Question

which point defect lowers the density of a crystal

Answer 1

Its Schottky defect which lower the density of crystal.

Answer 2

Schottky defect lowers the density of a crystal.

It is a point defect in ionic crystals which arises when oppositely charged ions leave their lattice sites, creating vacancies.

To maintain an overall neutral charge in the ionic solid, these vacancies are formed in stoichiometric units.

Then the atoms in the surrounding move in to fill these vacancies. As a result of which new vacancies are formed.

In non-ionic crystals it is also known as lattice vacancy defect.