[lrw1793]

The structures of the oxides and sulphides of Ca, Ni and Cd are given
Rocksalt: CaO, CdO, NiO, CaS
ZnS (zinc-blende/wurtzite): CdS
nickel arsenide: NiS

Explain fully the variation in structures making use of the following ionic radii (Å) (for 6-coordination):
Ni2+    0.69
Ca2+   1.00
Cd2+   0.95
O2-     1.40
S2-     1.84
 
I thought this was to do with r+/r- ratio but that doesn't seem to work so I'm stumped as to how to begin working this one out.