LC Electronic Properties Flashcards
why cant ions usually conduct electricity
they only have enough energy to vibrate in the lattice not to leave it
what is the formula for ionic conductivity
σ=neμ
n= no. charge carriers
e = charge on carriers
μ = mobility
why is ionic coduction much easier at high T
defects are involved
what are the minimum requirements of a conductor
either;
some sites are vacant
some interstitial sites are occupied
how do solid electrolytes conduct electricity
they contain tunnels or layers for ions to travel through
give an example of a solid electrolyte and why it has such properties
α-Li2SO4
SO4 2- anions rotate to aid cation motion
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
what is the formula to calculate the number of schottky defects
Ns=N e^(-Es/2RT)
what is the formula to calculate the number of frenkel defects
Nf = (NNi)^1/2 e^(-Ef/2RT)
what is the Arrhenius equation for ionic conductivity
σ=ne𝜇0 e^(-Es/2RT) e^(-Em/RT)
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
what is the energy difference between 2 bands called
band gap (Eg)
what is the energy spread of a band called
bandwidth
what 3 things do differences in conductivity depend on
band structure
how full the band is
size of band gap
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
what is the fermi level
highest occupies energy level at O K
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
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
what is the band gap of an insulator
> 3eV
what is the band gap of a semi conductor
0.5-3 eV
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
