Semus'o N. Ternary systems {Pr,Tb}-Al-{Si,Ge}. Phase equilibria, crystal structures and electrical properties

The isothermal sections (673 K) of the {Pr,Tb}-Al-{Si,Ge} phase diagrams in the range 0-50 at.% R were constructed based on X-ray diffraction data. The formation of 25 compounds (14 new compounds) in these systems was determined. Alumosilicides and alumogermanides are formed on the RX2-RAl2, RX2-Al and RX-Al (X - Si or Ge) cross-sections. In the systems with other rare-earth metals 27 new ternary compounds were found. All known 145 rare-earth alumosilicides and alumogermanides crystallize in 23 structure types and form 17 rows of isotypic compounds. The existence of six new rows with the Y2Al3Si2, YAlGe, ZrSi2, PrGe1,91, Pr4Al3Ge3 and CrB structure types was established. Short distances between the Al, Si or Ge atoms (11-13% contraction) indicate that covalent bonds are present in addition to metallic bonding. The aluminium, silicium and germanium atoms form pairs, infinite straight or zigzag chains, planar or puckered layers, three-dimensional frameworks or finit layers and frameworks. New structure type Pr4Al3Ge3 belongs to the homologous series based on an intergrowth of AlB2, LiBaSi and W - type stabs. For the PrAl2Si2, PrAlSi, PrAlGe and PrAl1,5Ge0,5 compounds electroresistivity and thermopower in the range 80-360 K were determined. Linear increase of resistivity and small value of thermopower indicate the metallic nature of the praseodimium alumosilicides and alumogermanides.