  We performed ac-susceptibility measurements of magnesium diboride powder
samples under pure hydrostatic pressures up to 0.4 GPa and under
quasi-hydrostatic pressure conditions up to 8 GPa in a helium gas pressure cell
and a diamond-anvil cell (DAC), respectively. Furthermore, the DAC has been
used to investigate the lattice compression by X-ray diffraction. Under helium
gas pressure the superconducting transition temperature shows a linear decrease
dTc/dp = -1.13 K/GPa. The pressure effect is fully reversible with a return to
the initial Tc of 37.5 K after pressure release. In the diamond-anvil cell the
application of pressure leads to a stronger Tc-decrease than in the helium gas
pressure cell, and after the release of pressure a degradation effect with
lower Tc and a broader transition compared to the first measurement at ambient
pressure occurs. Such degradation may be explained by shear stresses and
uniaxial pressure components, which cannot be avoided in a DAC at low
temperatures. Considering the anisotropic AlB2-type structure with alternating
honeycomb boron and magnesium layers, MgB2 seems to be very sensitive to such
non-hydrostatic pressure components, which would explain the spread of dTc/dp
values reported in the literature.
