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.
