  The microstructure of the MgB2 superconductor sintered at high temperature
under a high pressure of about 3 GPa was investigated by using a
high-resolution transmission electron microscope (HRTEM). The TEM images did
not show any pores in the specimen. All grains were compactly connected, and no
discernable empty spaces or impurities at the boundaries existed over the
regions investigated. The HRTEM image showed clearly each constituent atom that
formed the basal hexagonal plane, without any defect in a single grain. The
a-axis lattice parameter, 0.307 nm, from this direct measurement was shorter
than the value, 0.314 nm obtained from samples prepared using diffusion
techniques. A minor impurity phase, which was most probably MgB4 and did not
form interfacial layers was also observed, but was well isolated from the main
MgB2 phase. Our results verify that the MgB2 powder was sintered under high
temperature and high pressure into its theoretical density without any porosity
or grain growth.
