  The accurate charge density of MgB2 was observed at room temperature(R.T.)
and 15K by the MEM(Maximum Entropy Method)/Rietveld analysis using synchrotron
radiation powder data. The obtained charge density clearly revealed the
covalent bonding feature of boron forming the 2D honeycomb network in the basal
plane, on the other hand, Mg is found to be in divalent state. A subtle but
clear charge concentration was found on boron 2D sheets at 15K, which should be
relating to superconductivity.
