THE MECHANICAL RELAXATION STUDY OF POLYCRYSTALLINE MgC_(1-x)Ni_3

The mechanical relaxation spectra of a superconducting and a nonsuperconducting MgC-{1-x}Ni-3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300K labeled as P-1 and 125K as P-2) for the superconducting sample. For the nonsuperconducting one, the position of P-1 shifts to 250K, while P-2 is almost completely depressed. It is found that the peak position of P-2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P-2 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 150K and 50K observed by many other experiments.