To determine the incorrect statement, let's analyze each option:$\\$Option 1:$\\$Schottky defect involves the loss of equal numbers of cations and anions from the lattice,$\\$which decreases the density of the crystal. This statement is correct.$\\$Option 2:$\\$NaCl is an insulator because it does not conduct electricity in solid form.$\\$Silicon is a semiconductor, as it has a band gap that allows it to conduct electricity under certain conditions.$\\$Silver is a conductor due to its free electrons.$\\$Quartz is a piezoelectric crystal, meaning it generates an electric charge under mechanical stress.$\\$This statement is correct.$\\$Option 3:$\\$Frenkel defect occurs when a cation leaves its lattice site and occupies an interstitial site.$\\$This defect is favored in ionic compounds where there is a large difference in size between cations and anions,$\\$not when they are almost equal. This statement is incorrect.$\\$Option 4:$\\$FeO$_{0.98}$ indicates a non-stoichiometric compound with a metal deficiency defect.$\\$This occurs when some $Fe^{2+}$ ions are missing, and to maintain charge balance, some $Fe^{2+}$ ions are oxidized to $Fe^{3+}. \\$This statement is correct.$\\$Therefore, the incorrect statement is Option 3.