Characterization of the structure of YD3 by neutron powder diffraction

Neutron-powder-diffraction (NPD) measurements of the hexagonal rare-earth trideuteride gamma-YD3 were undertaken between 10 and 400K. Rietveld refinements indicated that the YD3 structure possesses <P(3)over bar c1> (D-3d(4)) symmetry, in agreement with previous NPD studies of YD3 and HoD3. The unit cell is a (root 3 x root 3)R30 degrees expansion of the conventional hcp unit cell in the ab plane. To accomodate the D atoms, the c axis for YD3 is elongated by similar to 15% [c(0)/a(0) = 1.8019(1), a(0) = 3.6627(1) Angstrom, at 295 K] compared with that for Y. The D atoms occupy unusual interstitial positions of a slightly distorted, hcp metal lattice, instead of the ideal octahedral (o) and tetrahedral (t) sites. In particular, the hypothetical o-site D atoms are displaced vertically toward the Y-defined basal planes, with correlated placements at either near-plane or in-plane threefold metal (m) sites. The t-site D (D-t) atoms are displaced horizontally, also in a correlated fashion, with respect to the m-site D (D-m) atoms. This structure is maintained over the entire temperature range from 10 to 400 K. Moreover, the data suggest the presence of an order-disorder equilibrium amongst the D-m atoms. The disorder involves c-axis displacements of a fraction of D-m atoms from near-plane to in-plane positions, concomitant with an equal number of compensating D-m atoms shifting from in-plane to near-plane positions. The displacements are facilitated by the relatively open channels formed along the c direction by the Y and D-t atoms.