In CaF2 crystal ca2+ ions are present in fcc arrangement.Calculate no of F- ions in unit cell if they occupy all tetrahedral voids.

Asked by prateek vaish | 25th Jul, 2013, 02:01: PM

Expert Answer:

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. 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. Each Ca+2 ion is surrounded by 8F- ions and each F- ions is surrounded by 4Ca+2 ions.

No. of Ca+2 ions in unit cell = 8(at corners) x 1/8 + 6 (at face centres) x ½ = 4

No. of F ions in unit cell = 8 (within the body) x 1 = 8

Answered by  | 30th Aug, 2013, 11:05: AM

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