Synthesis, Crystal Structure, and Magnetism of A2Co12As7 (A = Ca, Y, Ce–Yb)

Abstract

Ternaryintermetallics,A2Co12As7 (A¼Ca, Y,Ce–Yb), havebeensynthesizedbyannealingmixturesof elements inmoltenBiat1223K.Thematerialsobtainedcrystallizeinthe P63/m variantoftheZr2Fe12P7 structure type.Theunitcellvolumeshowsamonotonicdecreasewiththeincreasingatomicnumberof the rare-earthmetal,withtheexceptionofCe-,Eu-,andYb-containingcompounds.Anexaminationof these outlierswithX-rayabsorptionnearedgestructures(XANES)spectroscopyrevealedmixedvalence of Ce,Eu,andYb,withtheaverageoxidationstatesof þ3.20(1), þ2.47(5),and þ2.91(1),respectively,at room temperature.MagneticbehaviorofA2Co12As7 is generallycharacterizedbyferromagneticordering of Co3d moments at100–140K,followedbylow-temperatureorderingofrare-earth4f moments. The 3d-4f magnetic couplingchangesfromantiferromagneticforA¼Pr–Sm toferromagneticforA¼Ce and Eu–Yb. Polarizedneutronscatteringexperimentswereperformedtosupportthepostulatedferro-and ferrimagnetic groundstatesforCe2Co12As7 and Nd2Co12As7, respectively.