Hydrogen Neel temperature

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

The temperature and thickness dependence of the electric resistivity of Sm, Dy and Tm thin films (25-370 nm thick) was measured between 4.2 and 300 K by Dudas and Feher (1984, 1987), Dudas et al. (1985, 1986, 1987a-d, 1990), and Janos et al. (1987). For Dy films the Neel temperature and the residual resistivity ratio increase both as thickness increases. The thickness dependence of the spin-disorder resistivity is reported in fig. 6. In the case of Tm samples it is observed that the Neel temperature increases (from 49 to 54 K) and in-disorder resistivity decreases (from 35 to lOpQm) as thickness increases (fig. 7). Such results, and correlations with the crystalline orientations (basal plane of crystallites parallel or perpendicular to the substrate surface) correct the errors made by other investigators (see Gasgnier 1980). On the other hand the resistance ratio as a fimction of the temperature exhibits different kinds of curves as the thickness varies as measured for Tm films (fig. 8). Different magnetic transitions have been also observed. One can conclude that numerous kinds of anomalies were caused by hydrogen in solution in the metallic matric, in agreement with Dudas (1991), and also by structure... 

The Neel point of cobalt monoxide is 291 K. For obvious reasons this oxide cannot be heated in hydrogen without reduction to the metal. A sample of surface 5.8 m2 g 1 was pretreated in situ by heating it in purified helium at 823 K. The catalyst was then cooled to near room temperature before the admission of hydrogen. Figure 20 shows A k for CoO as a function of extrinsic field up to 18 kOe at 301 K. At this temperature ka was about 7.2 /u.mol m-2 s l. Figure 21 shows results for the same sample at 275 K. Figure 22 shows k0 and AkH at 17.3 kOe over the temperature range 275-300 K. 

The foregoing remarks do not hold, of course, for the dihydrides of the triva-lent lanthanides. They exhibit metallic conduction (10), as would be expected, since their conduction band is only somewhat depleted. One would expect them to display a tendency to order at low temperatures, but it seems not unreasonable to expect that this tendency would be weaker than the corresponding element, in view of the decreased electron concentration, and the ordering would hence occur at lower temperatures. This was in fact observed for HoH2, which exhibits (6) a Neel point at 8° K., as coippared to 135° K. for Ho. It is also observed in the present work for the terbium dihydrides, whose Neel points are 40° to 50° K., whereas that for the element is 241° K. These properties are compatible with the notion that hydrogen in all the lanthanide hydrides is anionic. On this basis the dihydrides appear as an intermediate form between the truly metallic elements on the one hand and the truly ionic or saline trihydrides on the other. 

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