E03mtm - Engineering 45 The Structure and Properties of...

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Page 1 Engineering 45 The Structure and Properties of Materials Midterm Exam March 19, 2003 Problem 1: (a) Describe the ß-ZnS structure and show how it is related to the FCC structure. (b) The ß-ZnS structure is adopted by many covalently bonded compounds, like GaAs. Why would this structure be favored? [Ga has valence 3, As has valence 5.] (c) The ß-ZnS structure is also adopted by some ionic compounds, such as AgI. What features of the ß-ZnS structure makes it suitable for ionic materials? (d) Consider a series of binary compounds (AB) that bond ionically, let R A be the radius of the A ion and R B the radius of the B ion. As the ratio R A /R B decreases from 1.0, the structure tends to change from CsCl to NaCl to ß-ZnS. Why?
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Engineering 45: Spring, 2003 Midterm Exam Page 2 Problem 2: (a) In a given crystal, interstitial species diffuse much more rapidly than substitutional species. Why? (b) Plot, schematically ln(D) vs. 1/T for an interstitial and a substitutional species
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E03mtm - Engineering 45 The Structure and Properties of...

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