Indications of a phase transition in carbonaceous MgB2 above 9GPa at 300K, after stress relaxation by laser heating, was reported. The transition was detected using Raman spectroscopy and X-ray diffraction. The observed changes were consistent with a second-order structural transition involving a doubling of the unit cell along c and a reduction of the B 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 could play an important roles in the observed phenomena.

Possible Superlattice Formation in High-Temperature Treated Carbonaceous MgB2 at Elevated Pressure. O.Tschauner, D.Errandonea, G.Serghiou: Physica B, 2006, 371[1], 88-94

Figure 12

Variation of the a Lattice Parameter as a Function of Temperature