X-ray diffraction study of MgB2 at low temperatures

We have performed powder X-ray diffraction of MgB2 superconductor between 10 and 300 K. Temperature dependence of integrated intensities of both (002) and (110) X-ray reflections shows a peak at around T-C, superconducting transition temperature. The integrated intensity of the X-ray reflection is related to the phonon frequency through a Debye-Waller factor. Lattice parameters a and c show negative thermal expansions at low temperatures. The negative thermal expansion might be due to an electronic origin and not directly related to the superconducting transition.