Trihydride state

Epitaxial films also switch reversibly between the dihydride and trihydride state although some irreversible relaxation occurs during the first loading cycle (see Fig. 7.13). This irreversible relaxation is due to the ridges which develop in addition to those already formed during deposition. The process through which an epitaxial film can accommodate high reversible strains/stresses without deterioration of its crystallinity was recently described by Kerssemakers et al. [78, 79]. 

Recently, R nanoparticle layers [e.g., Y Bour et al. (2001), Gd Aruna et al. (2004)] have also been utilized as the active layer in the switchable mirrors. In these New generation switchable mirrors , in addition to the size dependent switchable properties, utilization of nanoparticle layers showed that it was possible to tune the band gap of the trihydride state by varying the nanoparticle size (Aruna et al., 2004) and sense very low concentrations of hydrogen (Bour et al., 2001). In a recent study, the use of Pd nanoparticle over layer to enhance the hydrogen induced switching properties of the Gd polycrystalline films has been reported (Aruna et al., 2005c). The studies are discussed in detail in section 4. 

Fig. 93. Yttrium concentration (y) dependence of the (i) lattice parameters (a,c) and (ii) molar volume of Lai j,Yj,H c (Vtn) for th as-deposited, dihydride and trihydride states. The open symbols are the literature values (Gschneidner and Calderwood, 1982 Mueller et al., 1968). The vertical lines represent the phase boundaries fa-the various structural phases (solid lines) and mixed phase region (dotted lines) (van Gogh et al., 2001).

A range of external variables is known to alter dramatically the EAC of nitrogenase preparations in vitro, including concentrations of ATP and reduc-tant, and ATP/ADP ratio (Davis et ai, 1975). In all cases, treatments which increase the rate of electron flow to the MoFe protein also raise the EAC. To explain this effect, Chatt (1980) has suggested that the functional site of nitrogenase contains molybdenum in the mono-, di-, or trihydride state, following complexing with an appropriate number of protons. The minimum stoichiometry of one per N2 would then be accounted for if it were assumed that N2... 

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