in size. These grains are separated physically from each other by thin layers of
intergranular materials of approximately 0.2 mm in thickness. The nature and composition
of these layers depends on the additives used for doping ZnO. Very sensitive
measurements [40, 42] showed that the intergranular layer can be as small as 2 Å.
Spinel particles of Zn7Sb2O12 which are thought to play a passive electrical role,
were also observed [39, 41, 48] in the intergranular phase and their origin was traced
back to the presence of Sb2O3 additive in the ZnO composition during the sintering
process. In addition to the spinel crystallites, a pyrochlore phase (Bi2{Zn4/3Sb2/3}O6)
was identified in the intergranular layer and a bismuth rich phase at the triple point
which is formed with three or more ZnO grains [47].