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

R₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ (R = La or Ce; <i>x</i> ≤ 0.9; <i>y</i> ≤ 0.2) and R₃₃Fe_{13–<i>x</i>}Al_<i>x</i>B₁₈C₃₄ (R = Ce or Pr; <i>x</i> < 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 <i>Im</i>3̅<i>m</i> (<i>a</i> = 14.617(1) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and <i>a</i> = 14.246(8) Å, <i>Z</i> = 2, <i>R</i>₁ = 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–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ 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 <i>T</i>_C = 180 K is due to magnetic moments on the Fe₁₃ clusters

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

R₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ (R = La or Ce; <i>x</i> ≤ 0.9; <i>y</i> ≤ 0.2) and R₃₃Fe_{13–<i>x</i>}Al_<i>x</i>B₁₈C₃₄ (R = Ce or Pr; <i>x</i> < 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 <i>Im</i>3̅<i>m</i> (<i>a</i> = 14.617(1) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and <i>a</i> = 14.246(8) Å, <i>Z</i> = 2, <i>R</i>₁ = 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–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ 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 <i>T</i>_C = 180 K is due to magnetic moments on the Fe₁₃ clusters