lecture 6🅱️

Question 1

reasons as to why solids may contain defects

Answer 1

• 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)

Question 2

what external factors can damage crystal solids

Answer 2

impact
degradation

Question 3

are defects always bad

Answer 3

no,, theyre important

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

Question 4

what physical and chemical properties do defects influence

Answer 4

conductivity
mechanical strength
reactivity

Question 5

what are defects characterised by

Answer 5

their dimensionality

their arrangement in space

Question 6

types of defects

Answer 6

point defect

extended defect

Question 7

whats a point defect

Answer 7

occur at a single site in the crystal structure

Question 8

whats an extended defect

Answer 8

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

throughout the crystal

Question 9

what is a intrinsic defect

Answer 9

a defect that occurs in a pure substance

Question 10

what is an extrinsic defect

Answer 10

defect that occurs due to the presence of impurities

Question 11

explain point defects fully

Answer 11

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

Question 12

intrinsic point defect memory

Answer 12

from inside
occurs in a pure substance

Question 13

extrinsic point defect memory

Answer 13

from the outside

aka a defect due to impurities present

Question 14

point defect short

Answer 14

• single site
• occur randomly
• intrinsic or extrinsic

Question 15

what would an intrinsic point defect be then

Answer 15

intrinsic = in a pure substance

point = single site, random

defect in a single site in the pure solid

Question 16

types of intrinsic point defects

Answer 16

Schottky defect
Frenkel defect

Question 17

do we learn about extended defects

Answer 17

nope <3

Question 18

what is a Schottky defect

Answer 18

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

Question 19

schottky memory

Answer 19

missing S from skl

Question 20

schottky defect occurs in what special way

Answer 20

it occurs stoichiometrically

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

Question 21

what does the schottky defect occuring stoichiometrically mean

Answer 21

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

Question 22

how do we represent a vacancy

Answer 22

a box where the particle should be

a v

Question 23

what is a defect notation

Answer 23

kroger vink notation

notation used to describe defects in crystal structures.

Question 24

describe how u use a kroger notation

Answer 24

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)

Question 25

what would V*Cl mean Kroger vink

Answer 25

a vacancy at a Cl- anion site with a +1 relative charge

+1 bc ur going from -1 to a valancy which is 0

Question 26

what would a kroger vink notation of V’Na mean

Answer 26

a valancy at a Na cation site with a -1 relative charge

-1 bc ur going from +1 to 0

Question 27

what would a kroger vink notation of Ni x Cu mean

Answer 27

a Ni atom in a Cu site with no change in charge

Question 28

what is a schottky defect

Answer 28

type of intrinsic point defect

atoms/ions are missing from their expected position in an otherwise ideal structure

Question 29

frenkel defects

Answer 29

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)

Question 30

what structures are more likely to have a frenkel defect

Answer 30

structures with lower coordination numbers

wurzite
zinc blende // spheralite

Question 31

how do we draw a frenkel defect

Answer 31

a box that represents a vacancy with an arrow pointing to an atom//ion in an interstitial site

Question 32

frenkel kroger vink notation of MgO when Mg moves

Answer 32

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

Question 33

what is an extrinsic defect

Answer 33

defect due to impurities

Question 34

can a solid every be 100% pure

Answer 34

nope,, impurities will always be present

Question 35

are impurities always always bad

Answer 35

nope,, some are introduced intentionally and this is called doping

Question 36

why is doping useful

Answer 36

it can be used to target certain chemical and physical properties

can change the electronic properties in semiconductor materials

Question 37

what is a substitutional defect

Answer 37

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.

Question 38

do impurities only occupy ionic//atom sites

Answer 38

nope

they can be present in interstitial sites aswell

Question 39

are substituional defects stoichiometric,, aka do they keep the charge balanced

Answer 39

nope!!

not always,, the charge needs to be balanced if the balance is disturbed due to an impurity

Question 40

substitutional defect

Answer 40

impurity is substituted into where an atom//ion usually is.

Question 41

if a substitutional defect disturbs the charge balance,, what needs to be done

Answer 41

a vacancy must be made