The starting materials used were ZnO and MnCO3 powders with
purities of 99.999% and 99.99% respectively. Mixtures of powders
containing 5 and 20 wt% of MnCO3 were prepared by milling in a
centrifugal ball mill with 20 mm agate balls for 5 h. The final powder
was compacted under a compressive load to form disk shaped
samples of about 7 mm diameter and 2 mm thickness. Three samples
with 5 wt% of MnCO3 were annealed at 1240, 1270 and 1320 ◦C
respectively and a sample with 20 wt% MnCO3 was annealed at
1270 ◦C. All thermal treatments were performed during 15 h and
under argon flow.