lecture 6🅱️ Flashcards
reasons as to why solids may contain defects
- entropically favoured (increase disorder and entropy)
- no material can be 100% pure
- to achieve a truley ordered crystal structure,, the components have to be assembled extremely slowly to ensure every particle is in its ideal position
- crystal structures can be damaged by external factors (impact degradation)
what external factors can damage crystal solids
impact
degradation
are defects always bad
no,, theyre important
they influence physical and chemical properties like conductivity, mechanical strength and reactivity
what physical and chemical properties do defects influence
conductivity
mechanical strength
reactivity
what are defects characterised by
their dimensionality
their arrangement in space
types of defects
point defect
extended defect
whats a point defect
occur at a single site in the crystal structure
whats an extended defect
occur in 1d 2d and 3d through the crystal
throughout the crystal
what is a intrinsic defect
a defect that occurs in a pure substance
what is an extrinsic defect
defect that occurs due to the presence of impurities
explain point defects fully
occur at a single site in the structure
dont extend further in the solid in any direction
- occur randomly throughout the crystal structure
- can be intrinsic point defects of extrinsic point defects
intrinsic point defect memory
from inside
occurs in a pure substance
extrinsic point defect memory
from the outside
aka a defect due to impurities present
point defect short
- single site
- occur randomly
- intrinsic or extrinsic
what would an intrinsic point defect be then
intrinsic = in a pure substance
point = single site, random
defect in a single site in the pure solid
types of intrinsic point defects
Schottky defect
Frenkel defect
do we learn about extended defects
nope <3
what is a Schottky defect
intrinsic point defect
- defect where an atom or ion is missing from its expected position in an otherwise ideal structure.
ideal structure bc its intrinsic = defect in a pure substance
schottky memory
missing S from skl
schottky defect occurs in what special way
it occurs stoichiometrically
the overall charge balance of the structure is not affected,, its maintained
what does the schottky defect occuring stoichiometrically mean
the overall change balance is not affected. the balance between cation and anion are the same before and after the defect occurs.
in nacl: sodium vacancy = cl vacancy bc theyre 1:1 in charge
in CaF2: Ca vacancy = 2 F vacancies
how do we represent a vacancy
a box where the particle should be
a v
what is a defect notation
kroger vink notation
notation used to describe defects in crystal structures.
describe how u use a kroger notation
M for the species (atom, valancy, intersitial site)
C for the charge (charge of the species compared to what was there before!!! compare what was to what is present due to the defect)
*=+. ‘ = - x = no change in charge
S for the site (indicates the site is occupied)
what would V*Cl mean Kroger vink
a vacancy at a Cl- anion site with a +1 relative charge
+1 bc ur going from -1 to a valancy which is 0
what would a kroger vink notation of V’Na mean
a valancy at a Na cation site with a -1 relative charge
-1 bc ur going from +1 to 0
what would a kroger vink notation of Ni x Cu mean
a Ni atom in a Cu site with no change in charge
what is a schottky defect
type of intrinsic point defect
atoms/ions are missing from their expected position in an otherwise ideal structure
frenkel defects
type of instrinsic point defect
intrinsic = in an pure substance
point = only occurs in a single site
when an atom/ion is displaced into an interstitial site leaving a valancy in its original position
- charge balance + stoichiometry remains unchanged
- cation, anion or both an be displaced
- more common in structures with lower coordination numbers (intersitial sites are easily accommadated)
what structures are more likely to have a frenkel defect
structures with lower coordination numbers
wurzite
zinc blende // spheralite
how do we draw a frenkel defect
a box that represents a vacancy with an arrow pointing to an atom//ion in an interstitial site
frenkel kroger vink notation of MgO when Mg moves
V’‘Mg + Mg ** i
aka theres a valancy in an Mg cation site with a relative charge of -2
theres a Mg cation in an interstitial site with a relative charge of +2
what is an extrinsic defect
defect due to impurities
can a solid every be 100% pure
nope,, impurities will always be present
are impurities always always bad
nope,, some are introduced intentionally and this is called doping
why is doping useful
it can be used to target certain chemical and physical properties
can change the electronic properties in semiconductor materials
what is a substitutional defect
type of extrinsic impurity
occurs when an impurity is incorporated into a regular atomic/ionic site in the structure.
aka an impurity is put into a site where a atoms / ion is usually occupying.
do impurities only occupy ionic//atom sites
nope
they can be present in interstitial sites aswell
are substituional defects stoichiometric,, aka do they keep the charge balanced
nope!!
not always,, the charge needs to be balanced if the balance is disturbed due to an impurity
substitutional defect
impurity is substituted into where an atom//ion usually is.
if a substitutional defect disturbs the charge balance,, what needs to be done
a vacancy must be made
