Observation of a low-symmetry crystal structure for superconducting MgCNi₃ by Ni K-edge x-ray absorption measurements

The local atomic structure of MgCNi₃ has been determined over the temperature range of 3 to 300 K by Ni K-edge x-ray absorption fine structure measurements. No anomalies were observed near T_c∼7 K. However, below T* ∼70 K the symmetry of Ni₆ octahedra is lower than the cubic Pm- 3m determined by conventional diffraction measurements. We suggest that the onset of local distortions could be closely related with removal of the degeneracy in the electronic states dominated by the Ni d bands. The results are discussed in relation to nuclear magnetic resonance, x-ray photoemission measurements, and the superconductivity.