Question

In an `LiI` crystal, `I^(ɵ)` ions from a cubical close-packed arrangement, a nd `Li^(o+)` ion occupy octahedral holes. What is the relationship between the edge length of the unit cells and radii of the `I^(ɵ)` ions if `a = 60` "pm".

Answer 1

Cubical close packing imlied that the unit cell of the `I^(ɵ)` ion is fcc, and for that (from Table `1.12` and Fig. `1.67`)
`r_(o+) + r_(ɵ) = a//2 = (600)/(2) = 300` pm
From `OV`,
`(r_(o+))/(r_(ɵ)) = 0.414`
`r_(o+) = 0.414 xx r_(ɵ)`
`= 0.414 xx 300 = 124.2` pm