eo2mtm.ans - Engineering 45 The Structure and Properties of...

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The Structure and Properties of Materials Midterm Exam March 18, 2002 Problem 1: (a) Draw the face-centered cubic structure (fcc), and identify the positions of the octahedral and tetrahedral voids within it. Straightforward (b) Describe how the NaCl structure is related to fcc. The NaCl structure is a common structure for binary compounds with ionic bonding, but not covalent bonding. Why? The NaCl structure of an AB compound has one element (A) on the fcc sites and the other (B) in the octahedral voids. The structure is a natural for ionic bonding since the nearest neighbors of the A atoms are all B, and the nearest neighbors of the B atoms are all A. It is not favorable for covalent compounds since each atom has six neighbors, making it difficult to saturate covalent bonds by sharing electrons. (c) Describe how the ß-ZnS structure is related to fcc. The ß-ZnS is a common structure for binary compounds that are covalently bonded. Why? The ß-ZnS structure of an AB compound has one element (A) on the fcc sites and the other (B) in one-half of the tetrahedral void sites (need to indicate which ones by drawing, listing site indices or noting that the ABC stacking of fcc becomes AaBbCc where a, b, c indicate the tetrahedral void sites above each atom in the stacking of close- packed planes). It is a common structure for covalent compounds since each atoms has exactly 4 nearest neighbors and, if A and B have an average of 4 valence electrons (valences 4-4, 3-5 or 2-6), one can saturate bonds by sharing electrons among four equivalent covalent bonds to each of the four neighbors. Page 1
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eo2mtm.ans - Engineering 45 The Structure and Properties of...

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