The structure of HoD3

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Abstract
X-ray diffraction studies have shown (Sturdy and Mulford, 1956 ; Pebler and Wallace, 1962) that the metal ions in the heavy lanthanide trihydrides (beginning with Sm) are in a close packed hexagonal arrangement with c/a approximately equal to 1.80. It has generally been assumed that hydrogen in these and the more dilute hydrides reside in the octahedral and tetrahedral interstices. Unpublished lattice energy calculations by Pebler and Wallace showed that this cannot be correct since the hexagonal trihydride is energetically unstable relative to the cubic arrangement. The present work was undertaken as a follow-up of the earlier study of TbD2 and HoD2 (Cox et al., 1963) and was initiated to provide information concerning the location of the deutérium ions in the hexagonal trideuterides. HoD3 was chosen for study and neutron powder diffraction patterns were obtained using the Westinghouse Test Reactor. The unit cell found for HoD3 using neutrons is three times larger than the pseudo-cell found using x-ray procedures (an = 6.308 Å, cn = 6.560 Å). Thus the unit cell contains 6 holmium and 18 deuterium ions. The space group for HoD 3 is P3c1 (D43d) with occupancy of positions as follows : 6 Ho in (f) : ± (2/3, 0, 1/4 ; 0, 2/3, 1/4 ; 1/3, 1/3, 1/4). 12 D in (g): ± (xyz; y, x-y, z; y-x, x, z; y, 1/2 + z; x, x-y, 1/2 + z; y-x, y, 1/2 + z) with xg = 0.356, yg = 0.028, zg = 0.096. 4 D in (d) : ± (1/3, 2/3, z ; 1/3, 2/3, 1/2 + z) with zd = 0.167. 2 D in (a) : ± (0, 0, 1/4). The trihydrides of the following metals have the same x-ray pseudo cell : Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. From general considerations it appears that these hydrides are isostructural with HoD3

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