CHE-30043 Materials Chemistry & Catalysis :Solid State Chemistry lecture 1

Rob JacksonLJ1.16, 01782 733042

r.a.jackson@keele.ac.ukwww.facebook.com/robjteaching

@robajackson

Background reading

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Recommended‘Reactions and Characterisation of Solids’

Sandra Dann (RSC Tutorial Chemistry Text No 2 (2000))

Supplementary (detailed)‘Solid State Chemistry: An Introduction’

Lesley Smart, Elaine Moore (4th edition, CRC Press (2012))

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Lecture 1 contents

1. Key concepts of basic crystallography– Refer to che-20031 slides

2. Bonding in different structures– Metals, ionic structures, covalent structures,

molecular structures

3. Correlation of structure & bonding with properties

4. Electrical conductivity in solids

Key concepts of basic crystallography

• Important concepts from che-20031:– Unit cells– Crystal class– Lattice type– Bravais lattices

• Review these!

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Metal Structures - Characteristics

• Most adopt close-packed structures.• Bonding is non-directional.• This is reflected in mechanical

properties.• Cations are close packed and

surrounded by a ‘sea’ of valence electrons – electrical conductors.

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Examples of metal structures

• Alkali metals tend to adopt the body centred cubic (BCC) structure:

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/metals.html

2d view

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Other metal structures: hcp and ccp

http://www.seas.upenn.edu/~chem101/sschem/metallicsolids.html

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Ionic structures - characteristics

• Structures minimise ionic repulsion.• Structures influenced by ionic radii.• Strong directional bonding.• Valence electrons are involved in ionic

bonding (electron transfer).• Electrical insulators.

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The diversity of structures found

http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html

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Some ionic structures - 1

We will consider a few examples to illustrate the diversity of structures.

• e.g. the metal halide structures, where MX can have at least 4 different structures depending on M and X.

NaCl and CsCl – why different?

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NaCl and CsCl structures compared

Note: CsCl is not BCC – why?

CsCl

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Some ionic structures - 2

• MX2 also has several forms, but an important structure is the fluorite structure, named after the mineral, fluorite, CaF2.

• This is an important structure adopted by many technologically important materials, including ZrO2 and UO2.

The fluorite structure: cations at alternate cube centres (shown for MF2)

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Covalent Structures

• Covalently bonded materials.• Strong bonds – can lead to structures of

high strength.• Valence electrons involved in shared

covalent bonds.• Electrical insulators or semiconductors.

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Examples of covalent solids

• The diamond phase of carbon is a good example.

• Each C atom is bonded to 4 others through sp3 hybrid bonding orbitals, giving a 3-dimensional network

• Other examples include Ge, Si, B, P, As, Se, Te.

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Carbon phases – bonding and properties

• Diamond – rigid 3-dimensional (sp3) covalent network with all bonds equivalent.

• Graphite – layer structure – covalent (sp2) bonding within each layer, and relatively weak van der Waals forces between the layers. This gives the material its distinct properties.

• What about C60?

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Diamond and graphite structures compared

Diamond structurecontinuous 3d network

Graphite structure2 dimensional layer structure

http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/diamond.htm

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Buckminsterfullerene (C60)

• From the structure, how many C atoms is each atom bonded to?

• From this information, what type of hybridisation would you expect for the C bonding orbitals?

http://www.chm.bris.ac.uk/motm/buckyball/c60a.htm

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Molecular Solids

• Solids formed from molecules which retain their identity and shape in the solid

• Structures held together by van der Waals forces and electric multipole moments (depends on symmetry)

• Bonding is weak – low melting points

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Examples of molecular solids

• Classic example is solid CO2, ‘dry ice’

• Linear O=C=O molecules in FCC lattice• Structure ‘melts’ (sublimes*) at low

temperatures, indicating that the bonding is weak

• Other examples – solid benzene, etc.* goes from solid to gas phase direct

The solid CO2 structure

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• The individual CO2 molecules form an FCC lattice.

• The structure is held together by van der Waals forces.

• Tsub = -78.5°C

(194.5 K)

(atmospheric pressure)

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Example: the structure of solid CCl4

• CCl4 melts at -23C (250K).

• Below this temperature, it forms a simple cubic structure with a CCl4 molecule at each lattice site.

• Molecules retain their identity in the solid.

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Bonding in Solids summarised

• Metals – cations held together by delocalised valence electrons

• Ionic solids – balance of repulsive and attractive interactions

• Covalent solids – covalent bonds between atoms

• Molecular solids – non-bonded electrostatic interactions

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Electrical conductivity in solids

• Does a given solid state material behave as conductor, insulator or semiconductor?

• How is this related to its electronic structure?

• We will separately look at:– Metals– Insulators (ionic solids, covalent solids)– Semiconductors (to be defined later)

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Electrical Conductivity and Temperature

• In a metal, conductivity decreases with temperature. Why?

• In insulators and semiconductors, conductivity increases with temperature.

• Why the difference between metals and insulators/semiconductors?

Electron conductivity in metals – (i)

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http://www.saburchill.com/physics/chapters/0037.html

Electrons in a metal with no current flowing. Valence electrons are free to move, but randomly.

Electrons in a metal with a current flowing. Valence electrons move in the direction of the current

Electron conductivity in metals – (ii)

• Why does the electrical conductivity of a metal reduce with increasing temperature?

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The cations move about their lattice positions and ‘get in the way’ of the electrons.

Summary of lecture

• Key concepts from crystallography have been listed.

• Some metallic, ionic, covalent and molecular structures have been reviewed.

• Electrical conductivity in materials has been introduced, with metals as an example.

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