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CHEM313 > SC > Flashcards

SC Flashcards

1
Q

define crystal

A

a regular, periodic arrangement of atoms, ions or molecules

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2
Q

define motif

A

a periodic pattern that becomes associated with each point in the lattice

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3
Q

deffine lattice

A

defines the periodicity of the structure

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4
Q

define crystal structure

A

regular pattern defined by the lattice and the motif

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5
Q

define unit cell

A

a repeating unit that can generate the entire structure by translation in 3 dimensions

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6
Q

what does an atom in the corner of a unit cell contribute

A

1/8

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7
Q

what does an atom in the edge of a unit cell contribute

A

1/4

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8
Q

what does an atom in the face of a unit cell contribute

A

1/2

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9
Q

what does an atom fully within a unit cell contribute

A

1

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10
Q

what are miller indices used for

A

to label planes across a unit cell

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11
Q

what are the 2 true close packed systems

A

hexagonal and cubic close pack

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12
Q

what is the packing efficiency of a truly close packed structure

A

0.74

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13
Q

what is the formula for packing efficiency

A

n atoms in UC x V atom / V (UC)

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14
Q

what is the coordination number of a truely close packed system

A

12

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15
Q

what pattern does cubic close pack follow

A

ABC

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16
Q

what pattern does hexagonal close pack follow

A

AB

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17
Q

why is body centred cubic packing not truly close pack

A

only has PE of 0.68 and a coordination number of 8

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18
Q

what is the PE and CN of simple cubic packing

A

PE - 0.52

CN - 6

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19
Q

what effect does corrugation have on hexagonal close packed structures

A

low corrugation

more brittle

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20
Q

what effect does corrugation have on cubic close packed structures

A

low corrugation

more ductile

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21
Q

how does the corrugation of body centred cubic differ to hexagonal and cubic close pack

A

higher corrugation
harder than CCP
more ductile than HCP

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22
Q

what is an alloy

A

a metallic containing atoms of 2 or more metals

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23
Q

what are the 3 types of alloy

A

pure metal
substitutional alloy
interstitial alloy

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24
Q

how does an alloy change properties

A

disrupts periodicity

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25
Q

what is the structure of a substitutional alloy

A

one atom swapped for another in the close packed structure

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26
Q

what is the structure of an interstitial alloy

A

new ion fits in the holes in the close packed structure

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27
Q

what is the sphere to hole ratio for tetrahedral holes

A

1:2

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28
Q

what is the hole to sphere ratio for octahedral holes

A

1:1

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29
Q

what is the hole to sphere ratio for cubic holes

A

1:1

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30
Q

what is the formula for radius ratio

A

y = rsmall/large

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31
Q

which ion is usually largest

A

cation

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32
Q

What is a higher order ionic solid

A

Ionic solids with two or more elements

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33
Q

What formula does the spinel structure have

A

AB2X4

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34
Q

What are a and B in the spinel structure

A

Cations

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35
Q

What is X in a spinel structure

A

The anion

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36
Q

How many formula units are in a unit cell of a spinel structure

A

Eight formula units

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37
Q

How are the anions packed In a spinel structure

A

Cubic close packed

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38
Q

Where did the a cations go in spinel structure

A

1/8 of the tetrahedral holes

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39
Q

Where do the B cations go in the spinel structure

A

In half of the octahedral holes

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40
Q

What are the two formula versions of the spinel structure

A

A2+ B3+ X2- (MgAl2O4)

A4+ B2+ X2- (GeFe2O4)

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41
Q

What is a mixed cation phase

A

Having multiple B atoms

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42
Q

How is inverse spinel different to standard spinel

A

A cations fill half of the octahedral sites

B cations fill all tetrahedral and the other half of octahedral holes

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43
Q

How does ionic radius Affect spinel and inverse spinel structures

A

Tetrahedral is holes are smaller than octahedral holes so usually the more charged ion occupies the tetrahedral holes

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44
Q

How do electrostatics affect spinel and inverse spindle structures

A

The higher charged ion should prefer high coordination (octahedral)

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45
Q

What is the general formula for a perovskite

A

ABX3

46
Q

What are the three variations in formula for a perovskite

A

+1/+5
+2/+4
+3/+3

47
Q

When are perovskites Often formed

A

When the size difference between a and B is large

48
Q

Where do Atoms a and X go in a perovskite

A

In a cubic closepack array

49
Q

Where do B cations go in a perovskite structure

A

In the octahedral holes

50
Q

Why do perovskites distort

A

Not all preferences can be met so the structure distorts to better meet these preferences

51
Q

What is the formula for tolerance factor

A

T=rA+rX / root2 (rB + rX)

52
Q

What is the structure of lead perovskites

A

Pb2+ occupies A site

Lone pair offsets structure from the centre

53
Q

What is YBCO

A

YBa2Cu3O7-d

54
Q

what information does x ray diffraction give

A

average long range structure of an ordered solid

55
Q

why can it be difficult to characterise defects in a structure by x ray diffarction

A

defects are not regularly ordered

56
Q

when does diffraction happen

A

when waves pass through a grating whose spacing is in the same order of magnitude to that of the wavelength

57
Q

when are bright spots shown on a diffraction pattern

A

when the waves interfere constructively

58
Q

when are dull areas shown on a diffraction pattern

A

when the waves interfere destructivly

59
Q

what does braggs law do

A

relates spacing of a crystal plane with te observed angle of refraction

60
Q

what is the formula for braggs law

A

lambda = 2 d(hkl) sin(theta)

61
Q

what is the pattern of given by x ray diffraction known as

A

the recipricol lattice

62
Q

how do reciprocal lattice vectors relate to the real lattice

A

they are reciprocal

63
Q

what information is given in single crystal diffraction

A

positions of dots give information about the plane distances in the unit cell. Intensities tell us about electron densities

64
Q

how does powder diffraction differ from single crystal diffraction

A

produces rings instead of individual spots

65
Q

what is the disadvantage of powder diffraction

A

difficult to get info about intensities due to overlap

66
Q

what are advantages of powder diffraction

A

simple and fast

doesnt need a high quality crystal

67
Q

what is a colour centre

A

when a free electron is trapped in a vacant anion site it acts like an electron in a box and transitions between energy levels - characteristic of host lattices

68
Q

which compounds often have colour centres

A

alkali halides

69
Q

how can powder diffraction data be analysed

A

fingerprinting
unit cell determination
phase purity
crystal structure determination

70
Q

how does crystallite size effect diffraction reflections

A

smaller crystallites give broader diffraction peaks

71
Q

how can the unit cell be determined from the diffraction

A

combining bragg and the lattice planes dhkl

72
Q

what is a systematic absence

A

a missing reflection due to destructive interference

73
Q

what condition must a miller indices complete to be body centred

A

H+K+L = 2n

74
Q

what condition must a miller indices complete to be face centred

A

H,K,L = all odd or all even

75
Q

what is the formation of defects driven by

A

gain in entropy

76
Q

what is an intrinsic defect

A

a defect in a pure material - no change in composition

77
Q

what is an extrinsic defect

A

a defect caused by change in composition (doping)

78
Q

what is a point defect

A

a defect occuring at a particular point

79
Q

what are extended defects

A

clusters of defects

80
Q

what are the 3 types of point defects

A

vacancy
substitutional
interstitial

81
Q

what is a vacancy defect

A

a normally filled site is vacant

82
Q

what is a substitutional defect

A

when a site occupied by a different species to normal

83
Q

what is an interstitial defect

A

when a site that is usually empty is filled

84
Q

what is a schttky defect

A

a pair of vacancy defects (1 anion and 1 cation)

85
Q

what is the formula for the number of schottky defects

A

Ns = N e(-^Hs/2RT)

86
Q

what is a frenkel defect

A

when an atom or ion moves into an interstitial site from its normal site

87
Q

what is a solid solution

A

a material with a high dopant conc (0.1-1%)

88
Q

what is charge compensation

A

when a material is doped so defects form to maintain a neutral charge

89
Q

what is the formula for ionic conductivity

A

sigma = nZemu

90
Q

what is a stacking fault

A

when a stacking doesnt follow the correct pattern like ABCBCABC

91
Q

what is an antiphase boundary

A

where the stacking pattern inverts ABCABCCB

92
Q

how can short range structure be characterised

A

Pair distribution function
solid state nmr
x ray basorbtion spectroscopy
TEM

93
Q

what image does TEM give

A

2D projection of structure including defects 1A resolution

94
Q

what information does diffuse scattering give

A

short range structure

95
Q

what is total scattering

A

bragg and diffuse

96
Q

how can PDF be analysed

A

peak positions give interatomic separations

area provides coordination number

97
Q

what is the general formula of zeolites

A

M(x/n)[(AlO2)x(SiO2)y].mH2O

98
Q

what are the primary building blocks of zeolites

A

[SiO4]4-

[AlO4]5-

99
Q

what is the structure of silicates

A

corner sharing tetrahedra

Si-O-Si link is flexible

100
Q

what is the structure of a sodalite cage

A

24 tetrahedra in 4 and 6 memebered rings

has internal diameter of 6A

101
Q

what is the structure of sodalite

A

sodalite rings bound at 4 memebered rings

102
Q

what is the size of the entry point of the sodalite cage

A

2.6A

103
Q

what is zeolite A

A

sodalite cages linked by O bridges

104
Q

what is the entry restriction on zeolite A

A

4.1 A

105
Q

what is the structure of zeolite X and Y

A

sodalite cages linked by 4-memebred rings and O links

has large pores

106
Q

what is the Si/Al ratio in sodalite X

A

1-1.5

107
Q

what is the Si/Al ratio in sodalite X

A

1.5-3

108
Q

what are some uses of zeolites

A

can exchange cations (water softening)

selectively absorb polar molecules

109
Q

how can zeolites be used in hydrocarbon separations

A

only allows linear chains through small cavities not branched structures

110
Q

how can zeolites be tuned

A

Al/Si ratio - effects structure hydrophobicity and staility

counter ion - effects cgae size acidity and polarity

111
Q

how can zeolites be used heterogeneous catalysts

A

can be bronsted or lewis acids#

selectivly made para xylene

CHEM313 (5 decks)

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