Synthesis, crystal structure, and magnetism of A₂Co₁₂As₇ (A=Ca, Y, Ce-Yb)

Ternary intermetallics, A₂Co₁₂As₇ (A=Ca, Y, Ce-Yb), have been synthesized by annealing mixtures of elements in molten Bi at 1223 K. The materials obtained crystallize in the P6₃/m variant of the Zr₂Fe₁₂P₇ structure type. The unit cell volume shows a monotonic decrease with the increasing atomic number of the rare-earth metal, with the exception of Ce-, Eu-, and Yb-containing compounds. An examination of these outliers with X-ray absorption near edge structures (XANES) spectroscopy revealed mixed valence of Ce, Eu, and Yb, with the average oxidation states of +3.20(1), +2.47(5), and +2.91(1), respectively, at room temperature. Magnetic behavior of A₂Co₁₂As₇ is generally characterized by ferromagnetic ordering of Co 3d moments at 100-140 K, followed by low-temperature ordering of rare-earth 4f moments. The 3d-4f magnetic coupling changes from antiferromagnetic for A=Pr-Sm to ferromagnetic for A=Ce and Eu-Yb. Polarized neutron scattering experiments were performed to support the postulated ferro- and ferrimagnetic ground states for Ce₂Co₁₂As₇ and Nd₂Co₁₂As₇, respectively.