Structures of Ionic Solids - Practice Questions & MCQ

NaCl (Rock Salt)

• In it CI^- ions have CCP structure that is, FCC and Na⁺ ions occupies octahedral holes (voids) here.
• Both Na⁺ and CI^- have co-o-ordination number six. The number of formula units per unit cell is four. Example, halides of alkali metals (except CsX) like LiX, NaX, KX, other halides like AgCl, AgBr, NH₄Cl oxides like TiO, FeO, NiO.
• r_e + r_a = a/2  r_Na+/r_cl- must be 0.414 \text however, it is 0.525.

CsCl

• Here Cl^- ions are at the comers of a cube and Cs⁺ ions are in the cubic void (centred position) i.e. BCC like structure.
• Here co-ordination number of both Cs⁺ and Cl^- is 8 and here the number of formula units per unit cell is 1. r_e + r_a = √3a/2.
• Here r_Cs⁺/r_Cl⁺ should be 0.732 but it is 0.93, e.g. CsX, TiBr, NH₄Cl, NH₄Br
  
  

ZnS (Zinc-Blende)

• Here S^2- occupies CCP or FCC arrangement while Zn²⁺ ions occupy alternate tetrahedral voids.
• Only half of the total voids are occupied.
• Here co-ordination number of both Zn²⁺ and S^2- is four and here the number of formula units per unit cell is four. For example, ZnS, CuBr, Cul, Agl, etc.
  r_e + r_a = √3a/4
  

CaF₂ (Fluorite Structure)

• Here Ca²⁺ occupies CCP and F^- ions occupy all the tetrahedral voids.
• Here coordination number of Ca²⁺ is 8 while for F^- it is four and the effective number of Ca²⁺ and F^- ions in a unit cell are 4 and 8 respectively, here the number of formula units per unit cell is four.
  r_e + r_a = √3a/4
  

Na₂O (Anti-Fluorite Structure)

• Here negative ions (O^2-) occupy CCP while cations Na⁺ occupy all the tetrahedral voids.
• Here co-ordination number of Na' is four while for O^2- it is eight, here number of formula units per unit cell is four and the effective number of Li⁺ and O^2- ions in a unit cell are 8 and 4 respectively.
  r_e + r_a = √3a/4
  

Normal Spinal Structure (XY₂O₄)

• Spinal is MgAl₂O₄, and spinals, in general, have the formula AB₂O₄. Here A is divalent cation (Mg²⁺, Ca²⁺) and B is trivalent cation Al³⁺.
• Here Mg²⁺ occupy 1/8th of tetrahedral voids while oxide ions 1/ 8 of Al³⁺ occupy 1/2 of the octahedral voids.
• As magnetic materials, these are used in telephones, memory loops of computers.

Inverse Spinal Structure (Fe₃O₄, Magnetite)

• In Fe₃O₄, Fe³⁺ and Fe²⁺ are present in 2:1 ratio.
• Here oxides ions are in CCP while Fe²⁺ ions occupy 1 octahedral voids while Fe³⁺ occupy 1 octahedral and 1 tetrahedral voids.
• Here O^2- ions form the FCC unit cell.
• Here the formula ratio of
  Fe²⁺ : Fe³⁺ : O^2- = 1 : 2 : 4
• MgFe₂O₄, Pb₃O₄, Mn₃O₄ have also this type of structure.

Effect of T and P on Crystal Structure

• On increasing the temperature of the CsCl structure coordination number decreases from 8:8 to 6:6.
  
    8:8                 6:6
• On subjecting the NaCl structure to high pressure it will increase coordination number from 6:6 to 8:8.
  
    6:6                                          8:8