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

0 - no band gap

what is the trend for conductivity down group 14 and why

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

Pure material

size of Eg
temperature

give and example of an intrinsic semiconductor

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