Crystal structures of PrAl sub x Ge sub 2-x compounds

The PrAl sub x Ge sub 2-x cross-section of the Pr-Al-Ge system at 1073 K was studied. The homogeneity ranges of two ternary praseodymium alumogermanides, PrAl sub 1.55-1.48 Ge sub 0.45-0.52 and PrAl sub 1.42-0.98 Ge sub 0.58-1.02 , were determined and their crystal structures refined from X-ray single-crystal diffraction data. The former has a hexagonal structure of the AlB sub 2 type (hP3, P6/mmm, a = 4.3223(3), c = 4.2585(4) A for x = 1.476(2)) and the latter a tetragonal structure of the LaPtSi type (tI12, I4 sub 1 md, a = 4.2534(5), c = 14.641(2) A for x = 1). For both structures, the Al-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 phase PrAl sub 2 was found to be less than 2 at.%, that of Al in off-stoichiometric PrGe sub 2-x less than 8 at.%. Al(Ge)-centered R sub 6 trigonal prisms constitute a common geometrical feature of the structures of the germanides RGe sub 2-x and alumogermanides RAl sub x Ge sub 2-x formed by the light rare-earth elements. Substitution of Ge for Al, or replacement of a large rare-earth element by a smaller one, leads to a reduction of the prism volume.