CHE-30043 Materials Chemistry & Catalysis : Solid State Chemistry lecture 1 Rob Jackson LJ1.16,...
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Transcript of CHE-30043 Materials Chemistry & Catalysis : Solid State Chemistry lecture 1 Rob Jackson LJ1.16,...
CHE-30043 Materials Chemistry & Catalysis :Solid State Chemistry lecture 1
Rob JacksonLJ1.16, 01782 733042
[email protected]/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))
che-30043 lecture 1 3
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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