Hydrides orthorhombic structure

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 third subgroup consists of Eu and Yb dihydrides, which have an orthorhombic structure and resemble the alkaline-earth hydrides, CaHj, SrHj and BaHj (see 1.8.3.3). 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

Hexagonal AB5 compounds form orthorhombic hydrides. Basal plane expansion, caused by hydrogen occupation of interstitial sites (4), change the compound s structure and can be ordered or disordered. For example, Kuijpers and Loopstra (4) found, by neutron diffraction, that the deuterium atoms in PrCo5D4 were ordered on certain interstitial octahedral and tetrahedral sites. The sites occupied by hydrogen in various AB5 hydrides are not fully understood because of insufficient neutron diffraction data. The available information is considered in the section on configurational entropies. 

Eu—H system.—The lighter rare earth trihydrides have a face centered cubic structure whilst those of samarium and the heavier rare earths are hexagonal close packed. Europium and ytterbium, however, form orthorhombic dihydrides [236]. Although it has been possible to prepare [237] a hydride of ytterbium with empirical formula YKH2.55 all attempts to prepare the europium trihydride failed. 

Ciba, Ltd. (1965) Dutch Patent Application 6,405,130. Chem. Abstr.,63,15103. [262] Ciechanowicz, M., Skapski, A. C. and Troughton, P. G. H. (1976). The crystal structure of the orthorhombic form of hydridodicarbonylbis(triphenylphosphine)-iridium(I) successful location of the hydride hydrogen atom from x-ray data. Acto Crystallogr. B, 32, 1673-80. [44]... 

Calcium, Sr, Ba and presumably Ra react directly with to form ionic hydrides that have the orthorhombic PbClj structure resembling the heavier alkaline-earth halides (e.g., BaClj, BaBtj, Balj, SrBtj). 

Hydrides are made from 1 1 intermetallics of groups V and VIII transition metals. For example, TiFe, TiCo and TiNi have the cubic CsCl structure, but on hydriding the first two form monohydrides and dihydrides that are orthorhombic and monoclinic, respectively, although they may be viewed as distorted CsCl structures. No hydride phase is formed from TiNi at RT (or above), but a solid solution of maximum composition TiNiH, 7 forms . The ZrCo and HfCo intermetallics also have the CsCl structure but transform to the orthorhombic CrB structure on formation of a monohydride... 

Hydrides. The thermodynamic characteristics of the Nb-H system have been investigated and the crystal structure of P-NbH, which has a face-centred orthorhombic lattice, has been determined." The crystal structure of Ta2D has been,reported." ... 

This section refers to intermetallics of type MX2 that do not take the Laves-phase structure. Two materials crystallizing in the orthorhombic CeCu2 structure and some R-hydrides have been studied by xSR. 

Orthorhombic black phosphorus will take up arsenic in solid solution and it forms a definite compound AsP. The latter can also be made by reaction (4.37). An analogous reaction (4.38) may occur with antimony hydride. Arsenic phosphide, (AsP) , resembles orthorhombic black phosphorus in physical properties and has a similar puckered layer structure. Arsenic nitride (AsN) also appears to have a structure of this kind and a similar layer structure might be expected for (PN) (see Chapter 7). 

The RMs compounds have the hexagonal CaCus type structure and they form orthorhombic hydride phases (Bowman et al., 1973 Kuijpers and Loopstra, 1974). Pressure-composition isotherms for the representative LaNi,-H2 system are shown in fig. 26.12 (van Vucht et al., 1970). The hydride formed has a... 

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