Crystal structures of PrAlxGe2-x compounds

The PrAlxGe2-x cross-section of the Pr-Al-Ge system at 1073 K was studied. The homogeneity ranges of two ternary praseodymium alumogermanides, PrAl1.5 5-1.48Ge0.45-0.52 and PrAl1.42-0.98Ge0.58-1.02, were determined and their c rystal structures refined from X-ray single-crystal diffraction data. The f ormer has a hexagonal structure of the AlB2 type (hP3, P6/mmm, a=4.3223(3), c=4.2585(4) Angstrom for x=1.476(2)) and the latter a tetragonal structure of the LaPtSi type (tI12, I4(1)md, a=4.2534(5), c=14.641(2) Angstrom for x =1). For both structures, the AI-Ge interatomic distances are close to the sum of the covalent radii, whereas the Pr-Al(Ge) distances agree well with the sum of the metallic radii. The solubility of Ge in the cubic Laves phas e PrAl2 was found to be less than 2 at.%, that of Al in off-stoichiometric PrGe2-x less than 8 at.%. Al(Ge)-centered R-6 trigonal prisms constitute a common geometrical feature of the structures of the germanides RGe2-x and a lumogermanides RAlxGe2-x formed by the light rare-earth elements. Substitut ion of Ge for Al, or replacement of a large rare-earth element by a smaller one, leads to a reduction of the prism volume. (C) 2000 Elsevier Science S .A. All rights reserved.