a. `MgO` has the structure of `NaCl` and `TICI` has the structure of `CsCl` . What are the coordiantion number of the ions in `MgO` and `TICI`?
If the closed-packed cations in an `XY`-type solid have a raidus of `73.2` pm, what would be the maximum and minimum sizes of the anions filling voids?
c. `Fe_(2)O_(3)` (haematite) forms ccp arrangement of `O^(2-)` ions with `Fe_(3+)` ions occupying initerstitial positions. Predict whether `fe^(3+)` ions are in the `OV` or `TV`. Given `r_(Fe^(3+)) = 0.7 Å` and `r_(O^(2-)) = 1.4 Å`
d. A solid `XY` has `CsCl`-type structure. The edge length of the unit cell is `400 pm` Calculate the distance of closest approach between `X^(o+)` and `Y^(ɵ)` ions.

Answer

a. i. `CN` of `Mg^(2+) = 6`
`CN` of `O^(2-) = 6` ii. `CN` of `Tl^(o+) = 8`
`Cn` of `Cl^(ɵ) = 8`, b. `100 pm (min), 176.81 pm (max)` c. `OV_(s)`, `346.4 pm`

Explanation

`NaCl`-type structure
i. It mean `6 : 6` coordination
`:. CN` of `Mg^(2+) = 6`
`CN` of `O^(2-) = 6`
ii. `CsCl`-type structure.
It means `CN = 8 : 8`.
`:. CN` of `TI^(o+) = 8`.
`CN` of `Cl^(ɵ) = 8`
b. For close-packed `XY` type solid,
The radius ratio `(r_(o+)//r_(ɵ))` lies in the range of `0.414-0.732`.
`:.` Minimum value of `r_(ɵ) = (r_(o+))/(0.732) = (73.2)//(0.732)` pm
`= 100` pm
Maximum value of `r_(ɵ) = (r_(+))/(0.414) = (73.2)/(0.414)` pm
`176.81` pm
`r_(Fe^(3+))/r_(O^(2-)) = (0.7)/(1.4)Å = 0.5` which lies in the range of
`0.414-0.732`. Hence, `Fe^(3+)` ion will be in `OV_(s)`
For bcc-type `(CsCl)` structure
`r_(o+) + r_(ɵ) = (sqrt3)/(2) a = (1.732)/(2) xx 400 pm = 346.4` pm