Question

3. (a) VO, TiO and NiO all have defect rock salt structures. Explain what this statement means. (b) In NiO, the Ni-O internuclear separation is 209 pm. Determine the volume of a unit cell of NiO and its density (assume a non-defect structure). Given that the observed density of NiO is 6.67 g cm-3 , determine the extent of vacancies in the NiO lattice. Express your answer as a percentage.

Answer 1

NiO : NaCl structure

side (a) of which = 2(rNi + rO) = 419 ×10^-12 m = 4.19×10^-8 cm
a^3 = 7.356×10^-23 cm^3 [The O ions do not touch across the face diagonal]
Z = 4 MX units for a NaCl lattice
Mass ( inside cell) = Z×(58.69 +16.00)/6.022×10^23 = 4.961×10^-22 g
D = 4.961×10^-22 /7.356×10^-23 = 6.74 g cm^3
The deficiencies are usually NiOx (x = <1.0) but I guess 6.67/6.74 = 98.9% or 1.1% vacancies
or [Z×(58.69 +x16.00)/6.022×10^23] /7.356×10^-23 = 6.67 g cm^-3 where x is the fraction of O sites occupied

x = 6.67