Valence and magnetic transitions in YbMn2Si2-xGex

Full text: Rare-earth intermetallic compounds containing ytterbium exhibit a wide range of interesting and unusual physical and magnetic properties. This occurs mainly as a result of their mixed valence states (II/III) or changes from one valence state to the other. Nowik et al. have recently investigated the magnetic phase transitions in the YbMn2Si2-xGex series (tetragonal ThCr2Si2-type structure) by magnetisation and Moessbauer effect studies on 57Fe doped samples. A number of transitions were observed in the limiting YbMn2Ge2 and YbMn2Si2 compounds. We have carried out a neutron diffraction investigation of a series of YbMn2Si2-xGex samples (x=0, 0.4, 1.0, 1.4, 2.0) over the temperature range 1.5-600 K to delineate details of the valence and related magnetic transitions. As well as determining the antiferromagnetic structures of YbMn2Ge2 and YbMnSi2, we have also established that the Yb sublattice orders antiferromagnetically below Tn3 ∼ 5 K. Our neutron diffraction measurements are consistent with a trivalent Yb state in YbMn2Si2, whereas analysis of the unit cell volumes leads to a Yb valence of about 2.35 in YbMn2Ge2. The change in valence state occurs around a critical Ge concentration xc ∼1.4