Diffraction interstitial hydrogen atom

Table II. Mean Distances 1 (A) Involving the Interstitial Hydrogen Atoms Determined from Neutron Diffraction Studies of Compounds 3 and 4...

T. F. Koetzle (1981). J. Am. Chem. Soc., 103, 1458-1466. An interstitial hydrogen atom in a hexanuclear metal cluster X-ray and neutron diffraction analysis of [(Ph3P)2N]"[HC06(CO),5r... 

NiII0 6 (aQ = 3.72 A). A powder neutron diffraction study (20) of this interstitial compound showed the hydrogen atoms to occupy only octahedral sites of the fee nickel lattice. 

The distribution of hydrogen atoms among the available interstitial sites of the metal host structure is of both fundamental and practical interest with regards to the stabilities and compositions of the various hydride phases. Although x-ray diffraction is commonly used to determine hydride structures, this technique cannot identify the hydrogen site occupancy except under extremely favorable conditions (which are quite rare). Powder neutron diffraction is the conventional method to locate deuterium atoms in metal deuterlde structures. Because the Incoherent... 

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. 

The hydrogen concentration 1 FI atom/f.u. corresponds to full occupancy of the interstitial 2b site according to the neutron diffraction experiment [17]. This interstitial site can be seen as a pseudo-octahedron with two rare-earth ions and four iron atoms 8j at the comers. Interstitial atoms occupy 2b sites adjacent to the rare earth, creating a change of crystal field that reflects the local symmetry and inducing significant changes of the MCA. 

The magnets of Sm-Fe-X (X=C,N) compounds having Th2Zni7 crystal structure, however, are prepared by gas-solid reactions. The third element is gradually introduced into specific sites in the crystal by a bulk difiusion process. In the 2-17 structure, when hydrogen (X=H) is introduced, the H atoms exist in both interstitial 9e and 18g sites in the structure in the cases of C and N, the atoms mainly occupy the 9e site, with a small fraction in the 18g site as shown in some neutron diffraction studies (Jaswal 1992, Isnard et al. 1992, Kajitani et al. 1993). 

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