16
1 – Description of ionic crystals
Exercise 1.2 – Notation for doping reactions
A – MgO doped with Al 2 O 3
1. Write the reaction for doping magnesium oxide MgO with aluminum oxide
Al 2 O 3 for the two following situations:
a. the aluminum occupies an interstitial position,
b. the aluminum substitutes for magnesium.
For each case, give the electroneutrality equation.
2. Given that the cationic radii of magnesium and aluminum are 86 and 68 pm,
respectively, which position (interstitial or substitution) would aluminum
preferentially occupy?
Data
MgO crystallizes in a NaCl-type cubic structure and exhibits a dominant
Schottky-type intrinsic disorder.
B – NiO doped with Li 2 O
Give the reaction for doping nickel monoxide NiO with lithium oxide Li 2 O
in the presence of oxygen, given that the solid solution obtained is a p-type
semiconductor and that the lithium substitutes for the nickel. Write the electroneutrality equation.
Data
Assume that cationic Frenkel disorder is dominant in NiO.
C – La 2 O 3 doped with SrO
1. Give the reaction for doping La 2 O 3 with strontium oxide SrO. Assume that
La 2 O 3 is dominated by anionic Frenkel disorder. Write the reactions that
involve only atomic structure defects.
2. Assuming that Schottky disorder dominates in LaMnO 3 ,
a. give the equilibrium relation associated with Schottky disorder in LaMnO 3
and the corresponding equilibrium constant.
b. We study three different ways to introduce the dopant SrO into the LaMnO 3
structure. We consider that the solid solutions obtained take the form
of an ideal cubic perovskite structure ABO 3 . This structure is shown in
figure 3.