lecture 6🅱️ Flashcards

1
Q

reasons as to why solids may contain defects

A
  • 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)
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2
Q

what external factors can damage crystal solids

A

impact
degradation

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3
Q

are defects always bad

A

no,, theyre important

they influence physical and chemical properties like conductivity, mechanical strength and reactivity

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4
Q

what physical and chemical properties do defects influence

A

conductivity
mechanical strength
reactivity

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5
Q

what are defects characterised by

A

their dimensionality

their arrangement in space

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6
Q

types of defects

A

point defect

extended defect

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7
Q

whats a point defect

A

occur at a single site in the crystal structure

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8
Q

whats an extended defect

A

occur in 1d 2d and 3d through the crystal

throughout the crystal

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9
Q

what is a intrinsic defect

A

a defect that occurs in a pure substance

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10
Q

what is an extrinsic defect

A

defect that occurs due to the presence of impurities

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11
Q

explain point defects fully

A

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
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12
Q

intrinsic point defect memory

A

from inside
occurs in a pure substance

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13
Q

extrinsic point defect memory

A

from the outside

aka a defect due to impurities present

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14
Q

point defect short

A
  • single site
  • occur randomly
  • intrinsic or extrinsic
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15
Q

what would an intrinsic point defect be then

A

intrinsic = in a pure substance

point = single site, random

defect in a single site in the pure solid

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16
Q

types of intrinsic point defects

A

Schottky defect
Frenkel defect

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17
Q

do we learn about extended defects

18
Q

what is a Schottky defect

A

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

19
Q

schottky memory

A

missing S from skl

20
Q

schottky defect occurs in what special way

A

it occurs stoichiometrically

the overall charge balance of the structure is not affected,, its maintained

21
Q

what does the schottky defect occuring stoichiometrically mean

A

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

22
Q

how do we represent a vacancy

A

a box where the particle should be

a v

23
Q

what is a defect notation

A

kroger vink notation

notation used to describe defects in crystal structures.

24
Q

describe how u use a kroger notation

A

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)

25
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
26
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
27
what would a kroger vink notation of Ni x Cu mean
a Ni atom in a Cu site with no change in charge
28
what is a schottky defect
type of intrinsic point defect atoms/ions are missing from their expected position in an otherwise ideal structure
29
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)
30
what structures are more likely to have a frenkel defect
structures with lower coordination numbers wurzite zinc blende // spheralite
31
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
32
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
33
what is an extrinsic defect
defect due to impurities
34
can a solid every be 100% pure
nope,, impurities will always be present
35
are impurities always always bad
nope,, some are introduced intentionally and this is called doping
36
why is doping useful
it can be used to target certain chemical and physical properties can change the electronic properties in semiconductor materials
37
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.
38
do impurities only occupy ionic//atom sites
nope they can be present in interstitial sites aswell
39
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
40
substitutional defect
impurity is substituted into where an atom//ion usually is.
41
if a substitutional defect disturbs the charge balance,, what needs to be done
a vacancy must be made