Crystal structures of PrAlxGe2−x compounds

Abstract

The PrAlxGe2−x cross-section of the Pr–Al–Ge system at 1073 K was studied. The homogeneity ranges of two ternary praseodymium alumogermanides, PrAl1.55–1.48Ge0.45–0.52 and PrAl1.42–0.98Ge0.58–1.02, were determined and their crystal structures refined from X-ray single-crystal diffraction data. The former has a hexagonal structure of the AlB2 type (hP3, P6/mmm, a=4.3223(3), c=4.2585(4) Å for x=1.476(2)) and the latter a tetragonal structure of the LaPtSi type (tI12, I41md, a=4.2534(5), c=14.641(2) Å 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 PrAl2 was found to be less than 2 at.%, that of Al in off-stoichiometric PrGe2−x less than 8 at.%. Al(Ge)-centered R6 trigonal prisms constitute a common geometrical feature of the structures of the germanides RGe2−x and alumogermanides RAlxGe2−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.