Hf-N systems

Figure 15 Wedge-type sample of the Hf-N system with various diffusion layers ...

Besides the f.c.c. 8HfNj the Hf-N system contains two subnitride phases [41], riHf3N2 ( and Hf4N3 j. Only a few phase equilibria studies have been performed for this system and they are cited in the ASM Handbook [10]. The decomposition temperatures of the subnitride phases need to be re-investigated. 

Another structure is the simple hexagonal structure (hex) such as that of tungsten mononitride (8WN) where the metal atom layers form a sequence of layers AA or BB. Such structures are not close-packed and do not form octahedral sites the available interstitial sites are trigonal prisms (see Fig, 3.13 of Ch. 3). This structure cannot form if the ratio of the nitrogen/ metal atomic radii is small, as is the case in the Zr-N and Hf-N systems. 

Having these severe approximations in mind, SCC-DFTB performs surprisingly well for many systems of interest, as discussed above. However, it has a lower overall accuracy than DFT or post HF methods. Therefore, applying it to new classes of systems should be only done after careful examination of its performance. This can be done e.g. by conducting reference calculations on smaller model systems with DFT or ab initio methods. A second source of errors is related to some intrinsic problems with the GGA functionals also used in popular DFT methods (SCC-DFTB uses the PBE functional), which are inherited in SCC-DFTB. This concerns the well known GGA problems in describing van der Waals interactions [32], extended conjugate n systems [45,46] or charge transfer excited states [47, 48],... 

Hf-N The system is complex with several intermediate phases Hf3N2, Hf4N3, HfN (40-52.5 at.% N), NaCl-type. 

BrF 98.92 20/ -33 + bromofluorinations, frequently in situ preparation of the agent from N-bromo-substituted amides and HF, HF-containing systems or AgF 4,7... 

The 5n2 reactions of y-substituted allyl chlorides where the nucleophile is a free anion and an ion pair were studied theoretically at the HF/6-31+G level of theory.10 The calculations showed that the free ion, S N2 reaction proceeds through a transition state (7) with significant positive charge on Ca and little conjugation with the n-system. Placing substituents on Cy of the free ion reactions gave computed Hammett p values of -3.3 and -4.6 for the trans- and ds-conformation reactions, respectively. The transition states (8) for the ion pair reactions, on the other hand, have considerable positive charge on both Ca and Cy and the computed Hammett p value is +18.8. 

A recent study of some other excited states of NH2 by Brown and Williams463 uses the UHF method. Of particular interest is the 2B2 state, with J (N—H) = 3.04 bohr, 0 = 26.6°. Thomson and Brotchie464 have repeated the calculations with a basis set giving near-HF accuracy and confirm these results. However, the species is not bound, and it seems to correspond to a H2 + N system. However, it is also possible for the SCF to converge to another apparently bound state, which population analysis shows to be an erroneous conclusion. However, some analogous states of BF2 are bound (see below). 

If the solid solution of nitrogen in Zr, o -Zr(N), which exists up to a composition Zr2N , is not considered as a nitride phase, the Zr N system is characterized by the presence of only one nitride phase, 5-ZrNi c up to a composition of approximately [N]/[Zr] = 1. It should be mentioned that none of the subnitride phases that exist in the neighboring systems Ti-N and Hf-N occur here. In Massalski s compilation ... 

Figure 6 Phase diagrams of group 4 transition metal-nitrogen systems, (a) Zr-N. (Ref. 11. Reproduced by permission of Materials Information Society/ASM International), (b) Hf-N. (Reprinted from Ref 28. 1991, with permission from Elsevier)...

Hafnium(IV) bis(perfluorooctanesulfonyl)amide complex Hf[N(S02C8F17)2]4 is a highly reactive and recyclable Lewis add catalyst in Friedel-Crafts acylation and Prins reactions in fluorous biphasic systems at low catalyst loadings (<1 mol%) (Equation 4.23). In these reactions, Hf[N(S02C8F17)2]4 is selectively soluble in the fluorous phase and can be recovered after reaction by simple phase separation. In addition, the catalyst can be reused without any loss of activity [44],... 

Hao, X., Yoshida, A., and Nishikido, J. 2005. Hf[N(S02CgPi7)2]4-catalyzed Priedel-Crafts acylation in a fluorous biphase system. Tetrahedron Lett. 46 2697-2700. 

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