Metals interstitial compounds

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). 

JS Haggerty, DW Lee. Plastic deforination of ZtB2 single crystals. J Am Ceram Soc 54 572, 1971. J BUUngham, PS BeU, MH Lewis. The crystal growth of transition metal interstitial compounds hy floating zone technique. J Cryst Growth 13/14 693, 1973. 

The transition metal structures consist of close-packed (p. 26) arrays of relatively large atoms. Between these atoms, in the holes , small atoms, notably those of hydrogen, nitrogen and carbon, can be inserted, without very much distortion of the original metal structure. to give interstitial compounds (for example the hydrides, p. 113). 

In tire transition-metal monocarbides, such as TiCi j , the metal-rich compound has a large fraction of vacairt octahedral interstitial sites and the diffusion jump for carbon atoms is tlrerefore similar to tlrat for the dilute solution of carbon in the metal. The diffusion coefficient of carbon in the monocarbide shows a relatively constairt activation energy but a decreasing value of the pre-exponential... 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

Polycrystalline oxide materials, both undoped and doped, have been extensively examined for use as photoanodes. Ti02 electrodes have been prepared by thermal oxidation of a Ti plate in an electric furnace in air at 300-800°C (15-60 min) and in a flame at 1300°C (20 min) [27-30]. XRD analysis of thermally oxidized samples indicates the formation of metallic sub-oxide interstitial compounds, i.e. TiOo+x (x < 0.33) or Ti20i y (0 < y < 0.33) and Ti30 together with rutile Ti02 [27]. The characteristic reflection of metallic titanium decreases in intensity after prolonged oxidation (60 min) at 800° C indicating the presence of a fairly thick oxide layer (10-15 pm). Oxidation at 900°C leads to poor adhesion of the oxide film... 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

In interstitial compounds, however, the nonmetal is conveniently regarded as neutral atoms inserted into the interstices of the expanded lattice of the elemental metal. Obviously, this is an oversimplification, as the electrons of the nonmetal atoms must interact with the modified valence and conduction bands of the metal host, but this crude picture is adequate for our purposes. On this basis, Hagg made the empirical observation that insertion is possible when the atomic radius of the nonmetal is not greater than 0.59 times the atomic radius of the host metal—there is no simple geometrical justification for this, however, as the metal lattice is concomitantly expanded by an unknown amount. These interstitial compounds are sometimes called Hagg compounds.9,10 They are, in effect, interstitial solid solutions of the nonmetal in the metal (as distinct from substitutional solid solutions, in which actual lattice atoms are replaced, as in the case of gold-copper and other alloys Section 4.3). 

Finally, there is another property of the interstitial compounds that should be mentioned. In contrast to the ionic halides of the same metal, they usually have not a stoichiometric composition, but form phases with broad regions of existence see Section 24) with, many positions, available for negative ions, remaining unoccupied. 

The properties of the interstitial compounds thus can be explained by assuming a combination of metallic and ionic bonding. As in all ionic bonds, there may be some covalency. The question of how far the non-metallic bonds are still ionic, or partially covalent, will not be discussed. 

The compounds with larger negative ions, like sulphides, selenides and phosphides, in many respect resemble the interstitial compounds. Like the latter, they often show metallic properties and have variable compositions, but their crystal structures are of quite a different type. 

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