ANTIFLUORITE STRUCTURE (REVERE OF FLUORITE) Na2O:

The compound having A₂B formula are compounds having anti fluorite structure :

Anti fluorite structure is having arrangement of cations and anions opposite to the fluorite structure Li₂O has an anti fluorite structure.

(1)  In the crystal structure of Li₂O, the O^-2 ions constitute a cubic close packed lattice (fcc structure) and the Li⁺ ions occupy all the tetrahedral voids

(2)  Each oxide ion, O^-2 ion is in contact with 8 Li⁺ ions and each Li⁺ ions having contact with 4 oxide ion. Therefore, Li₂O has 4:8 coordination
(3) Stoichiometric ratio  of Na₂O is 2:1
(4)  radius ratio , Packing efficiency, density and void %:

 Other Examples: – Na₂O, K₂O, K₂S, Na₂S, Rb₂O, Rb₂S

Note: Metals like Al, Ag, Au, Cu, Ni, and Pt have ccp structure and Be, Mg, Co and Zn have a hcp structure. And Noble gases (except He has a hcp structure) have ccp structure.

CALCIUM FLORIDE (FLORITE) STRUCTURE:

The salient features of fluorite structure are:

(1)  The Ca⁺² ions are arranged in ccp arrangement, i.e. these ions occupy all the corners and the centres of each face of the cube

(2)  The F^– ions occupy all the tetrahedral holes.

(3)  Since there are two tetrahedral holes for each Ca⁺² ion and F^- ions occupy all the tetrahedral holes, there will be two F^- ions for each Ca⁺² ions, thus the stoichiometry of the compound is 1:2.

(4) Each Ca⁺² ion is surrounded by 8F^- ions and each F^- ions is surrounded by 4Ca⁺² ions. The Coordination number of Ca⁺² ion is eight and that of F^- ion is four, this is called 8:4 Coordination.

 (5) Each unit cell has 4 calcium ions and 8 fluoride ions so formula of unit cell is Ca₄F₈ which is explained as  below

      No. of Ca⁺² ions = 8(at corners)´1/8 + 6 (at face centres)´1/2

      No. of F ions = 8 (within the body)´1 = 8

      Thus the number of CaF₂ units per unit cell is 4.

(6) Radius Ratio, packing efficiency ,void % and density:

Other examples: of structure are SrF₂, BaCl₂, BaF₂, PbF₂, CdF₂, HgF₂, CuF₂, SrCl₂, etc.