Flux Growth and Physical Properties of Rare Earth Aluminides and Tetrelides

The flux method is a very useful technique for the exploratory synthesis of rare earth intermetallics. Reactions of rare earth metals with iron and silicon (or germanium, tin, lead) in a 1:1 Mg/Al flux have yielded single crystals of a series of new complex intermetallic phases. The structures of products were determined by single crystal X-ray diffraction or neutron diffraction. Reactions with early rare earths (R=La-Nd, Sm) produce RFe2MgAl7 phases which are substituted variants of the LaFe2Al8 type. NdFe2MgAl7 and SmFe2MgAl7 show antiferromagnetic ordering with Néel temperatures (TN) of 7.8K and 12K respectively. An electrical resistivity upturn was observed at ~11K for SmFe2MgAl7, consistent with the TN. Reactions with later rare earth elements (Gd, Dy) produce R5(Mg/Al)5Fe4(Al/Si)18 which forms in a new P4/mmm symmetry structure (a=11.655(2)Å, c=4.0668(8)Å for Dy analog). The single crystal neutron diffraction data of the Dy analog distinguished the Mg, Al and Si sites, indicating a stoichiometry of Dy5Mg2.92Fe4Al9.72Si10.36. The Gd and Dy analogs exhibit antiferromagnetic transitions with TN= 11K and 6.9K respectively. Reactions with ytterbium, erbium, dysprosium, or yttrium produce R3FeAl4-xMgxTt2 (R = Yb, Dy, Er, Y; Tt = Si, Ge) which has a new structure type in the P4/mbm space group (a=13.3479Å, c=4.0996Å); neutron diffraction data confirmed the partial incorporation of Mg into one Al site, leading to a formula of Yb3FeAl3.72Mg0.28Si2. Er2Fe3Al8Si crystallizes as a substituted variant of the orthorhombic Nd2Co3Al9 type. No magnetic ordering was observed for Yb3FeAl4-xMgxSi2 and Er2Fe3Al8Si. Common structural features in the RFe2MgAl7, R5(Mg/Al)5Fe4(Al/Si)18 and Yb3FeAl4-xMgxSi2 phases include chains of face-sharing mono-capped FeAl6 trigonal prisms, and preferential siting of Mg on specific Al sites. DOS calculations indicate that substitution of Mg for Al slightly destabilizes the structure. The presence of vacancies on the rare earth sites or the silicon substitution for aluminum are likely able to compensate for the structure destabilization, leading to the formation of pseudo gaps close to the EF. Reactions with europium and light tetrelides Tt = Si or Ge with iron in the Mg/Al flux produce products that do not incorporate iron or aluminum. Instead, these reactions yield ternary Zintl phases Eu6Mg17Tt13, crystallizing in the hexagonal Ba5Mg18Si13 structure type. Study of the magnetic anisotropy reveals that Eu6Mg17Si13 orders ferromagnetically (TC=12K) with a slight hysteresis when the c axis is oriented parallel to the field. Reactions with europium and heavier tetrelides Tt = Sn or Pb yield ternary Zintl phases EuMgTt, crystallizing in the orthorhombic TiNiSi structure type. Magnetic and electrical studies indicate that both compounds show large magnetoresistance up to -30% and -25% at their Néel temperatures (10.9 K and 13.9 K), respectively.