SC Flashcards
define crystal
a regular, periodic arrangement of atoms, ions or molecules
define motif
a periodic pattern that becomes associated with each point in the lattice
deffine lattice
defines the periodicity of the structure
define crystal structure
regular pattern defined by the lattice and the motif
define unit cell
a repeating unit that can generate the entire structure by translation in 3 dimensions
what does an atom in the corner of a unit cell contribute
1/8
what does an atom in the edge of a unit cell contribute
1/4
what does an atom in the face of a unit cell contribute
1/2
what does an atom fully within a unit cell contribute
1
what are miller indices used for
to label planes across a unit cell
what are the 2 true close packed systems
hexagonal and cubic close pack
what is the packing efficiency of a truly close packed structure
0.74
what is the formula for packing efficiency
n atoms in UC x V atom / V (UC)
what is the coordination number of a truely close packed system
12
what pattern does cubic close pack follow
ABC
what pattern does hexagonal close pack follow
AB
why is body centred cubic packing not truly close pack
only has PE of 0.68 and a coordination number of 8
what is the PE and CN of simple cubic packing
PE - 0.52
CN - 6
what effect does corrugation have on hexagonal close packed structures
low corrugation
more brittle
what effect does corrugation have on cubic close packed structures
low corrugation
more ductile
how does the corrugation of body centred cubic differ to hexagonal and cubic close pack
higher corrugation
harder than CCP
more ductile than HCP
what is an alloy
a metallic containing atoms of 2 or more metals
what are the 3 types of alloy
pure metal
substitutional alloy
interstitial alloy
how does an alloy change properties
disrupts periodicity
what is the structure of a substitutional alloy
one atom swapped for another in the close packed structure
what is the structure of an interstitial alloy
new ion fits in the holes in the close packed structure
what is the sphere to hole ratio for tetrahedral holes
1:2
what is the hole to sphere ratio for octahedral holes
1:1
what is the hole to sphere ratio for cubic holes
1:1
what is the formula for radius ratio
y = rsmall/large
which ion is usually largest
cation
What is a higher order ionic solid
Ionic solids with two or more elements
What formula does the spinel structure have
AB2X4
What are a and B in the spinel structure
Cations
What is X in a spinel structure
The anion
How many formula units are in a unit cell of a spinel structure
Eight formula units
How are the anions packed In a spinel structure
Cubic close packed
Where did the a cations go in spinel structure
1/8 of the tetrahedral holes
Where do the B cations go in the spinel structure
In half of the octahedral holes
What are the two formula versions of the spinel structure
A2+ B3+ X2- (MgAl2O4)
A4+ B2+ X2- (GeFe2O4)
What is a mixed cation phase
Having multiple B atoms
How is inverse spinel different to standard spinel
A cations fill half of the octahedral sites
B cations fill all tetrahedral and the other half of octahedral holes
How does ionic radius Affect spinel and inverse spinel structures
Tetrahedral is holes are smaller than octahedral holes so usually the more charged ion occupies the tetrahedral holes
How do electrostatics affect spinel and inverse spindle structures
The higher charged ion should prefer high coordination (octahedral)
What is the general formula for a perovskite
ABX3
What are the three variations in formula for a perovskite
+1/+5
+2/+4
+3/+3
When are perovskites Often formed
When the size difference between a and B is large
Where do Atoms a and X go in a perovskite
In a cubic closepack array
Where do B cations go in a perovskite structure
In the octahedral holes
Why do perovskites distort
Not all preferences can be met so the structure distorts to better meet these preferences
What is the formula for tolerance factor
T=rA+rX / root2 (rB + rX)
What is the structure of lead perovskites
Pb2+ occupies A site
Lone pair offsets structure from the centre
What is YBCO
YBa2Cu3O7-d
what information does x ray diffraction give
average long range structure of an ordered solid
why can it be difficult to characterise defects in a structure by x ray diffarction
defects are not regularly ordered
when does diffraction happen
when waves pass through a grating whose spacing is in the same order of magnitude to that of the wavelength
when are bright spots shown on a diffraction pattern
when the waves interfere constructively
when are dull areas shown on a diffraction pattern
when the waves interfere destructivly
what does braggs law do
relates spacing of a crystal plane with te observed angle of refraction
what is the formula for braggs law
lambda = 2 d(hkl) sin(theta)
what is the pattern of given by x ray diffraction known as
the recipricol lattice
how do reciprocal lattice vectors relate to the real lattice
they are reciprocal
what information is given in single crystal diffraction
positions of dots give information about the plane distances in the unit cell. Intensities tell us about electron densities
how does powder diffraction differ from single crystal diffraction
produces rings instead of individual spots
what is the disadvantage of powder diffraction
difficult to get info about intensities due to overlap
what are advantages of powder diffraction
simple and fast
doesnt need a high quality crystal
what is a colour centre
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
which compounds often have colour centres
alkali halides
how can powder diffraction data be analysed
fingerprinting
unit cell determination
phase purity
crystal structure determination
how does crystallite size effect diffraction reflections
smaller crystallites give broader diffraction peaks
how can the unit cell be determined from the diffraction
combining bragg and the lattice planes dhkl
what is a systematic absence
a missing reflection due to destructive interference
what condition must a miller indices complete to be body centred
H+K+L = 2n
what condition must a miller indices complete to be face centred
H,K,L = all odd or all even
what is the formation of defects driven by
gain in entropy
what is an intrinsic defect
a defect in a pure material - no change in composition
what is an extrinsic defect
a defect caused by change in composition (doping)
what is a point defect
a defect occuring at a particular point
what are extended defects
clusters of defects
what are the 3 types of point defects
vacancy
substitutional
interstitial
what is a vacancy defect
a normally filled site is vacant
what is a substitutional defect
when a site occupied by a different species to normal
what is an interstitial defect
when a site that is usually empty is filled
what is a schttky defect
a pair of vacancy defects (1 anion and 1 cation)
what is the formula for the number of schottky defects
Ns = N e(-^Hs/2RT)
what is a frenkel defect
when an atom or ion moves into an interstitial site from its normal site
what is a solid solution
a material with a high dopant conc (0.1-1%)
what is charge compensation
when a material is doped so defects form to maintain a neutral charge
what is the formula for ionic conductivity
sigma = nZemu
what is a stacking fault
when a stacking doesnt follow the correct pattern like ABCBCABC
what is an antiphase boundary
where the stacking pattern inverts ABCABCCB
how can short range structure be characterised
Pair distribution function
solid state nmr
x ray basorbtion spectroscopy
TEM
what image does TEM give
2D projection of structure including defects 1A resolution
what information does diffuse scattering give
short range structure
what is total scattering
bragg and diffuse
how can PDF be analysed
peak positions give interatomic separations
area provides coordination number
what is the general formula of zeolites
M(x/n)[(AlO2)x(SiO2)y].mH2O
what are the primary building blocks of zeolites
[SiO4]4-
[AlO4]5-
what is the structure of silicates
corner sharing tetrahedra
Si-O-Si link is flexible
what is the structure of a sodalite cage
24 tetrahedra in 4 and 6 memebered rings
has internal diameter of 6A
what is the structure of sodalite
sodalite rings bound at 4 memebered rings
what is the size of the entry point of the sodalite cage
2.6A
what is zeolite A
sodalite cages linked by O bridges
what is the entry restriction on zeolite A
4.1 A
what is the structure of zeolite X and Y
sodalite cages linked by 4-memebred rings and O links
has large pores
what is the Si/Al ratio in sodalite X
1-1.5
what is the Si/Al ratio in sodalite X
1.5-3
what are some uses of zeolites
can exchange cations (water softening)
selectively absorb polar molecules
how can zeolites be used in hydrocarbon separations
only allows linear chains through small cavities not branched structures
how can zeolites be tuned
Al/Si ratio - effects structure hydrophobicity and staility
counter ion - effects cgae size acidity and polarity
how can zeolites be used heterogeneous catalysts
can be bronsted or lewis acids#
selectivly made para xylene
