CRYSTAL-CHEMISTRY AND MAGNETISM OF TERNARY ACTINOID BORON CARBIDES UB1-XC1+X AND U1-XMXB2C WITH M = SC, LU, AND TH

Within the homogeneous range of uranium monocarbide UB1-xC1+x, the crystal structures of stoichiometric UBC and of the carbon-rich solid solution UB0.78C1.22, have been refined from single-crystal X-ray counter data. From X-ray analysis crystal symmetry in both cases is consistent with the centro-symmetric space group Cmcm and there are no indications of superstructure formation. In contrast to the fully ordered atom arrangement revealed for stoichiometric UBC (a = 0.35899(4), b = 1.19781(12), c = 0.33474(3) nm), random occupation by boron and carbon atoms is observed for the boron site in UB0.78C1.22 (a = 0.35752(4), b = 1.18584(3), c = 0.33881(4) nm). For 279(278) reflections (|F0| > 3σ) the obtained reliability factors Rx = ∑|ΔF|/∑|F0| were Rx = 0.069 for UBC and Rx = 0.050 for UB0.78C1.22. Neutron powder diffraction experiments at 9 and 295 K unambiguously revealed full occupancy by the nonmetal atoms in UB0.78C1.22 and prove the statistical occupation of B and C atoms in the B-sites. For the orthorhombic symmetry Cmcm, refinement was not better than R1 = 0.044. A model calculation in monoclinic symmetry C12/m1, however, resulted in a significant reduction of the residual value to R1 = 0.030, releasing spatial constraints on the boron atoms. Thus the boron-boron chain in Cmcm (B-B = 0.1874 nm) is dissolved into boron pairs (B-B = 0.1706 nm) which are loosely bound at a distance of 0.2043 nm. The formation of C-B-B-C groups corresponds to the structure types of ThBC and Th3B2C3. The magnetic behavior has been investigated in the temperature range from 4.2 K to 1000 K for UB1-xC1+x (UBC-type) and U1-xMxB2C (ThB2C-type for the high temperature modification and 1-UB2C-type for the low temperature modification) with U partially substituted by Th or Sc, Lu. From magnetic susceptibilities, the alloys UB1-xC1+x reveal temperature independent paramagnetism with typical intermediate valence fluctuation behavior (TSF ∼ 350 K). ThB2C and 1-UB2C both are temperature independent paramagnets, whereas h-UB2C is a ferromagnet with the rather high Curie temperature TM = 80(2) K. TM and the saturation magnetiziation per U atom both successively decrease on substitution of U by Th, Sc, or Lu in UB2C, whereas the U-moments remain practically unchanged at μeff(U) ∼ 1.9 μB. Uranium L3-XANES (X-ray Absorption Near Edge Structure) spectroscopy revealed increased d-band localization, comparable to uranium-transition metal alloys, in nonmagnetic UB1-xC1+x (x = 0, 0.22). No superconductivity was observed down to 1.5 K; no hydrogen uptake was observed for UB2C and ThB2C even under hydrogen pressures as high as 7 × 107 Pa at 670 K. © 1993 Academic Press, Inc.