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CHEM313 > Topic 2 - Higher Order Structures > Flashcards

Topic 2 - Higher Order Structures Flashcards

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1
Q

What assumptions are made of ionic solids?

A

We assume their structure is determined by electrostatic considerations:

  • Arrangement is stabilised by maximising anion-cation interactions and minimising anion-anion repulsion
  • Size of ions is important in determining structure
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2
Q

What is the sphere:hole ratio of tetrahedral holes and what is the size of these holes?

A

1:2 sphere:hole ratio
Size = 0.225r

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3
Q

What is the sphere:hole ratio of octahedral holes and what is the size of these holes?

A

1:1 sphere:hole ratio
Size = 0.414r

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4
Q

What’s significant about eutactic structures?

A

Eutactic structures are formed when the structure expands to accommodate ions in interstitial sites, forces original ions to no longer touch.

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5
Q

What is the radius rule? (Radius ratio)

A

The radius rule predicts the coordination (preferred interstitial sites) of the secondary ion

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6
Q

What assumptions are made in the radius rule?

A

Electrostatic bonding dominates

Ions are perfect hard spheres of a known radius

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7
Q

What does the radius rule not account for?

A

Does NOT account for attractive and repulsive forces between ions, considerations of orbital overlap and covalency.

It is simply a geometric rule used as a guide to determine stability

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8
Q

What’re the threshold values of the radius rule for determining the preferred interstitial site?

A
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9
Q

What is the structure and CN of Rocksalt?

A

AB

FCC array of anions with all octahedral holes filled by cations

6,6 coordination

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10
Q

What is the structure and CN of Nickel arsenide?

A

AB

HCP array of anions (As) with all octahedral holes occupied by cations (Ni)

6,6 coordination
- Octahedral Ni
- Trigonal prismatic As

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11
Q

What’s significant about Nickel arsenide?

A

Charge separation:
- As exists as 1- and 3-, giving As^2- overall

Short M-M contact:
- preferred as compound is not strongly ionic, As is large and is polarisable
- also depends on covalency

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12
Q

What is the structure and CN of Fluorite and Antifluorite?

A

AB2 or A2B

FCC array of cations (Ca2+) with all tetrahedral holes occupied by anions (F-).
- Oct holes are vacant

8,4 coordination
- 2:1 ration for tetrahedral holes

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13
Q

What’s significant about the Fluorite and Antifluorite structures?

A

Structure gives Bight Blue colour

When coordination of Fluorite is inverted, it gives Antifluorite structure

There is no HCP equivalent of these structures

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14
Q

What is the structure and CN of Zinc blende and Wurtzite?

A

AB

Zinc blende:
FCC array of anions with half tetrahedral holes occupied by cations

Wurtzite:
HCP array of anions with half tetrahedral holes occupied by cations

4,4 coordination in both structures

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15
Q

What’s significant about Zinc blende and Wurtzite?

A

Is also known as sphalerite or cubic ZnS

3-5 semiconductors adopt this structure
- isoelectronic compounds

Zinc blende and Wurtzite are polymorphs of ZnS

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16
Q

What is the structure and CN of Lithium bismuthide?

A

AB3

FCC array of Bi^3- anions with all Oct and Tet holes filled by Li+

CN of Bi^3-:
14 coordinate
- 8 from cube
- 6 from FC

CN of Li+:
4 and 6 coordinate

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17
Q

What application does the lithium bismuthide structure have?

A

K3C60 is a superconductor
- FCC disordered C60 (4 per UC)
- K+ in all Oct and Tet holes (12 per UC)

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18
Q

What is the structure and CN of Caesium chloride?

A

AB

Simple cubic (primitive) array of Cl- anions with large Cs+ cation occupying cubic hole

8,8 coordination

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19
Q

What’s significant about CsCl structure?

A

That it is NOT BCC, as large Cs+ cation occupies central cubic hole in UC

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20
Q

What is the structure and CN of Rhenium trioxide?

A

AB3

Simple (primitive) cubic array of cations (Re6+)

6 coordinate octahedral Re6+
Linear 2 coordinate O2-

Corner sharing ReO6 octahedra

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21
Q

What’s significant about the structure of Rhenium trioxide?

A

It is a substructure of perovskite
- ion can be inserted inside structure
- ABX3 where A & B are both cations, X is anion

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22
Q

What structures covered are FCC/CCP?

A

Rocksalt
Zinc blende
Fluorite / Antifluorite
Lithium bismuthide

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23
Q

What structures covered are HCP?

A

Nickel arsenide
Wurtzite
Rutile

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25
What structures covered are simple cubic?
CsCl Rhenium trioxide
26
Determine the FCC structure of MBr using the radius ratio rule. M+ radius = 0.6A Br- radius = 1.96A
0.6 / 1.96 = 0.306 0.414 - 0.225 = tetrahedral holes occupied by cation Hence M+ must occupy tetrahedral holes Zinc blende - cation occupies half of Tet holes, matches stoichiometry of 1:1, as Tet holes have 2:1 ratio of close packed ions.
27
Whats the general formula of spinels?
AB2X4 MgAl2O4 - parent mineral
28
What is the structure of spinels?
FCC array of anions (O2-) A cations (Mg2+) in 1/8 tetrahedral holes B cations (Al3+) in 1/2 octahedral holes
29
How many formula units are there per unit cell in spinels?
8 formula units per UC - gives 8Mg 16Al 16O2
30
How many spheres (atoms) and holes are there in a spinel unit cell?
32 spheres (atoms) per UC - 64 tetrahedral holes (2:1) - 32 octahedral holes (1:1)
31
What different types of spinels are there? What do they mean?
2-3 spinels - A cation is 2+, B cations is 3+ 4-2 spinels - A cation is 4+, B cation is 2+
32
How do inverse spinels differ to regular spinel structures?
The distribution in the two types of cations is different However, the overall occupancy of sites are the same in each structure. - 1/2 Oct and 1/8 Tet holes occupied
33
What is the cation distribution in regular spinels?
8 A in 1/8 Tet holes 16 B in 1/2 Oct holes
34
What is the cation distribution of inverse spinels?
8 A in 1/4 Oct holes 8 B in 1/8 Tet holes 8 B in 1/4 Oct holes (1/2 Oct and 1/8 Tet holes filled total)
35
What factors determine spinel or inverse spinel structure?
Ionic radius: - Tet holes are smaller - Suggests smaller, highly charged cation occupancy Electrostatics: - Highly charged cation should prefer higher coordination. (Ionic radius and Electrostatic factors compete w/each other) CFSE (TM cations): - Relative stabilisation due to partially occupied d-orbitals affects preference for Oct sites. - Dominates when relevant.
36
How do you determine the octahedral site preference due to CFSE of a TM cation?
Octahedral site preference energy (OSPE) OPSE = CFSEoct - CFSEtet (for each cation) Greater ∆Oct CFSE = greater stability of crystal when in octahedral site
37
How do lithium ion batteries work on charging and discharging power?
On charging, Li+ diffuses through the electrolyte from the cathode to the anode - the reverse occurs on discharge
38
What key factors determine the usability of a lithium ion battery?
Li+ must be mobile in electrode material & materials must be able to store Li+ ions Key factors for cathodes: - charge density - charge rates - stability - cost - toxicity
39
LiCoO2 is the most common cathode material, why?
LiCoO2 forms a layered solid with an ordered Rocksalt structure Li+ can diffuse in two dimensions between layers of Co octahedra It allows for reversible interact action of Li+ ions with high cycling efficiency Also has a low thermal runaway temperature - tho Co is expensive
40
Whats an issue with LiMn2O4 for use as a cathode?
they have poor cycling stability partially due to Jahn-Teller distortion from Mn3+ - also undergoes side reactions with electrolyte
41
What’re the benefits of using LiMn2O4 as a Li+ ion cathode?
Mn is cheaper and less toxic than Co Also have higher thermal runaway temperatures
42
When are perovskites often formed?
Often formed when the difference in size of A and B cations is large
43
What’s the general formula of perovskites?
ABX3
44
What combinations of cations can be used in perovskites?
1+/5+ 2+/4+ 3+/3+
45
Whats the structure of perovskites?
A and X ions form FCC array B cations reside in octahedral holes
46
What factors cause distortion in perovskites?
Charge, size & coordination/geometry
47
Why does distortion occur in perovskites?
The structure distorts to better accommodate the preferences of the ions The composition and stoichiometry is unchanged
48
What can be used to determine the extent of distortion in perovskites?
The Tolerance factor, t
49
What’re the thresholds for the tolerance factor?
1 < t < 1.06 = hexagonal / tetragonal perovskite 0.9 < t < 1 = ideal cubic structure 0.8 < t < 0.9 = orthorhombic perovskite t < 0.8 = other structure types - e.g. LiNbO3, ilmenite
50
What happens when a tolerance factor of a crystal falls outside the thresholds?
The compound is unstable
51
What is the tolerance factor based on?
The tolerance factor is only based on ion size - no account for electrostatic interactions
52
Whats the significance of lead perovskites?
Lead-containing perovskites have important technological applications which relate to their dielectric, ferroelectric and piezoelectric (converts mechanical stress to electrical energy) properties.
53
How is polarisation of lead-containing perovskites achieved?
Pb2+ cations occupy the A-site (central) where the 6s lone pair drives the displacement from the centre of its coordination, polarising the material. Ti4+ is also off-centred, contributing to polarisation. Both cations are displaced in the same direction, with O2- anions shifting the opposite direction.
54
Whats the problem with Pb-containing perovskites?
Lead is highly toxic and affects the environment This makes Pb-free ferroelectrics a priority target for new materials
55
Why have organic-inorganic halide perovskite seen research interest recently?
Because of their potential applications as absorbers in solar cells (>22% energy conversion from solar energy - 3.8% in 2009)
56
How can the band gap in organic-inorganic halide perovskites be tuned?
By varying alkyl chain length
57
What is the chemical formula of YBCO?
YBa2Cu3O7-∂ (Where ∂ = sigma)
58
Whats the significance of YBCO?
It is the first high temperature superconductor Tc = 90K for ∂=0
59
What does changing the oxygen content (∂) in YBCO do?
As ∂ increases, oxygen content decreases. This causes greater +ve charge across the material. Hence, Cu oxidation state changes to accommodate

CHEM313 (9 decks)

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