NaCl (Rock Salt) Structure

• $Cl^{-}$ forms FCC lattice (Z = 4)
• $Na^{+}$ occupies ALL octahedral voids (4 voids → 4 $Na^{+}$)
• Formula per cell: 4 NaCl
• CN = 6:6 (octahedral, each ion surrounded by 6 of the other)
• Radius ratio: 0.414 ≤ $r^{+}$/$r^{-}$ ≤ 0.732
• Examples: NaCl, KCl, MgO, CaO, FeO

CsCl Structure

• $Cl^{-}$ at 8 corners of cube; $Cs^{+}$ at body centre
• CN = 8:8 (cubic coordination)
• Z = 1 formula unit per cell
• Radius ratio: $r^{+}$/$r^{-}$ > 0.732
• Examples: CsCl, CsBr, CsI
• Note: NOT a true BCC (different ions at corners vs body centre)

ZnS (Zinc Blende) Structure

• $S^{2-}$ forms FCC lattice (Z = 4 → 8 tetrahedral voids)
• $Zn^{2+}$ occupies HALF (4) of tetrahedral voids (alternate voids)
• CN = 4:4 (tetrahedral)
• Radius ratio: 0.225 ≤ $r^{+}$/$r^{-}$ ≤ 0.414
• Examples: ZnS, CuCl, SiC

Fluorite ($CaF_{2}$) and Antifluorite ($Na_{2}O$)