  Dense pure and doped (Mg1-xAx)B2 samples with A = Na, Ca, Cu, Ag, Zn and Al
were synthesized at high pressure - high temperature in a multi anvils press
(3.5-6 GPa, 900-1000C) for x up to 0.20. They were studied by x-ray
diffraction, scanning electron microscopy and their superconducting properties
were investigated by a.c. susceptibility, magnetization and transport
measurements. Only Al is really substituted on the Mg site. The others elements
form secondary phases with B or Mg. No large effect is observed on the
superconducting properties Tc, jc critical current, Hirr and Hc2.
