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

R33Fe14–xAlx+yB25–yC34 (R = La or Ce; x ≤
0.9; y ≤ 0.2) and R33Fe13–xAlxB18C34 (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 Ce33Fe13.1Al1.1B24.8C34, 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 Ce33Fe14–xAlx+yB25–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 Ce3+/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 TC = 180 K is due
to magnetic moments on the Fe13 clusters.