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CHEM313 > LC Electronic Properties > Flashcards

LC Electronic Properties Flashcards

1
Q

why cant ions usually conduct electricity

A

they only have enough energy to vibrate in the lattice not to leave it

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2
Q

what is the formula for ionic conductivity

A

σ=neμ
n= no. charge carriers
e = charge on carriers
μ = mobility

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3
Q

why is ionic coduction much easier at high T

A

defects are involved

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4
Q

what are the minimum requirements of a conductor

A

either;
some sites are vacant
some interstitial sites are occupied

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5
Q

how do solid electrolytes conduct electricity

A

they contain tunnels or layers for ions to travel through

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6
Q

give an example of a solid electrolyte and why it has such properties

A

α-Li2SO4

SO4 2- anions rotate to aid cation motion

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7
Q

how does vacancy conduction vary with temperature

A

at high T intrinsic (schottky) defects dominate due to natural entropic disorder
at low T extrinsic (impurity) defects dominate

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8
Q

what is the formula to calculate the number of schottky defects

A

Ns=N e^(-Es/2RT)

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9
Q

what is the formula to calculate the number of frenkel defects

A

Nf = (NNi)^1/2 e^(-Ef/2RT)

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10
Q

what is the Arrhenius equation for ionic conductivity

A

σ=ne𝜇0 e^(-Es/2RT) e^(-Em/RT)

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11
Q

why are bands formed

A

in MO theory σ and σ* are split as you form an infinitely long chain of atoms we get a band of energy levels

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12
Q

what is the energy difference between 2 bands called

A

band gap (Eg)

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13
Q

what is the energy spread of a band called

A

bandwidth

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14
Q

what 3 things do differences in conductivity depend on

A

band structure
how full the band is
size of band gap

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15
Q

give an example of a good conductor and explain why

A

Li - 8 coordinate
Good overlap -> large bandwidth -> s and p bands merge
sp band can hold 8N e- in Li it holds N
e- can easily be promoted above the fermi level

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16
Q

what is the fermi level

A

highest occupies energy level at O K

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17
Q

give an example of an insulator and explain this property

A

diamond - 4 coordinate
poor overlap -> small bandwidth
2 distinct bands are formed separated by band gap
the gap is too large for electrons to be promoted between bands

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18
Q

how does a semi-conductor compare to an insulator

A

similar structure but smaller band gap

at T>Eg e- can be promoted and conduction can take place

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19
Q

what is the band gap of an insulator

A

> 3eV

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20
Q

what is the band gap of a semi conductor

A

0.5-3 eV

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21
Q

what is the band gap of a conductor

A

0 - no band gap

22
Q

what is the trend for conductivity down group 14 and why

A 
become more conductive down the group
due to;
larger AOs - more overlap
less electronegative
therefore bandwidth increases
23
Q

what is an instrinsic semi conductor and what does its conductivity depend on

A

Pure material

  • size of Eg
  • temperature
24
Q

give and example of an intrinsic semiconductor

A

Si, Ge, GaAs

25
Q

how are extrinsic semiconductors different to intrinsic

A

contains mobile charge carriers (holes and electrons)

- due to doping

26
Q

what are n and p type extrinsic semiconductors

A

n type - contains negative charge carriers

p type - contains positive charge carriers

27
Q

what is the formula for conductivity in an intrinsic semiconductor at different T

A

σ=σ0 e^(-Eg/2kB*T)

kB = boltzmann

28
Q

what is the fermi-dirac distribution

A

shows the proportion of states in the conduction band that may be populated at T<0

29
Q

what is the fermi-dirac distribution formula

A

f(E) = e^(-Eg)/2kBT

30
Q

why are extrinic semiconductors electronically conductive

A

dopants
increase number of charge carriers
changes highest occupied level

31
Q

explain p type doping with an example

A

silicon 3s2 3p2 doped with boron 2s2 2p1
1 less e- in band structure
introduces acceptor level above valance band lowering energy for conduction

32
Q

explain n type doping with an example

A

silicon 3s2 3p2 doped with phosphorus 3s2 3p3
1 more e- in band structure
introduces donor level below conductance band lowering energy for conduction

33
Q

what happens at a p-n junction

A

there is a concentratio gradient of charge carriers

34
Q

what is photoconductivity and when does it take place

A

some semi conductors conduct more under light

if hv>Eg electrons can move into the conductance band

35
Q

describe the properties of III-V semiconductors

A

similar to IV semi conductors - same diamond structure
-isoelectronic to IV structures
Band gap increases down group

36
Q

describe and explain the conductivity of gallium arsenide

A

forms 2 sp bands separated by and band gap
electronegativity means;
Ga contributes more to conduction band
As contributes more to valance band

37
Q

when are d bands formed in transition metal ions

A

when d orbital overlap is good

38
Q

what are the 2 ways of overlap in transition metal oxides in rocksalt

A

dxy overlap

dx2-y2 overlap with px

39
Q

how does d orbital overlap change across the period

A

as zeff increases d orbitals are pulled closer to the nucleus so overlap is less so bandwidth is less

40
Q

what behaviour do TiO and VO have and why

A

metallic

due to high orbital overlap

41
Q

how is electron replusion taken into account

A

hubbard energy (U) - energy cost for putting 2 electrons onto the same atom in a lattice

42
Q

how does hubbard energy determine properties

A

Ubandwidth electrons are localised

43
Q

give 4 factors that favour d band formation

A

small charge on cation
cation is early on row
cation is in second and third row
anion is reasonably electropositive

44
Q

what 3 characteristics do superconductors have

A

conductivity increases at Tc
conduct with zero resistance
expel magnetic field

45
Q

what is a phonon

A

a lattice vibration that reduces conductivity

46
Q

how many phonons are present in super conductors

A

none

47
Q

what is the isotope effect

A

Tc depends on isotopic mass of element in a similar way to vibrational frequency - implies link

48
Q

describe BCS theory

A

1 e- passes through superconductor
distorts lattice and forms region of positive charge
next e- is attracted to the positive region
passes through freely

49
Q

give an example of a high Tc superconductor

A

YBCO

50
Q

how does the value of delta effect conductivity in YBCO

A

as delta increases Tc drops massively

CHEM313 (5 decks)

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