chem584.ionicsolids - Structures & Properties of Ionic...

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1 1 Ionic Radii and Ionic Solid structures Point defects: How are they different from those in metals? Impurities: How are they accommodated in the lattice? How do they affect properties? Mechanical Properties: What special provisions/tests are made for ceramic materials? Structures & Properties of Ionic Solids 2 Textbooks: Solid State Chemistry 1. Shriver, Atkins, Inorganic Chemistry (3rd ed, 1999) W.H. Freeman and Company (Chs. 2, 18 . ..). 2. A.R. West, Basic Solid State Chemistry (2nd ed. 1999) Wiley, New York, 1999. 3. A. R. West, Structural Inorganic Chemistry , 5 th ed., Oxford University Press, Oxford, 1984. 4. U. Muller, Inorganic Structural Chemistry, 2 nd ed., Wiley, NY, 2006.
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2 3 The Ionic Model of Solids The 'Ionic Model' (Goldschmidt') Ions are essentially Charged, Incompressible, Non-Polarizable Spheres More sophisticated model: a central hard, unperturbable core, where most electron density is concentrated a soft, polarizable outer sphere, which contains very little electron density Pauling's Rules: Goldschmidt's structural principles for ionic crystals were summarized by Pauling in a series of Rules. 4 Pauling Rule 1: Coordination Polyhedra A coordination polyhedron of anions is formed around every cation (and vice-versa): Only stable if cations are in contact with each neighbors. (i.e., “Tight packing”) Ionic crystals may thus be considered as sets of linked polyhedra. The cation-anion distance is regarded as the sum of the ionic radii.
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3 5 Pauling Rule 1: Coordination Polyhedra Common Coordination Polyhedra: 6 Pauling Rule 1: Coordination Polyhedra The Coordination Number of the Cation will be Maximized subject to the criterion of Maintaining Cation-Anion Contact. Determined by comparison of the ratio of the ionic radii, r+/r- with values derived from the geometric contact criterion: The Radius Ratio Rules are correct ONLY ~50% of the time!
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4 7 Pauling Rule 2: Local Electroneutrality Stable ionic structures preserve Local Eletroneutrality. Iions in a crystal are surrounded by ions of opposite charge so as not to produce large volumes of similar charge in the crystal - this maximizes Madelung potential 8 Pauling Rule 3: Polyhedral Linking The stability of structures with different types of polyhedral: vertex-sharing > edge-sharing > face-sharing 1. effect is largest for cations with high charge and low coordination number 2. especially large when r+/r- approaches the lower limit of the polyhedral stability Why?
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chem584.ionicsolids - Structures & Properties of Ionic...

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