Question

1. In rock-salt phases, cations are more mobile than the anions. (19/22 pts) a) For the 2D slices of the rock salt structure shown below, sketch and identify the 2 types of intrinsic point defects possible for pure rock salt phases. (5 pts) b) What effect, if any, do you expect an increase in temperature to have on the conductivity and c) Consider rock salt phases with composition MgO, NaF and NaCl. Which of these do you expect d) Small amounts of impurities can impact the conductivity. Consider AgCI, MgCl2 and Na2O why? (2pts) to have the highest and lowest ionic conductivity and why? (6 pts) impurities in the archetypal rock salt phase NaCl. Which would you expect to have the largest and the least impact on the conductivity? In each case, explain your reasoning. (6 pts)

Answer 1

a)ans:

Rock salt has two possible intrinsic point defect which arise due to vacancy of cation site

1. Schottky defect 2. Frenkel Defect

anion cation Schottky defect Cation and anion are vacant at denoted site Frenkel Defect Cation changes fromo cation site intersticial site creating vacancy at cation lattice

b) Ans. The conductivity of ionic conductor increases with increase in temperature.

The expression for variation of ionic conductivity is

$\sigma =\frac{\sigma _{0}}{T}exp\left ( \frac{-E_{a}}{T} \right )$

At low T conductivity is dominated by mobility of extrinsic defects and at High T, conductivity is due to thermally formed (intrinsic) defects.

With increase in T, the mobility of ions increases hence, conductivity will increase.