Hydrides structural disorder

From either hydrogenated binary Mg3lr alloy or by sintering powder mixtures of the elements at up to 773 K under a H2 pressure of up to 150 bar. Red colored hydride partially disordered structure (Figure 45) from npd on deuteride, P2 /c, Z = 8 contains four symmetry independent Ir hydride complexes a spy [IrHs]" " and three sad [lrFl4] complexes of which two are disordered, and five hydride anions are exclusively bonded to Mg all complexes are surrounded by Mg in distorted cubic... 

Band structure calculations of metal hydrides have provided understanding of the bonding characteristics of these materials and clarified the mechanisms involved in various physical properties such as superconductivity. Since the electronic structure of stoichiometric hydrides has been discussed extensively in the literature, in this article we will focus on the methodology and results of disordered hydrides. This disorder can occur on the metal site of the hydride by considering random substitutions of the host metal or on the hydrogen sublattice where vacancies appear. 

N. Hanada, S. Orimo, H. Fujii, Hydriding properties of ordered-/disordered-Mg-based ternary Laves phase structures, J. Alloys Compd. 356-357 (2003) 429-432. 

As mentioned in Sect. 3.2.3 a drawback of this method is the formation of a byproduct, which is salt, which reduces the overall hydrogen content of the synthesized mixture. Another peculiarity of this synthesis method is a complete lack of XRD peaks corresponding to a crystalline synthesized borohydride in the mixture in the right-hand side of the reaction of (3.37). Only the peaks of either LiCl or NaCl are present. On the other hand, the Raman spectra seem to indicate the presence of borohydride. Nakamori et al. [173, 174] interpreted this peculiar behavior as arising most likely due to disordering of the crystal structure of a synthesized borohydride, whatever it means for a sohd state hydride. Application of MCAS to the synthesis of Mg(BH )2 will be discnssed later in the text. 

These two structures illustrate a fundamental and disturbing point about diffraction experiments. It is not until the late stages of refinement, after considerable time and money has been spent on the experiment, that one sometimes discovers his inability to define accurately a salient feature of the structure—in this instance the hydride position in the triazenido complex. There is no way from the formula, space group, or films to have predicted this nor are there any usefully consistent methods that enable one to predict, especially in common low-symmetry space groups, when disorder will occur. 

The structure of (/i2-H)(H)Os3(CO)n is, unfortunately, subject to some disorder and it was not possible to locate the hydride ligands by direct methods. However, their locations could be determined unambiguously from the geometry of the cluster the derived ordered structure is illustrated in Figure 4. The position... 

One motivation to carry out a neutron diffraction investigation of H3Ni4(Cp)4 was to check the possibility of disorder of the hydride ligands over all four faces of the Ni4 tetrahedron. The hydrides were not located from the x-ray data (33, 34). Rather, their positions were inferred from the deviations of the structure from strict tetrahedral symmetry. The observed Cp(i)-Cn-Cp( ) angles (see Table IV) are distorted from the tetrahedral value such that Cp(2), Cp(3), and Cp(4) are bent away from Cp(l). The face defined by Ni(2), Ni(3), and Ni(4) therefore could be expected to be vacant. Our neutron results indicate, that this is indeed the case, with no evidence for disorder of the hydride ligands on the nuclear density maps. 

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. 

We will notice the following important circumstance in the area of disordered a-(3 phases, initial IMC crystal structure in the most instances does not differ from the structure of the metallic matrix in the hydride phases in IMC-hydrogen systems. In this case chemical potential o.h=Gh/Nh of the hydrogen component of the IMC hydride (that is, specific, per H atom, Gibbs energy GH) is as follows ... 

The character of the filler effect depends on the affinity of the silica surface for the copolymer, and the rigidity of macromolecules. For MEDDE-DVB and DMGE-DYB copolymers, the introduction of both methyl-, and methyl,hydride-containing silicas results in the formation of amorphous structures. A greater degree of disorder was detected in the presence of silicon hydride groups on the filler surface. 

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