In a NaCl crystal, a single Na⁺ vacancy (a missing Na⁺ ion) is detected. If only ONE additional vacancy (no interstitials) can form, which vacancy would best restore overall electrical neutrality to the lattice?
a) Creation of a second Na+ vacancy adjacent to the first one.
b) Creation of a Cl− vacancy elsewhere in the lattice.
c) Removal of a neighbouring NaCl ion pair, leaving both sites vacant.
d) Creation of a vacancy by removing a neutral Na atom from the crystal surface.