Metal interstitial

Handbook of Chemical Vapor Deposition 1.1 Refractory-Metal (Interstitial) Carbides... 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

Metal-excess oxides can change composition by way of metal interstitials or oxygen vacancies. The formation of cation interstitials in a nonstoichiometric oxide MO can be represented by... 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

It is usually assumed that the dominant defect species responsible for nonstoichiometry is constituted by oxygen vacancies (rather than metal interstitials). This assumption can be justified on the basis of X-ray and neutron diffraction and density measurements . ... 

Ionic hydrides T Metallic (interstitial) hydrides T Covalent hydrides ... 

Since s does not remain constant with temperature, there are two unknown parameters in each equation. An IBM-7090 computer was used to obtain the best value of s and Evv (or Eu) at each temperature. Much better agreement was obtained with the assumption of hydrogen vacancies rather than metal interstitials for the case of uranium hydride. A comparison of the theoretically derived curves with the experimental points for uranium hydride is shown in Figure 3. For the case of palladium hydride, equally good agreement was obtained with both assumptions. However, on the basis of x-ray data, it was decided that the nonstoichiometry was due to hydrogen vacancies. The calculated... 

The converse conclusion, that it would also be plausible that these lattices are built up from ions, is however certainly incorrect, since diamond, the hardest substance of all, is certainly not built up of quadrivalent negative and positive ions. Atomic bonding, which here and certainly also in the two first-mentioned compounds predominates, likewise leads to extreme hardness, which is also encountered in the last-mentioned metallic interstitial compounds (p. 322). 

Interestingly, members of this series of compounds are found containing transition-metal interstitial atoms as well. We can consider the effects of the... 

Metal interstitial with effective positive charge Nonmetal interstitial with effective negative charge m XI... 

Covalent (molecules containing covalently bound hydrogen to nonmetals, with individual molecules held together by intermolecular forces, e.g., CH4, SiHa) Metallic/interstitial (hydrogen molecules are contained in vacant interstitial sites of a transition-metal lattice, e.g., PdHo.e)... 

As an alternative to the above description which relates the shear plane to oxygen vacancies, these extended defects may be discussed in terms of metal interstitial... 

These alternative descriptions of shear-plane formation will be valuable in our discussion of mechanism in Section 4. In the account now presented of the relative stabilities of shear plane and point defect structures we will assume that vacancies are the predominant point defects. However, the arguments we present could be adapted for metal interstitial defect structures. 

A difficulty, however, arises in that detailed analysis of the data of Baumard et al. in the near-stoicheiometric regions suggests that the predominant point defects are metal interstitials rather than the oxygen vacancies assumed so far in... 

The most obvious heterogeneous mechanism follows from our discussion in Section 2 where we showed that shear planes could be related to metal interstitial defects, but that a pre-existing anti-phase boundary (APB) is required. Thus shear-plane formation may occur by metal interstitial capture at pre-exist ng APBs (Bursill et More particularly, it is proposed that metal interstitial ions produced by... 

The main structural features of the transition-metal interstitial carbides are as follows. The maximum carbon content depends on the c.p. layer sequence. The two octahedral interstices on either side of an h layer are located directly above one another, and only one of these is ever occupied. This restriction gives the following limiting formulae ... 

There are basically three types of hydrides ionic, covalent and transitional metal-interstitial hydrides. 

Many metal carbonyl clusters have interstitial atoms or groups located in the eenter of the polyhedron. Such interstitial atoms may be a light atom sueh as boron, carbon, or nitrogen a post-transition element such as germanium, tin, or antimony or a transition metal. Interstitial atoms most frequently provide all of their valence electrons as skeletal electrons since all of their valence orbitals are neeessarily internal orbitals because of the location of the interstitial atom in the center of the polyhedron. Exceptions to this rule may occur when some of the valence electrons of the interstitial atom occupy orbitals of symmetries which cannot mix with any of the molecular orbitals arising from the polyhedral skeletal bonding. 

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