Question

Name the crystal defect which lower the density of an ionic crystal

Answer 1

the crystal defect which lower the density of an ionic crystal is Schottky defect.
this defect is usually found in strong ionic compounds which have
higher coordination number
and smaller size of ions

Answer 2

The crystal defect that lowers the density of ionic crystal is "schottky defect" 
This defect is created when opposite charged ions leave their sites and thus creating vacancy