Impurities Borides

Other electropositive elements have been used (e.g. Li, Na, K, Be, Ca, Al, Fe), but the product is generally amorphous and contaminated with refractory impurities such as metal borides. Massive crystalline boron (96%) has been prepared by reacting BCI3 with zinc in a flow system at 900°C. 

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. 

INTERSTITIAL. < 11 Descripiive of a nnnstoichiuiiiclric compound ol a metal and a nometal whose structure conforms to. 1 simple chemical formula, bill exists over a limited range of chemical composition. Imersiinal compounds are represented by borides, nitrides, and carbides ol the transition metals. 21 Descriptive n an atom of an impurity lhai causes a del cel... 

Elemental boron is a refractory material that is usually isolated either as a shiny black crystalline solid or a softer, browner, more impure amorphous solid. Reduction of readily available boron compounds containing boron oxygen bonds to elemental boron is energy intensive and costly. This has limited the extent of conunercial use of this material. Many related refractory boron compounds have been prepared and characterized including metal borides, boron carbides, boron nitrides, and various boron metal alloys. These refractory materials and elemental boron are also discussed in some detail in the article Borides Solid-state Chemistry. Other general references are available on elemental boron and other refractory boron compounds. " ... 

It is very often difficult to establish the existence and composition of borides owing to one or more of the following reasons. The great reactivity of boron toward, for instance, the crucible material leads to the occurrence of impurities in the products. The impurities are often said to stabilize a structure type, while in reality a ternary compound has been formed although unintentionally. It is also difficult to analyze chemically or by microprobe the boron content owing to low accuracy of the methods. Indeed, in many cases a single-crystal-structure determination is, together with complementary structural information obtained by other... 

Materials that do not occur in nature or are rare must be synthesized (so calling them minerals is a misnomer) and we describe the processes used for their synthesis. Carbides, nitrides, and borides are becoming more common, but are generally expensive and require special processing environments. For many nonoxides the main impurities are often components of the starting material that have not reacted, e.g., A1 in AIN or Si in SijN4. 

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