Sinclair- Band Theory and Hopping Flashcards

1
Q

General requirement for high conductivity in TMOs

A

Need mixed metal valence TMs. Like Cu2+/Cu3+

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Methods of getting mixed valency in TMOs

A

Process in an oxidising or reducing atmosphere. Chemical doping

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Oxidation method for mixed valency

A

Sample gains oxygen to become non-stoichiometric. Example NiO becomes NiO1+δ when processed at 1500C in air. Get Ni2+ (d8) and Ni3+ (d7). Goes from pale green insulator to black semiconductor. Right of d block tend to be oxidised

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Reduction method for mixed valency

A

Sample loses oxygen to become non-stoichiometric. Example TiO2 goes to TiO2-δ when processed at 1500C in Ar (inert atmosphere). Get Ti3+ (d1) and Ti4+ (d0). Goes from white insulator to blue semiconductor. Left of d block tend to be resuced

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Normal difference in radii allowed for doping

A

About +/-15%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What does aliovalent doping mean?

A

Dopant has different charge to host ion. Need for doping method for mixed valency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Acceptor doping for mixed valency

A

Dopant ion of lower charge. E.g Li+ doping of NiO turns it into a black semiconductor. Get mixed Ni2+ and Ni3+. Hole doping, p-type conductor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Donor doping for mixed valency

A

Dopant ion of higher charge. E.g Nb5+ doping of Ti site in StTiO3 can induce semiconductivity. Get mixed Ti3+ and Ti4+. Electron doping, n-type conductor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is a hopping semiconductor?

A

One in which electrons or holes can hop between ions of the same type but different valence.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How is conductivity of hopping semiconductor influenced by temperature?

A

σ=nqμ still holds. Hopping is thermally activated so mobility of charge carrier is temperature dependent (contrast to band model where μ of charge carriers has little temperature dependence).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What happens when NiO heated in air?

A

Normally rocksalt structure. It picks up O2 molecules which adsorb on the surface, dissociate and pick up 2e- to form O2-. The electrons come from oxidation of Ni2+ to make Ni3+ (d7). To partially balance extra layer of O2- ions at surface Ni2+ ions migrate leaving cation vacancies throughout the crystal. Each O2- generated creates one Ni vacancy somewhere and two Ni3+ ions.
Ni(1-x)O becomes Ni2+(1-3x)Ni3+(2x)O

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How does hopping conduction work in NiO after heating in air?

A

Electrons hop from Ni2+ to an adjacent Ni3+ (or holes hop in opposite direction. Electrons localised on Ni2+ so concept of conduction and valence bands doesn’t apply. Main process
Ni2+ -> Ni3+ + e-. Activation energy needed to cause hopping. Number of charge carriers, n, is [Ni3+]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

NiO heated in air graph of lnσ vs 1/T

A

Straight line negative gradient. Slope -E/k

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Why is it difficult to control degree of oxidation for NiO heated in air?

A

Depends on oxygen partial pressure, temperature and solid/gas equilibria/kinetics. Means n varies and therefore so does σ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How to control oxidation of NiO

A

Dope with Li+ to obtain Ni(1-x)LixO.
2Ni2+ -> Ni3+ + Li+. Compound becomes
Ni2+(1-2x)Ni3+(x)Li+(x)O
Means [Ni3+] and σ controlled by x so Li+. It forms a solid solution with rocksalt structure and Ni3+, Ni2+ and Li+ distributed at random over crystal octahedral sites. Fixed x at fixed field E means n constant. μ is lowish and Ea 0.2 to 1eV

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Why can Li+ doped NiO be used for thermistors?

A

σ=f(T). Can use its lnσ vs 1/T graph. Measure conductivity and read off temperature. σ reproducible over 1000s of cycles

17
Q

Polaron model

A

Polaroid Ar due to interaction of mobile electron (or hole) with lattice ions which cause localised lattice distortions. When distortion sufficiently strong, electron or hole may be trapped at a particular lattice site via polarisation and can only move by thermally activated hopping through the solid.

18
Q

Polarons in Fe3O4

A

Fe2+ (d6) next to Fe3+ (d5). Fe2+ is larger ion so has longer bond lengths with O. Electron can move from Fe2+ to Fe3+ much quicker than cation and anion movement. The hopping e- can polarise the surrounding lattice. As electron moves through the solid the distortion follows it. This lower mobility of the e- and increases its effective mass m*. Is called a polaron and can be considered as a quasi-particle

19
Q

The two extreme cases for polarons

A

e- localised on either metal ion
e- delocalised between metal ions
An oxygen separates the adjacent metal ions

20
Q

Energy vs distortion coordinate, D, graph

A

Two y=x^2 curves. One has minimum on right and the other on the left but both same level above x axis. The curves cross at the y axis. Activation energy height difference between minimum and point of intersection. Means Ea is proportional to D^2 and small changes in ionic radii have dramatic effect on Ea for hopping conduction

21
Q

What is distortion coordinate equal to?

A

Difference in ionic radius between the two metal ions

22
Q

Normal and inverse spinel structures

A

Normal is [A][B2]O4 where A is 2+ and in tet sites, B is 3+ and in oct sites.
Inverse is [B][A,B]O4 where A is 2+ and in half of oct sites, B is 3+ with half occupying all tet sites and half occupying half of oct sites.
Example inverse is [Fe3+][Fe2+,Fe3+]O4
Octahedral sites always half full and tetrahedral sites 1/8 full

23
Q

What happens if the localised state for a polaron is more stable?

A

The e- is effectively trapped by the local distortion it creates around a given lattice site. This is the small polaron model for an e- in a metal orbital in an oxide. The extra electron changes the oxidation state of the atom in the solid. This changes the ionic radius which produces a local distortion that can trap the charge l

24
Q

How to identify distinct M2+ and M3+ ions

A

Use x-ray absorption spectroscopy (XAS) to examine the crystallography.

25
Q

What happens if the e- or h+ polaron can become delocalised?

A

All ions have some fraction of oxidation state. For Fe3O4 all ions on oct sites appear identical as Fe2.5+ instead of distinct Fe2+ and Fe3+

26
Q

Phase transition in Fe3O4

A

Structural phase transitions often induce changes in electrical properties. Changes from an insulator to a semiconductor at 120K and conductivity increases by 3 orders of magnitude. At higher temperatures all Fe-O bonds in oct sites are the same Fe2.5+-O and there is easy e- hopping process and is semiconductor. At lower T cooperative lattice distortions create distinct oct sites so distinct Fe2+-O and Fe3+-O bonds, e- hopping difficult, insulator

27
Q

Why is inverse spinel Fe3O4 a semiconductor/metal and normal spinel Mn3O4 a poor semiconductor/insulator?

A

In inverse Fe3O4, there are short distances between the oct sites which have Fe2+ or Fe3+ so the hopping
Fe2+=Fe3+ + e- is easier
In normal Mn3O4the distance between tet and oct sites is long so the hopping
Mn2+ = Mn3+ + e- is harder