Question

3) Respond the following:

a)Describe a Schottky Defect in Li2O

b)Describe a Frenkel Defect in NiO

c)How would you expect the number of vacancies per unit volume to change in a metal as the temperature increases?

d)Explain why the strength of a simple thermoplastic polymer, such as a polyvinyl chloride, is expected to be very low compared to a thermoset polymer.

e) Would it be easier to crystalize polytetrafluoroethylene (teflon) (C2F4) or polystyrene (C8H8)? Why?

Answer 1

(a) Schottkey defect in Li₂O:

Schottky defects consists of unoccupied anion and cation sites in a stoichiometric ratio. For a simple ionic crystal of type A?B+, a Schottky defect consists of a single anion vacancy (A) and a single cation vacancy (B), or

v•A + v?B following Kröger–Vink notation. For a more general crystal with formula AxBy, a Schottky cluster is formed of x vacancies of A and y vacancies of B, thus the overall stoichiometry and charge neutrality are conserved.

Schottky defects are observed most frequently when there is a small difference in size between cations and anions. This is produced as the result of the thermal incorporation of unoccupied lattice sites from the exterior of the crystal. The lattice undergoes thermal vibration and thermal expansion when the temperature is raised above 0 K. When it happens the pair of vacancies are incorporated in the crystal. So electrical neutrality is maintained inside the crystals.

•LiI has relatively low ionic conductivity, but was used in heart pacemaker batteries in the early 1970’s, where a low current, small, long lasting, and generate no gases during discharge. A B C Li // LiI // I2 and polymer where the cathode is a conducting polymer with embedded iodine, poly-2-vinyl-pyridine •Electrode reactions are:

2Li+(s)+ 2e- Cathode

Anode A: 2Li(s)   2I-à(s) + 2e- (s)

•LiI contains intrinsic Schottky defects and the small Li+ cations are able to pass through the solid electrolyte, while the electrons travel through the circuit to perform work.