Question

which point defect in its crystal unit increases the density of solid

Answer 1

When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have interstitial defect. This point defect increases the density of the substance.

Answer 2

When the molecules or the atom occupy the intestinal site it is said that the crystals experience an increase in the density of the solid.  

The occupation of more than 2 atoms in a crystal lattice in a crystal there occurs a formation of the intestinal defect. This intestinal defect increases the energy flow and makes the lattice highly unstable.  

This high build-up of energy leads to massive accidents for instance in nuclear reactors which leads to Wigner effect.