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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.

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