Other Binary Carbides

3 Other binary carbides. Boron and carbon form a series of boron carbide phases of which B4C appears to be the most ordered. There are several competing structural models for boron carbide which have been investigated by B and C 

The C NMR spectra of the ionic carbides CaC2 and BaC2 have been determined by Wrackmeyer et al. (1990) who report 8iso values of 206.2 ppm for CaC2 and 232.1 ppm for BaC2, both shifts with respect to TMS. The NMR results indicate that the environment of the carbon atoms in these compounds is not axially symmetric, and that the carbide unit in these ionic carbides is not comparable with the carbon-carbon triple bond in alkynes. 

A practical problem with the N NMR literature arises from the wide range of secondary shift references which have been employed, making it necessary to correct shift values quoted directly against the secondary shift reference before the spectra from different studies can be compared. A common secondary standard is desirable, even if 

Multinuclear Solid-State NMR of Inorganic Materials Table 9.4. N isotropic chemical shifts of pure nitrides and related phases. 

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Other Binary Compounds.—Scandium nitride and zirconium and titanium carbide do not conform with the theoretical radii. It is possible that these crystals do not consist essentially of Sc+3, N 3, Ti+4, Zr+4 and C-4 ions, especially since zirconium and titanium nitride, ZrN and TiN, also form crystals with the sodium chloride structure but possibly also the discrepancy can be attributed to deformation of the anions, which have very high mole refraction values. 

This chapter is concerned with the thermal decompositions of oxides and peroxides. There are obviously very important connections with the reactions of hydroxides (Chapter 8) and so-called peroxysalts, which contain hydrogen peroxide of crystallization (included in Chapter 7 on hydrates). Hydrated oxides vary from compounds accurately represented by the stoichiometric formula M(OH) , to phases which contain discrete HjO molecules. The chemistry of oxides should also be considered in the context of the other binary compounds (e.g. hydrides, nitrides, carbides, sulphides etc.) dealt with in Chapter 10. 

Numerous other binary compounds are obtained by direct interaction at elevated temperatures examples are the semi-conducting sulfides,20 M2S3, which can also be made by reaction of MC13 with H2S at 1100°. Group V compounds, MX, where X = N, P, As, Sb or Bi, which have the NaCl structure borides, MB4, MB6 and carbides, MC2 and M2C3 (pages 290 and 1075). 

Other Binary Compounds. Various borides, sulfides, carbides, nitrides, etc., have been obtained by direct interaction of the elements at elevated temperatures. Like other actinide and lanthanide metals, thorium also reacts at elevated temperatures with hydrogen. Products with a range of compositions can be obtained, but two definite phases, ThH2 and Th4H15, have been characterized. 

The Sc-Co-C phase equilibria at 600°C have been established by Pecharskaya (1989) and are shown in fig. 49. The data about the existence at 600 C of SC4C3 binary carbide are not in agreement with the other literature sources (see sect. 2.14). Earlier the existence of three ternary compounds were reported in the system. Their compositions and the crystal structure data are given in table 18. 

While most of the binary carbides and nitrides considered above form unlimited homogeneous solid solutions, some other 5 and p elements (B, Be, Al, Mg, etc.) have only a low solubility in these phases. As their content in carbides and nitrides increases, ternary compounds with very specific crystal structures are formed which were reviewed by Alyamovsky et al (1981) and Goldschmidt (1967). It is well known (see Samsonov, Serebryakov and Neronov (1975)) that B or transition metal borides do not form unlimited solid solutions, when interacting with MX phases (X = C, N) and single-phase TiNjBj, compounds exist over a narrow composition range for example, when z + y = 0.62-0.94, y < 0.03 (see Alyamovsky, Zainulin and Shveikin (1976)). As the B/N ratio increases. 

Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfHl.86. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Niobium combines with carbon, boron, silicon and other elements at very high temperatures, forming interstitial binary compounds of varying compositions. With carbon, it forms niobium carbide having compositions varying from NbCo.7 to NbC [12069-94-2]. With boron, the products are orthorhombic niobium boride, NbB [12045-19-1], and the hexagonal diniobium diboride, Nb2B2[12007-29-3]. 

Most chemical properties of technetium are similar to those of rhenium. The metal exhibits several oxidation states, the most stable being the hep-tavalent, Tc +. The metal forms two oxides the black dioxide Tc02 and the heptoxide TC2O7. At ambient temperature in the presence of moisture, a thin layer of dioxide, Tc02, covers the metal surface. The metal burns in fluorine to form two fluorides, the penta- and hexafluorides, TcFs and TcFe. Binary compounds also are obtained with other nonmetaUic elements. It combines with sulfur and carbon at high temperatures forming technetium disulfide and carbide, TcS2 and TcC, respectively. 

Vanadium combines with other nonmetals at elevated temperatures forming binary compounds. Such compounds include nitride, VN carbide VC, and the sulfides, VS (or V2S2), V2S3, and V2S5. 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

The Union of Two Elementary Substances.— The most obvious way in which to prepare a binary compound is by the union of the two constituent elements, though in many cases this is not the most practicable way. Sometimes, the elements are first prepared in pure form and are then combined in other cases, the preparation of the elements and their union is effected in one operation, as in the manufacture of calcium carbide and carborundum. In general, the more dissimilar the two elements the more likely they are to combine readily, but elements of the same general kind sometimes combine with ease, as is the case with chlorine and iodine, sulfur and phosphorus, or sodium and lead. 

Although these compounds have been investigated many times several open questions and unexplored properties still remain. For the most part, the phase equilibria in the binary systems have been established but other questions such as the correct homogeneity ranges of the various carbide and nitride phases as well as the decomposition temperatures of certain... 

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