Solid state – lesson 7 PDF

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SEG Lecture 7...

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Solid state – 12/10/15 Conductance=

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In metals, as soon as you go above 0K some electrons can subsceed the Ef. in semi conductors, as you increase the temperature electrons can move from the valence band to the conduction band. As you increase the temperature you increase the number of electrons that can move into the conduction band. Metals: increasing the temperature increases the resistance. Semi-conductors: increasing the temperature decreases the resistance. This doesn’t fit with the model unless you take into account the vibration of the atoms. Filling holes in ionic solids:

Radius ratio rules: - As the ratio of the cation and anion radii increases the coordination number of the cation will increase. - Trig shows that if the nearest neighbors touch, then a particular coordination number will be found for a particular range of radii.

- This knowledge can be used to determine the likely coordination number for a salt of a give formula type, based only on ionic radii.

Coordination number: the number of nearest neighbors Ie. How many ions are in contact with the ion in question Coordination geometry: shape of the polyhedron formed by the ions around a given ion. Notation for describing coordination: The short hand we use is to show the coordination number of the cation followed by the coordination number of the anion: Eg. NaCl is (6,6) XY ionic solids: - Radius ration rules predict that the coordination number of a cation increases as it gets bigger (relative to the anion).

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How good are the predictions? - Works best where there is a large difference in electronegativity - Predictions fail when the ionic model breaks down Ionic model: What it does - Provides an understanding of ionic solids - Uses the ionic radius to predict the structures of ionic solids. It uses 4 assumptions: 1. Ions are integrally charges solid spheres, the forces between ions are entirely electrostatic (no covalency). 2. Ions of one charge will be in contact with the nearest neighbours of opposite charge, in order to maximise attractive electrostatic forces. 3. The coordination number of an ion will be the maximum possible. 4. The packing of the atoms will minimise electrostatic repulsions between ions of like charges throughout the entire crystal.

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