  We report indications of a phase transition in carbonaceous MgB2 above 9 GPa
at 300 K after stress relaxation by laser heating. The transition was detected
using Raman spectroscopy and X-ray diffraction. The observed changes are
consistent with a second-order structural transition involving a doubling of
the unit cell along c and a reduction of the boron site symmetry. Moreover, the
Raman spectra suggest a reduction in electron-phonon coupling in the slightly
modified MgB2 structure consistent with the previously proposed topological
transition in MgB2. However, further attributes including deviatoric stress,
lattice defects, and compositional variation may play an important role in the
observed phenomena.
