At room temperature and pressure RbI crystallizes with the NaCl-type structure. (a) Use ionic radii to predict the length of the cubic unit cell edge. (b) Use this value to estimate the density. (c) At high pressure the structure transforms to one with a CsCl-type structure. (c) Use ionic radii to predict the length of the cubic unit cell edge for the high-pressure form of RbI. (d) Use this value to estimate the density. How does this density compare with the density you calculated in part (b)?

Answer 1

NaCl has an fcc structure.
This implies that in an RbI crystal lattice, the distance between cation and anion (ionic radii) is half the diagnol of the unit cell.

Hence, if ionic radii = r

=> √2a = 2r
=> a = √2r

Answer 2

b) d = (Z)(M.M)/(Na)(a^3)

You'll find 'a' in the above part. Note that Z = 4 for an fcc lattice.

c) & d) Repeat the same procedure for CsCl which is a bcc structure.