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

Abstract Ternary intermetallics, A 2 Co 12 As 7 (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 P 6 3 / m variant of the Zr 2 Fe 12 P 7 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 A 2 Co 12 As 7 is generally characterized by ferromagnetic ordering of Co 3 d moments at 100–140 K, followed by low-temperature ordering of rare-earth 4 f moments. The 3 d -4 f 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 Ce 2 Co 12 As 7 and Nd 2 Co 12 As 7 , respectively.