A systematic study of the structure and superconductive characteristics of high-pressure–high-temperature (2 GPa, 700–1000 °C)-synthesized and sintered MgB2 without additions from different initial powders was performed. Among various secondary phases Mg–B inclusions with a stoichiometry close to MgB12 were identified. With an increasing amount of these inclusions the critical current density increased. So these inclusions can be feasible pinning centres in MgB2. The highest jc values in zero field were 1300 kA cm−2 at 10 K, 780 kA cm−2 at 20 K and 62 kA cm−2 at 35 K and in 1 T field were 1200 kA cm−2 at 10 K, 515 kA cm−2 at 20 K and 0.1 kA cm−2 at 35 K for high-pressure-synthesized magnesium diboride and the field of irreversibility at 20 K reached 8 T. The average grain sizes calculated from x-ray examinations in materials having high jc were 15–37 nm.

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