  An anisotropic lattice anomaly near the superconducting transition
temperature, Tc, was observed in MgB2 by high-resolution neutron powder
diffraction. The a-axis thermal expansion becomes negative near Tc, while the
c-axis thermal expansion is unaffected. This is qualitatively consistent with a
depletion of the boron-boron s-band as the superconducting gap opens, resulting
in weaker bonding. However, the observed anomaly is much larger than predicted
by the Ehrenfest relation, strongly suggesting that the phonon thermal
expansion also changes sign, as commonly observed in hexagonal layered
crystals. These two effects may be connected through subtle changes in the
phonon spectrum at Tc.
