$\mathrm{BaCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}$
Interstitial water molecules can be found in $\mathrm{BaCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}$ salt. Between the first and second solvation shells, water molecules are primarily trapped. It can be found in the crystal lattice's interstitial locations. Two interstitial water molecules in barium chloride dihydrate.