Synthesis and X-ray diffraction pattern for MgCu2

To clarify the inconsistency between calculated and existing experimental X-ray diffraction (XRD) pattern for dicopper magnesium (MgCu2) and to identify the impurity phase in Cu-sheathed MgB2 superconductors, polycrystalline MgCu2 samples were synthesized and studied by XRD measurement. The XRD result shows that the (2 2 0), (4 2 2), (6 2 0), and (6 4 2) reflection lines, which are missing from the standard pattern of MgCu2 in the current Powder Diffraction File (PDF) database with file #01-1226, are all present in the XRD patterns of our MgCu2 samples. This result reveals that the long-time widely used XRD pattern in the PDF file #01-1226 is in fact an incomplete one and should be replaced by a more complete pattern. By comparison with our measured XRD pattern for MgCu2 samples, the reflection line located at 2θ ≈ 36° for Cu-sheathed MgB2 wires has been unambiguously identified as the (2 2 0) line of the MgCu2 impurity phase. This line was previously unidentifiable due to its absence in current PDF database. The calculated XRD pattern for MgCu2 was found to match well with the measured pattern in terms of both line position and intensity. Thus, this study has provided a more complete powder XRD pattern of MgCu2 than that in the current PDF database for future phase determination and crystal structure study on alloys and compounds containing MgCu2 phase.