Characterization of the structure of YD 3 by neutron powder diffraction

Abstract

Neutron-powder-diffraction (NPD) measurements of the hexagonal rare-earth trideuteride γ-YD 3 were undertaken between 10 and 400 K. Rietveld refinements indicated that the YD 3 structure possesses P 3 ̄ c1 ( D 3d 4) symmetry, in agreement with previous NPD studies of YD 3 and HoD 3. The unit cell is a (√3 × √3)R30 ° expansion of the conventional hcp unit cell in the ab plane. To accomodate the D atoms, the c axis for YD 3 is elongated by ~ 15% [ c 0 a 0 = 1.8019(1), a 0 = 3.6627(1) A ̊ , 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.