Borides binary compounds

Many binary and pseudo-binary compounds of the alkali metals are more conveniently treated within the context of the chemistry of the other element and for this reason discussion Is deferred to later chapters, e.g. borides (p. 145),... 

The deposition of a binary compound can be achieved by a coreduction reaction. In this manner, ceramic materials such as oxides, carbides, nitrides, borides, and silicides can be produced readily and usually more readily than the parent metal. A common example is the deposition of titanium diboride ... 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

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]. 

An example of (a) is the structure of NiAs, in which Ni has 6 As neighbours but is also bonded to 2 Ni (b) represents a normal binary compound with only A—X bonds, while an example of (c) is a boride such as FeB in which B is surrounded by 6 Fe but is also bonded to 2 B atoms. 

Recently, there has also been an increase in the importance of melts in their use as a reaction medium for chemical and electrochemical synthesis of compounds for functional and construction ceramics, e.g. double oxides with spinellitic and perowskite structure and binary compounds with prevailing covalent bond character, mainly borides and carbides of transition metals. 

In Chapters 20 and 21 we shall look at individual elements of the c -block in detail. However, a few general points are given here as an overview. In general, the metals are moderately reactive and combine to give binary compounds when heated with dioxygen, sulfur or the halogens (e.g. reactions 19.1-19.3), product stoichiometry depending, in part, on the available oxidation states (see below). Combination with H2, B, C or N2 may lead to interstitial hydrides Section 9.7), borides Section 12.10), carbides Section 13.7) or nitrides Section 14.6). 

Other Compounds. Direct reaction of the metals with other elements can lead to binary compounds such as borides, silicides, arsenides, sulfides, etc. Many of these are ionic and are rapidly hydrolyzed by water or dilute acids. At 300°, magnesium reacts with nitrogen21 to give colorless, crystalline... 

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. 

Gradual transitions from one to another type of boron aggregation are monitored at best in binary compounds such as e.g. the borides of the transition metals (T) in the concentration range between monoboride and diboride, forming a structural... 

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