The temperature dependences of the Raman spectrum and lattice parameters (figures 11 and 12) of polycrystalline material were investigated by means of Raman spectroscopy and X-ray diffraction. It was found that the lattice parameters exhibited an approximately linear change with temperature decrease, giving differing thermal expansions along the a- and c-axes; caused by the comparatively weak metal–B bonding. The grain size, as determined by means of X-ray diffraction, was about 45nm for both the [100] and [001] directions. There was no evidence of any structural transition as the temperature was changed from 300K to 12K. An anomalous Raman band at 603/cm was observed; consistent with the theoretical prediction for the E2g in-plane B stretching mode. The Raman frequency increased, and the line-width decreased, as the temperature decreased. It was suggested that a grain-size effect at the nm-scale would have a clear influence upon the frequency and line-width of the Raman spectrum.

The Raman Spectrum and Lattice Parameters of MgB2 as a Function of Temperature. L.Shi, H.Zhang, L.Chen, Y.Feng: Journal of Physics - Condensed Matter, 2004, 16, 6541-50

Figure 11

Variation of the c Lattice Parameter as a Function of Temperature