MSE 605, Crystallography & Crystal Chemistry
Name: _______________________________
Homework 4
Due 15 October 2015
The numbers following each question give the approximate percentage of marks
allocated to that question. SHOW ALL YOUR WORK!
1. Explain how network modifiers change the network structure of oxide glasses and
what changes can result in the properties of the glass from different network
modifiers.
Autumn 2015
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MSE 605, Crystallography & Crystal Chemistry
2. Draw representative parts of two-dimensional {2,3}, {3,2}, {3,3}, and {4,2} networks.
In each case, calculate the closure.
What can you determine about the topology of these networks (does sufficient
freedom exist to form an amorphous structure)?
Autumn 2015
32
4
MSE 605, Crystallography & Crystal Chemistry
3. Consider SiO2.
a. Determine the cation and anion charges (ZC and ZA) then use the univalent ionic
radii (rC and rA) and Pauling’s rules to find the coordinations (NC and NA) and the
electrostatic bond valence strength, s, of Si-O bonds.
b. Use Pauling’s 1960 equation to calculate the percent ionicity (P) of these bonds.
c. The Si-O bond length is 1.62 Å. Calculate the bond valence sums for both Si
and O in this structure. Use bond-valence parameters given at:
http://www.iucr.org/__data/assets/file/0006/81087/bvparm2013.cif.
Is Si overbonded or underbonded? What about O?
d. What sort of network does SiO2 form (i.e., what is the correct {m,n} notation)?
e. Considering your answers above, sketch part of a three-dimensional SiO2
network.
f. Calculate the topological degree of freedom SiO2 has to form an amorphous
structure.
g. How might your answer to (e) change if Na+ were added as a network modifier?
Explain.
r(Si+) = 0.65 Å
r(O-) = 1.76 Å
χ(Si) = 1.90
χ(O) = 3.44
Autumn 2015
10
5
8
5
15
5
10