Unexpected Magnetic Ordering on the Cr Substructure in UCr₂Si₂C and Structural Relationships in Quaternary U-Cr-Si-C...

Previous experimental and theoretical studies revealed that carbon insertion into the RCr₂Si₂ compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCr₂Si₂C (R = Y, La-Sm, Gd-Er) compounds in contrast to RCr₂Si₂ (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCr₂Si₂C compound. While specific heat and magnetic measurements suggest a Pauli paramagnetic behavior, neutron powder diffraction reveals an antiferromagnetic ordering of the chromium substructure at high temperature ( T_N > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCr₂Si₂C carbides involving a magnetic lanthanide, is discussed in relation with the singularity of its crystal structure among the series. Moreover, the crystallographic structures and the structural stability of UCr₂Si₂C and of two other quaternary U-Cr-Si-C compounds (i.e., UCr₃Si₂C and U₂Cr₃Si₂C₃), based on the full occupancy of interstitial sites by carbon atoms, are discussed and compared to those of the related ternary intermetallics. Finally, the low-temperature form of UCr₂Si₂, corresponding to a displacive transformation around 210 K of the ThCr₂Si₂-type structure, is reinvestigated by considering a higher symmetry monoclinic unit cell ( C2/ m) instead of the previously reported triclinic cell ( P1̅). The antiferromagnetic ordering at low temperature ( T_N = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.