Structure of Th₄D₁₅ from measurements at the Argonne ZING-P pulsed neutron source

The superconductor Th₄D₁₅ has been investigated by neutron diffraction with the spallation neutron source (ZING-P) at Argonne National Laboratory. Profile refinement of the powder data for d spacings between 0.65 and 1.95 Å, corresponding to a neutron wavelength range of 1.26-3.77 Å, confirms the structure suggested by Zachariasen [Acta Cryst. (1953), 6, 393-395]. The b.c.c. cell, I3 d (a₀ = 9-11 Å), of Th₄D₁₅ contains 16 Th atoms at 16(c) [x_Th = 0.2066 (3)] and 60 D atoms at 12(a) and 48(e) [x = 0.3706 (7), y = 0.2155 (7), and z = 0.4048 (7)]. The Th atoms are equally spaced (3.95 Å) along four non-intersecting chains that are parallel to the four body diagonals. The general position D atoms, 48(e), form small equilateral triangles along the chains approximately equidistant from each Th atom. Neighboring triangles along a chain are rotated ±19.5 (2.0)° with respect to each other. The D atoms at 12(a) are centered in near-regular tetrahedra of Th atoms, consisting of one member from each chain and sharing only corners with adjacent tetrahedra. Each Th atom is found in a cage of three 12(a) plus nine 48(e) D atoms. Results for Debye-Waller factors of the Th and D atoms at 295 and 92 K are presented and compared with values obtained for the Debye-Waller factors in Pd-H.