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

Previous experimental and theoretical studies revealed that carbon insertion into the RCrSi compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCrSiC (R = Y, La-Sm, Gd-Er) compounds in contrast to RCrSi (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCrSiC 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 > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCrSiC 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 UCrSiC and of two other quaternary U-Cr-Si-C compounds (i.e., UCrSiC and UCrSiC), 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 UCrSi, corresponding to a displacive transformation around 210 K of the ThCrSi-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 = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.