  The mechanical relaxation spectra of a superconducting and a
non-superconducting MgCNi3 samples were measured from liquid nitrogen
temperature to room temperature at frequency of kilohertz. There are two
internal friction peaks (at 300 K labeled as P1 and 125 K as P2) for the
superconducting sample. For the non-superconducting one, the position of P1
shifts to 250 K, while P2 is almost completely depressed. It is found that the
peak position of P2 shifts towards higher temperature under higher measuring
frequency. The calculated activation energy is 0.13eV. We propose an
explanation relating P2 to the carbon atom jumping among the off-center
positions. And further we expect that the behaviors of carbon atoms maybe
correspond to the normal state crossovers around 150 K and 50 K observed by
many other experiments.
