Question

why electrical conductivity increases in schottky defect but does not change in frenkel defect?

Answer 1

The reason behind this is that the electrical conductivity depends on the no. of ions and as we know that in Schottky defects usually anions gets displaced from their original position resulting in increased electrical conductivity.
In frenkel defect the cations are displaced to interstitial sites and thus conductivity remains same.

Hope you get your answer.

Answer 2

Ans:

In Schotky defect, when some ions are missing, due to that the vacancies allows the movement of electrons/ions. Due to that, conductivity increases

(p.s: dont follow the other answer given, its wrong)