Hydrides tetragonal distortion

The first successful synthesis of polycrystalline Mg CoH and its deuteride MgjCoDj was reported by Zolliker et al. [57]. The 2Mg-Co elemental mixture was sintered under 4.0-6.0 MPa of hydrogen at temperatures between 350 and 500°C. The formation of the hydride was not complete and the main remaining phases, MgH and Co had to be removed by a separate purification process. X-ray and neutron diffraction data suggested a tetragonally distorted CaF -type crystal structure... 

Nonstoichiometric hydrides that have cubic CaFj-type structures or a tetragonally distorted version form when Ti, Zr and Hf react with Hj. The hydride phases have wide composition ranges that depend on T. The limiting composition is MHj. 

The metals V, Nb, and Ta have b.c.c. structures. The a solid solution of H in this kructure extends to VHq.os, NbH(,., and TaHo.2. The next distinct phase is the/3 hydride, a non-stoichiometric phase which is a (tetragonal) distorted version of the b.c.c. solid solution and is stable over a wide range of composition (for example. 

H atom is tetrahedrally surrounded by U atoms. The / UH3 structure is also found [217] for PaHj. Thorium forms two hydrides ThH, which has a tetragonally distorted fluorite-like structure [218], and Th H s, which is cubic [219] and has each Th atom bonded to nine H atoms at distances of 2.29 and... 

The body-centered cubic Group VA metals dissolve a considerable amount of hydrogen before distorting to either a b.c. tetragonal or orthorhombic structure. These behave somewhat more like solid solutions of hydrogen in metal than do the more definite hydrides previously discussed. They appear to have the stoichiometric formula MH, but this has never been reported, the maximum being about... 

The 5f hydrides are summarized in Table 8.1. In contrast to the other elements Th forms a dihydride with a distorted (f.c. tetragonal) fluorite-like structure, resembling in this respect Ti, Zr, and Hf, but the structure becomes cubic if a little oxygen is present. Th also forms Th4His, in which there are two types of non-equivalent H atom, and the shortest Th—Th distance (3-87 A) is appreciably greater than in the metal (3-59 A) ... 

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