The ab-initio crystal structure determination of UPd₂Sn by synchrotron X-ray powder diffraction

Abstract

The structure of the heavy-fermion compound UPd2Sn has been determined by synchrotron x-ray radiation powder diffraction techniques. It is orthorhombic, space group Pnma [lattice parameters a = 9.9787(1), b = 4.58843(5), c = 6.89166(8) Å at room temperature] and Z = 4. All atoms, one U, one Sn, and two Pd are in 4d special positions at (x 1 4 z). The refinements were carried out by the Rietveld method with a pseudo-Voigt peak shape function. The final conventional R factors were: Rwp = 21.3% and RE = 14.3%. The U positions in the unit cell were unequivocally located, but because of the similarity in x-ray scattering factors it was not possible to determine whether the Pd atoms and the Sn atoms are ordered or disordered on the other three sites. The structural arrangement of UPd2Sn is of either MnCu2Al, or ordered NaTl, or disordered Fe3Al type, all these structures being cubic and b.c.c. related. The orthorhombic distortion is large and is probably due to a size effect of the U atoms which would be in a mixed ( 3+ 4+) valence state. © 1988.