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CHEM313 > Topic 2 - Higher Order Structures > Flashcards

Topic 2 - Higher Order Structures Flashcards

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?

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 caitons

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 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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24
Q

What structures covered are HCP?

A

Nickel arsenide
Wurtzite
Rutile

25
Q

What structures covered are simple cubic?

A

CsCl
Rhenium trioxide

26
Q

Determine the FCC structure of MBr using the radius ratio rule.

M+ radius = 0.6A
Br- radius = 1.96A

A

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
Q

Whats the general formula of spinels?

A

AB2X4

MgAl2O4 - parent mineral

28
Q

What is the structure of spinels?

A

FCC array of anions (O2-)

A cations (Mg2+) in 1/8 tetrahedral holes

B cations (Al3+) in 1/2 octahedral holes

29
Q

How many formula units are there per unit cell in spinels?

A

8 formula units per UC
- gives 8Mg 16Al 16O2

30
Q

How many spheres (atoms) and holes are there in a spinel unit cell?

A

32 spheres (atoms) per UC
- 64 tetrahedral holes (2:1)
- 32 octahedral holes (1:1)

31
Q

What different types of spinels are there? What do they mean?

A

2-3 spinels
- A cation is 2+, B cations is 3+

4-2 spinels
- A cation is 4+, B cation is 2+

32
Q

How do inverse spinels differ to regular spinel structures?

A

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
Q

What is the cation distribution in regular spinels?

A

8 A in 1/8 Tet holes

16 B in 1/2 Oct holes

34
Q

What is the cation distribution of inverse spinels?

A

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
Q

What factors determine spinel or inverse spinel structure?

A

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
Q

How do you determine the octahedral site preference due to CFSE of a TM cation?

A

Octahedral site preference energy (OSPE)

OPSE = CFSEoct - CFSEtet (for each cation)

Greater ∆Oct CFSE = greater stability of crystal when in octahedral site

37
Q

How do lithium ion batteries work on charging and discharging power?

A

On charging, Li+ diffuses through the electrolyte from the cathode to the anode - the reverse occurs on discharge

38
Q

What key factors determine the usability of a lithium ion battery?

A

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
Q

LiCoO2 is the most common cathode material, why?

A

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
Q

Whats an issue with LiMn2O4 for use as a cathode?

A

they have poor cycling stability partially due to Jahn-Teller distortion from Mn3+
- also undergoes side reactions with electrolyte

41
Q

What’re the benefits of using LiMn2O4 as a Li+ ion cathode?

A

Man is cheaper and less toxic than Co

Also have higher thermal runaway temperatures

42
Q

When are perovskites often formed?

A

Often formed when the difference in size of A and B cations is large

43
Q

What’d the general formula of perovskites?

A

ABX3

44
Q

What combinations of cations can be used in perovskites?

A

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

45
Q

Whats the structure of perovskites?

A

A and X ions form FCC array

B cations reside in octahedral holes

46
Q

What factors cause distortion in perovskites?

A

Charge, size & coordination/geometry

47
Q

Why does distortion occur in perovskites?

A

The structure distorts to better accommodate the preferences of the ions

The composition and stoichiometry is unchanged

48
Q

What can be used to determine the extent of distortion in perovskites?

A

The Tolerance factor, t

49
Q

What’re the thresholds for the tolerance factor?

A

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
Q

What happens when a tolerance factor of a crystal falls outside the thresholds?

A

The compound is unstable

51
Q

What is the tolerance factor based on?

A

The tolerance factor is only based on ion size
- no account for electrostatic interactions

52
Q

Whats the significance of lead perovskites?

A

Lead-containing perovskites have important technological applications which relate to their dielectric, ferroelectric and piezoelectric (converts mechanical stress to electrical energy) properties.

53
Q

How is polarisation of lead-containing perovskites achieved?

A

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
Q

Whats the problem with Pb-containing perovskites?

A

Lead is highly toxic and affects the environment

This makes Pb-free ferroelectrics a priority target for new materials

55
Q

Why have organic-inorganic halide perovskite seen research interest recently?

A

Because of their potential applications as absorbers in solar cells

(>22% energy conversion from solar energy - 3.8% in 2009)

56
Q

How can the band gap in organic-inorganic halide perovskites be tuned?

A

By varying alkyl chain length

57
Q

What is the chemical formula of YBCO?

A

YBa2Cu3O7-∂

(Where ∂ = sigma)

58
Q

Whats the significance of YBCO?

A

It is the first high temperature superconductor

Tc = 90K for ∂=0

59
Q

What does changing the oxygen content (∂) in YBCO do?

A

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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