produce magnetism in the substances.  

At least one unpaired electron

 O2, Cu2+, Fe3+, TiO, Ti2O3, VO, VO2, CuO Electronic appliances
Ferromagnetic Permanent magnetism even in the absence of magnetic field, Above a temperature called Curie temperature, there is no ferromagnetism. Dipoles are aligned in the same direction

 Fe, Ni, Co, CrO2 CrO2 is used in audio and video tapes
Antiferromagnetic This arises when the dipole alignment is zero due to equal and opposite alignment.   MnO, MnO2, Mn2O, FeO, Fe2O3; NiO, Cr2O3, CoO, Co3O4,  
Ferrimagnetic This arises when there is net dipole moment   Fe3O4, ferrites –

 

(4)     Dielectric properties :  When a non-conducting material is placed in an electrical field, the electrons and the nuclei in the atom or molecule of that material are pulled in the opposite directions, and negative and positive charges are separated and dipoles are generated, In an electric field :

(i)      These dipoles may align themselves in the same direction, so that there is net dipole moment in the crystal.

(ii)     These dipoles may align themselves in such a manner that the net dipole moment in the crystal is zero.

Based on these facts, dielectric properties of crystals are summarised in table  

  

Dielectric properties of solids

Property Description Alignment of electric dipoles Examples Applications
Piezoelectricity When polar crystal is subjected to a mechanical stress, electricity is produced a case of piezoelectricity. Reversely if electric field is applied mechanical stress developed. Piezoelectric crystal acts as a mechanical electrical transducer.

Piezoelectric crystals  with permanent dipoles are said to have ferroelectricity

Piezoelectric crystals with zero dipole are said to have antiferroelectricity

–

 Quartz, Rochelle salt

 

BaTiO3, KH2PO4, PbZrO3

 Record players, capacitors, transistors, computer etc. 
Pyroelectricity Small electric current is produced due to heating of some of polar crystals – a case of pyroelectricity – Infrared detectors

 

Important Tips

  • Doping : Addition of small amount of foreign impurity in the host crystal is termed as doping. It increases the electrical conductivity.
  • Ferromagnetic property decreases from iron to nickel (Fe > Co > Ni) because of decrease in the number of unpaired electrons.
  • Electrical conductivity of semiconductors and electrolytic conductors increases with increase in temperature, where as electrical conductivity of super conductors and metallic conductors decreases with increase in temperature.

 

Silicates

These are the compounds with basic unit of (SiO4)4– anion in which each Si atom is linked directly to four oxygen atoms tetrahedrally. These tetrahedral link themselves by corners and never by edges. Which are of following types :

(1)     Ortho silicates : In these discrete SiO44– tetrahedra are present and there is no sharing of oxygen atoms between adjacent tetrahedra e.g., Willamette (Zn2Si2O4), Phenacite (Be2SiO4) (Be2SiO4), Zircons (ZrSiO4) and Forestrite (Mg2SiO4).

(2)     Pyrosilictes : In these silicates the two tetrahedral units share one oxygen atom (corner) between them containing basic unit of (Si2O7)6– anion e.g., Thortveitite (Sc2Si2O7) and Hemimorphite Zn3Si2O7Zn(OH)2O

(3)     Cyclic or ring silicates : In these silicates the two tetrahedral unit share two oxygen atoms (two corners) per tetrahedron to form a closed ring containing basic unit of (SiO3)n2n– e.g., Beryl (Be3Al2Si6O18) and  Wollastonite (Ca3Si3O9).

(4)     Chain Silicates : The sharing of two oxygen atoms (two corners) per tetrahedron leads to the formation of a long chain e.g., pyroxenes and Asbestos  CaMg3O(Si4O11) and Spodumene LiAl(Si2O6).

(5)     Sheet Silicates : In these silicates sharing of three oxygen atoms (three corners) by each tetrahedron unit results in an infinite two dimensional sheet of primary unit (Si2O5)n2n–. The sheets are held together by electrostatic force of the cations that lie between them e.g., [Mg3(OH)2(Si4O10)] and Kaolin, Al2(OH)4(Si2O5).

(6)     Three dimensional or frame work silicates : In these silicates all the four oxygen atoms (four corners) of (SiO4)4– tetrahedral are shared with other tetrahedral, resulting in a three dimensional network with the general formula (SiO2)n e.g., Zeolites, Quartz.

Important Tips

Beckmann thermometer : Cannot be used to measure temperature. It is used only for the measurement of small differences in temperatures.  It can and correctly upto 0.01o

  • Anisotropic behaviour of graphite : The thermal and electrical conductivities of graphite along the two perpendicular axis in the plane containing the hexagonal rings is 100 times more than at right angle to this plane.
  • Effect of pressure on melting point of ice : At high pressure, several modifications of ice are formed. Ordinary ice is ice –I. The stable high pressure modifications of ice are designated as ice –II, ice – III, ice- V, ice – VI and ice – VII. When ice –I is compressed, its melting point decreases, reaching –22oC at a pressure of about 2240 atm. A further increase in pressure transforms ice – I into ice – IIIs whose melting point increases with pressure. Ice- VII, the extreme high-pressure modification, melts to form water at about 100°C and 20,000 atm pressure. The existence of ice-IV has not been confirmed.
  • Isotropic : The substances which show same properties in all directions.
  • Anisotropic : Magnitude of some of the physical properties such as refractive index, coefficient of thermal expansion, electrical and thermal conductivities etc. is different in different directions, with in the crystal