A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes

R₃₃Fe_14–xAl_x+yB_25–yC₃₄ (R = La or Ce; x ≤ 0.9; y ≤ 0.2) and R₃₃Fe_13–xAl_xB₁₈C₃₄ (R = Ce or Pr; x < 0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R–T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group Im3̅m (a = 14.617(1) Å, Z = 2, R₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and a = 14.246(8) Å, Z = 2, R₁ = 0.0142 for Ce₃₃Fe₁₃B₁₈C₃₄). Their structures can be described as body-centered cubic arrays of large Fe₁₃ or Fe₁₄ clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe₁₄ clusters in Ce₃₃Fe_14–xAl_x+yB_25–yC₃₄ are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce^3+/4+ valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce₃₃Fe₁₃B₁₈C₃₄ indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at T_C = 180 K is due to magnetic moments on the Fe₁₃ clusters.