LOW-TEMPERATURE STRUCTURE OF UNI2SN

The low-temperature structure of UNi2Sn has been identified by means of neutron powder diffraction. It is an orthorhombic structure, space group Pnma, with lattice parameters a = 9.6208(8), b = 4.3055(4) and c = 6.5895 (6) angstrom at 80 K, Z = 4. The orthorhombic deformation is caused by displacements of the atoms out of their positions in the high-temperature Heusler structure, which is cubic, space group Fm3mBAR. The pattern of these displacements can be described by two wavevectors q = 2pi/2a (1, 1, 0) and 2pi/a (1, - 1, 0), for polarization directions p = (0, 0, 1) and (1, 1, 0) respectively, in the cubic coordinate system. In the low-temperature refinement R(Bragg) = 4.05% and chi2 = 5.53 was obtained.