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CHE-30005 Solids, Surfaces and Catalysis : Solid State Chemistry lecture 1

CHE-30005 Solids, Surfaces and Catalysis : Solid State Chemistry lecture 1. Rob Jackson LJ1.16, 01782 733042 r.a.jackson@keele.ac.uk www.facebook.com/robjteaching. Background reading. Recommended ‘ Reactions and Characterisation of Solids’ Sandra Dann

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CHE-30005 Solids, Surfaces and Catalysis : Solid State Chemistry lecture 1

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  1. CHE-30005 Solids, Surfaces and Catalysis :Solid State Chemistry lecture 1 Rob Jackson LJ1.16, 01782 733042 r.a.jackson@keele.ac.uk www.facebook.com/robjteaching

  2. Background reading 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-30005 lecture 1

  3. Lecture 1 contents • Revision of basic crystallography: • Crystal structures reviewed • Bonding in solids • Correlation of structure & bonding with properties • Electrical conductivity in solids che-30005 lecture 1

  4. Revision of Crystallography • Unit cells are the ‘building blocks’ of crystals. • They are defined by two properties, the crystal class and the lattice type. • Crystal class defines the relative lengths of the sides of the cell and the angles between them (a, b, c, , , ). che-30005 lecture 1

  5. Cubic a=b=c, ===90° Tetragonal a=bc, ===90° Orthorhombic a bc, ===90° Monoclinic abc, ==90°, 90° Triclinic abc,  90° Hexagonal a=bc, ==90°, =120° Trigonal a=b=c, ==90° Crystal Class (7 in total) Also see http://cnx.org/content/m16927/latest/ che-30005 lecture 1

  6. Lattice Type • 4 lattice types, varying in number of lattice points and their position in the cell: Primitive (P) Body-Centred (I) Face-Centred (F) Side-Centred (A, B or C) che-30005 lecture 1

  7. Bravais Lattices • By combining the 7 crystal classes and the 4 lattice types, 14 Bravais lattices are obtained. • See Dann p11 table 1.4 for a list of Bravais Lattices. • Or see web reference on slide 5. • They are also shown on the next slide: che-30005 lecture 1

  8. The 14 Bravais lattices

  9. 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. che-30005 lecture 1

  10. Examples of metal structures • Alkali metals tend to adopt the body centred cubic (BCC) structure:  2d view http://www.science.uwaterloo.ca/~cchieh/cact/applychem/metals.html che-30005 lecture 1

  11. Other metal structures: hcp and ccp http://www.seas.upenn.edu/~chem101/sschem/metallicsolids.html che-30005 lecture 1

  12. 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. che-30005 lecture 1

  13. The diversity of structures found http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html che-30005 lecture 1

  14. 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? che-30005 lecture 1

  15. NaCl and CsCl structures compared CsCl Note: CsCl is not BCC – why? http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html#anchor4 che-30005 lecture 1

  16. 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. che-30005 lecture 1

  17. The fluorite structure, shown for CaF2 http://www.metafysica.nl/turing/preparation_3dim_3.html An alternative view of the structure will be drawn. che-30005 lecture 1

  18. Covalent Solids • Covalently bonded materials. • Strong bonds – can lead to structures of high strength. • Valence electrons involved in shared covalent bonds. • Electrical insulators or semiconductors. che-30005 lecture 1

  19. 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. che-30005 lecture 1

  20. 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? che-30005 lecture 1

  21. Diamond and graphite structures compared Diamond structure continuous 3d network Graphite structure 2 dimensional layer structure http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/diamond.htm che-30005 lecture 1

  22. 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 che-30005 lecture 1

  23. 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 che-30005 lecture 1

  24. 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 che-30005 lecture 1

  25. 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) The solid CO2 structure che-30005 lecture 1

  26. Example: the structure of solid CCl4 • CCl4 melts at -23C (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. che-30005 lecture 1

  27. 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 che-30005 lecture 1

  28. Electrical conductivity in solids • Do solids behave as conductors, insulators or semiconductors? • How is this related to their electronic structure? • We will look at: • Metals • Insulators (ionic solids, covalent solids) • Semiconductors (to be defined later) che-30005 lecture 1

  29. Electrical Conductivity and Temperature • In a metal, conductivity decreases with temperature. • In insulators and semiconductors, conductivity increases with temperature. • Why the difference? che-30005 lecture 1

  30. Conductors, Insulators or Semiconductors – some questions • Conduction will take place if electrons are free to move through the material. • Metals conduct because the valence electrons are delocalised – but why does conductivity decrease with temperature? • What is the difference between insulators and semiconductors? • To be answered in lecture 2. che-30005 lecture 1

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