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

Abstract

R(33)Fe(14-x)Al(x+y)B(25-y)C(34) (R = La or Ce; x ≤ 0.9; y ≤ 0.2) and R(33)Fe(13-x)Al(x)B(18)C(34) (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 Im3m (a = 14.617(1) Å, Z = 2, R(1) = 0.0155 for Ce(33)Fe(13.1)Al(1.1)B(24.8)C(34), and a = 14.246(8) Å, Z = 2, R(1) = 0.0142 for Ce(33)Fe(13)B(18)C(34)). Their structures can be described as body-centered cubic arrays of large Fe(13) or Fe(14) 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 Fe(14) clusters in Ce(33)Fe(14-x)Al(x+y)B(25-y)C(34) 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 Ce(33)Fe(13)B(18)C(34) 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 Fe(13) clusters.

Nd8Co4−Al Ge2C3: A case study in flux growth of lanthanide-rich intermetallics

“…Similar contamination from reduction of the alumina crucible is seen in the flux syntheses of Ce 33 Fe 14-x Al x B 25 C 34 , La 6 Fe 13-x Al x Sn y , and La 21 Mn 8 Ge 6.2 Al 0.8 C 12 [6][7][8]. While the aluminum in these phases is found to partially occupy a mixed site with a late transition metal or main group metalloid of similar size, incorporation of this dopant is often required to stabilize the structure; the yield is increased if Al is deliberately added as a dopant, and different products are formed in the absence of aluminum (for instance if steel crucibles are used).…”

“…Similar M@Ln n clusters are found in other lanthanide-rich compounds, with the size of the coordinating lanthanide cluster dependent on the relative radii of the M and Ln ions. For instance, octahedral C@Ce 6 and Al@Sm 6 units are found in Ce 33 Fe 13 B 25 C 34 and Sm 12 Fe 14 Al 5 , respectively [8,22]. Square antiprismatic Sb@Gd 8 clusters are found in Gd 5 Ir 2 Sb [25], and the compound La 8 Co 2 In 3 features In@La 11 units [26].…”

“…Examples include the corner-sharing TC 4 tetrahedra that link together to form the infinite polyanionic layers ∞ [(T 2 C 3 ) 6-] (T=Cr, Mo) of Ho 2 Cr 2 C 3 and Er 2 Mo 2 C 3 , and the corner-sharing ReC 5 trigonal bipyramids and ReC 4 squares that comprise the anionic network in U 5 Re 3 C 8 . Carbon-capped clusters of transition metals (which exhibit not only T-C but also T-T bonding) are seen in fluxgrown intermetallics La 21 Fe 8 Sn 7 C 12 (which features Fe 4 tetrahedra capped on all edges by carbon atoms) and Ce 33 Fe 13 B 25 C 34 (containing Fe 13 clusters capped by borocarbide chains) [4,8]. In recent years Dashjav et al have discovered two very interesting compounds in the Dy/Fe/C system, both of which feature Fe-Fe bonded units capped with carbon.…”

“…Our past investigations have demonstrated that eutectic mixtures of early lanthanide elements (Ln = La, Ce, Pr, Nd) with late first row transition metals (T = Fe, Co, Ni) are excellent solvents for the discovery and crystal growth of complex lanthanide-rich intermetallic compounds [2][3][4][5][6][7][8]. For example, both Nd and Co have high melting points, but when combined in a 64%/36% Nd/Co mole ratio a eutectic is formed with a low melting point (566°C) [9].…”

Flux growth and magnetic properties of rare earth cobalt germanide, RE6Co5Ge1+xAl3−x (RE=Pr, Nd; x≈0.8)

“…For example, isolated transition metal sites or small transition metal clusters in intermetallics such as NdCoC 2 , NdCoGe 3 , and R 2 Co 2 SiC (R=Pr, Nd) tend to be nonmagnetic [9][10][11]. On the other hand, phases containing larger transition metal building blocks, such as the Fe 13 clusters in Ce 33 Fe 13 B 18 C 34 , Fe layers in Nd 6 Fe 10 Al 3 , and Fe networks of Nd 2 Fe 14 B, will have strong magnetic moments on the transition metal atoms, and might even exhibit unusual magnetic phenomena such as multiple ordering transitions or spin glass behavior [12,13,3].…”