Synthesis, crystal structure, and magnetic properties of novel intermetallic compounds R₂Co₂SiC (R = Pr, Nd)

The intermetallic compounds R₂Co₂SiC (R = Pr, Nd) were prepared from the reaction of silicon and carbon in either Pr/Co or Nd/Co eutectic flux. These phases crystallize with a new stuffed variant of the W ₂CoB₂ structure type in orthorhombic space group Immm with unit cell parameters a = 3.978(4) Å, b = 6.094(5) Å, c = 8.903(8) Å (Z = 2; R₁ = 0.0302) for Nd₂Co₂SiC. Silicon, cobalt, and carbon atoms form two-dimensional flat sheets, which are separated by puckered layers of rare-earth cations. Magnetic susceptibility measurements indicate that the rare earth cations in both analogues order ferromagnetically at low temperature (T_C ≈ 12 K for Nd ₂Co₂SiC and T_C ≈ 20 K for Pr ₂Co₂SiC). Single-crystal neutron diffraction data for Nd₂Co₂SiC indicate that Nd moments initially align ferromagnetically along the c axis around ∼12 K, but below 11 K, they tilt slightly away from the c axis, in the ac plane. Electronic structure calculations confirm the lack of spin polarization for Co 3d moments.