  We studied the influence of sample preparation and defects in the
superconducting properties samples using atomic ratios of Mg:B=1:1 and
Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization
properties were examined in a SQUID magnetometer. The presence of Mg vacancies
was determined by Rietveld analysis. Most of the samples exhibited sharp
superconducting transitions with Tcs between 37- 39 K. We found a strong
correlation between the crystal strain and the Tc. This strain was related to
the presence of Mg vacancies. In addition, results showed that some samples
degraded with time when exposed to ambient conditions. In these samples the Tc
did not change with time, but the superconducting transition became broader and
the Meissner fraction decreased. This effect was only present in samples with
poor connectivity between grains and smaller grain sizes. The degradation was
related to a surface decomposition as observed by X-ray Photoelectron
Spectroscopy. No correlation was found between this effect and the presence of
Mg vacancies.
