CHE-30043 Materials Chemistry & Catalysis : Solid State Chemistry lecture 1 Rob Jackson LJ1.16, 01782 733042 r.a.jackson@keele.ac.uk www.facebook.com/robjteaching @robajackson

Supplementary (detailed) 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-30043 lecture 1

Lecture 1 contents Key concepts of basic crystallography Refer to che-20031 slides Bonding in different structures Metals, ionic structures, covalent structures, molecular structures Correlation of structure & bonding with properties Electrical conductivity in solids che-30043 lecture 1

Key concepts of basic crystallography Important concepts from che-20031: Unit cells Crystal class Lattice type Bravais lattices Review these! che-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

Other metal structures: hcp and ccp http://www.seas.upenn.edu/~chem101/sschem/metallicsolids.html che-30043 lecture 1

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-30043 lecture 1

The diversity of structures found http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture2/Lec2.html che-30043 lecture 1

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-30043 lecture 1

NaCl and CsCl structures compared Note: CsCl is not BCC – why? che-30043 lecture 1

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-30043 lecture 1

The fluorite structure: cations at alternate cube centres (shown for MF2) che-30043 lecture 1

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. che-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

(atmospheric pressure) The solid CO2 structure 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) che-30043 lecture 1

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-30043 lecture 1

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-30043 lecture 1

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) che-30043 lecture 1

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? che-30043 lecture 1

Electron conductivity in metals – (i) 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 che-30043 lecture 1

Electron conductivity in metals – (ii) Why does the electrical conductivity of a metal reduce with increasing temperature? The cations move about their lattice positions and ‘get in the way’ of the electrons. che-30043 lecture 1

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. che-30043 lecture 1