LC Electronic Properties Flashcards by willl ackerman

why cant ions usually conduct electricity

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

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what is the formula for ionic conductivity

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

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why is ionic coduction much easier at high T

defects are involved

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what are the minimum requirements of a conductor

either;
some sites are vacant
some interstitial sites are occupied

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how do solid electrolytes conduct electricity

they contain tunnels or layers for ions to travel through

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

α-Li2SO4

SO4 2- anions rotate to aid cation motion

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how does vacancy conduction vary with temperature

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

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what is the formula to calculate the number of schottky defects

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

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what is the formula to calculate the number of frenkel defects

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

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what is the Arrhenius equation for ionic conductivity

σ=ne𝜇0 e^(-Es/2RT) e^(-Em/RT)

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why are bands formed

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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what is the energy difference between 2 bands called

band gap (Eg)

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what is the energy spread of a band called

bandwidth

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what 3 things do differences in conductivity depend on

band structure
how full the band is
size of band gap

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give an example of a good conductor and explain why

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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what is the fermi level

highest occupies energy level at O K

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give an example of an insulator and explain this property

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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how does a semi-conductor compare to an insulator

similar structure but smaller band gap

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

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what is the band gap of an insulator

> 3eV

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what is the band gap of a semi conductor

0.5-3 eV

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what is the band gap of a conductor

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0 - no band gap

what is the trend for conductivity down group 14 and why

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

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

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Pure material

size of Eg
temperature

give and example of an intrinsic semiconductor

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Si, Ge, GaAs

how are extrinsic semiconductors different to intrinsic

contains mobile charge carriers (holes and electrons) | - due to doping

what are n and p type extrinsic semiconductors

n type - contains negative charge carriers | p type - contains positive charge carriers

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

σ=σ0 e^(-Eg/2kB*T) | kB = boltzmann

what is the fermi-dirac distribution

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

what is the fermi-dirac distribution formula

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

why are extrinic semiconductors electronically conductive

dopants increase number of charge carriers changes highest occupied level

explain p type doping with an example

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

explain n type doping with an example

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

what happens at a p-n junction

there is a concentratio gradient of charge carriers

what is photoconductivity and when does it take place

some semi conductors conduct more under light | if hv>Eg electrons can move into the conductance band

describe the properties of III-V semiconductors

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

describe and explain the conductivity of gallium arsenide

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

when are d bands formed in transition metal ions

when d orbital overlap is good

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

dxy overlap | dx2-y2 overlap with px

how does d orbital overlap change across the period

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

what behaviour do TiO and VO have and why

metallic | due to high orbital overlap

how is electron replusion taken into account

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

how does hubbard energy determine properties

Ubandwidth electrons are localised

give 4 factors that favour d band formation

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

what 3 characteristics do superconductors have

conductivity increases at Tc conduct with zero resistance expel magnetic field

what is a phonon

a lattice vibration that reduces conductivity

how many phonons are present in super conductors

none

what is the isotope effect

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

describe BCS theory

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

give an example of a high Tc superconductor

YBCO

how does the value of delta effect conductivity in YBCO

as delta increases Tc drops massively

LC Electronic Properties Flashcards

(50 cards)