No abstract

structure structure (solids and liquids) D 2000 -012Crystal Structures of PrAlxGe 2−x Compounds.-The PrAlxGe 2−x cross-section of the Pr-Al-Ge system is studied at 1073 K and the homogeneity ranges of two ternary series, PrAl 1.55−−1.48 Ge 0.45−−0.52 (I) and PrAl 1.42−−0.98 Ge 0.58−−1.02 (II) are determined. Compounds of type (I) have a hexagonal AlB 2 type structure (space group P6/mmm, Z = 1; single crystal XRD data for x = 1.476) and (II) crystallizes in the tetragonal LaPtSi type structure (I4 1 md, Z = 4; single crystal data for x = 1). -(GLADYSHEVSKII, E. I.; NAKONECHNA, N. Z.; CENZUAL, K.; GLADYSHEVSKII, R. E.; JOR-DAN, J.-L.; J. Alloys Compd. 296 (2000) 1-2, 265-271; Dep. Inorg. Chem., L'viv State Univ., L'viv 290005, Ukraine; EN)

The chemical class grouping rare-earth alumosilicides and alumogermanides includes 145 ternary compounds, characterized by simple stoichiometries and a rare-earth metal content between 20 and 60 at.%. The majority of the compounds are located along the RX2-RAl2, RX2-Al and RX-Al (R -rare-earth metal, X -Si or Ge) cross-sections of the ternary systems and 50 compounds contain 33 at.% R. The compounds crystallize with 23 different, but related, structure types, 10 of which (La2AlGe6, Ce3Al4Si6, CeAlSi2, Y2Al3Si2, YAl1.4Si0.6, YAlGe, Y2AlGe3, Yb7Al5Ge8, Pr4Al3Ge3 and Tb6Al3Si) are defined on a rare-earth alumosilicide or alumogermanide. For 18 structure types rows of isotypic compounds containing from 3 to 28 representatives are known, whereas only four types have one single representative. The CaAl2Si2 (28 compounds), YAlGe (15), α -ThSi2 (13) and AlB2 (11) structure types are common to rare-earth alumosilicides and alumogermanides. A differentiation with respect to the crystal structure is observed between compounds of light and heavy rare-earth metals for > 23% R atoms. In 18 structure types all or part of the X atoms centre trigonal prisms, formed either by R atoms, or by R and Al atoms in different ratios. The X and Al atoms are characterized by short interatomic distances and form pairs, chains, nets or three-dimensional frameworks. In some structures X and Al atoms are distributed statistically, however, phases with significant homogeneity ranges are not observed. Exceptions are the alpha-ThSi2 type phases with up to 17 at.% broad homogeneity ranges.