Pure-element precursors

Traditional solid-state synthesis involves the direct reaction of stoichiometric quantities of pure elements and precursors in the solid state, at relatively high temperatures (ca. 1,000 °C). Briefly, reactants are measured out in a specific ratio, ground together, pressed into a pellet, and heated in order to facilitate interdiffusion and compound formation. The products are often in powdery and multiphase form, and prolonged annealing is necessary in order to manufacture larger crystals and pure end-products. In this manner, thermodynamically stable products under the reaction conditions are obtained, while rational design of desired products is limited, as little, if any, control is possible over the formation of metastable intermediates. ... 

CVD processes have a greater flexibility of using a wide range of chemical precursors such as halides, hydrides, organo-metallic compounds and so forth which enable the deposition of a large spectrum of materials, including metals, non-metallic elements, carbides, nitrides, oxides, sulphides, as well as polymers. Up to now, around 70% of elements in the periodic table have been deposited by the CVD technique, some of which are in the form of the pure element, however, more often the compound materials. 

Occasionally some pure elements can also be used as precursors. For example, both Zn and Cd are vaporised as precursors for ZnS and CdS [23] respectively. 

The generation of such intermediate products does not take place in the case of MBE. Here the sources are either purely elemental or at least do not contain hydro-organic compounds. This means that reactive CHj or H radicals cannot be produced during or before MBE growth. The formation of such chemical byproducts in the form of carbon hydrides or hydrogen occurs during the pyrolysis of aU the precursors mentioned in Table 13.2 and likewise influence the surface formation of aU III-V and II-VI compounds. This influence on the atomic structure of the growing surface can take place in two ways ... 

The possibility of the incorporation of oxygen into the particle is particularly relevant for the carbides and nitrides of molybdenum and tungsten which possess a high affinity for this element.13,23 The oxygen may come from the carbonyl precursor, and result in oxycarbide or oxynitride formation in the core of the nanoparticle itself.16 Exposure to the ambient can also result in the formation of surface oxycarbides and oxynitrides with catalytic properties different from those of the pure nitride or carbide phase.15,24-26 However, heat treatment of these nanoparticles with a mixture of methane/hydrogen or ammonia/hydrogen should convert the surface to a pure nitride or carbide form. 

Few data are available on the hydrolysis of simple metal alkoxides of these elements. Alkoxides of alkaline and alkaline earth metals are mostly used as precursors for the preparation of complex oxides or solid oxide solutions. Commercial production of pure magnesium oxide by hydrolysis of Mg(OMe)2 with formation of transparent gel has been described [715], as well as hydrolysis of Mg(OC5H11i)2 with the following thermal treatment to produce a fine MgO powderthat sinters at low temperatures [1766]. Solutions prepared by dissolving magnesium in methoxyethanol are by far the most convenient precursors for preparation of magnesium oxide films. 

As can be seen from Scheme III, lanthanide halides are suitable precursors for the synthesis of homoleptic derivatives such as silylamides [114], cyclopen-tadienyls [115] and aryloxides [116]. Such organometallies can be readily obtained in a pure form by sublimating them from the reaction mixture. They themselves are important precursors in organometallic transformations (vide infra). Heteroleptic complexes of the type CpxLn(halide)y (x + y = 2,3) are important synthetic precursors with respect to formation of various Ln-X bonds via simple metathesis reactions [2-29]. Fig. 4 indicates the lanthanide element bonds which are involved in these ubiquitous heteroleptic cyclopentadienyl systems. 

The synthesis of pure precursor molecules is crucial, e.g., Ln(OR)3 or Ln(SeR)2. Well-defined compounds which contain specific element compositions are attractive for ceramic and electronic materials or catalysts in organic transformations. Hitchcock et al. reported the synthesis of monomeric homo-leptic lanthanide(III) aryloxides according to this route for the first time in 1983... 

Pyrophoric A compound that reacts with air in a way that results in spontaneous ignition. m-V compound semiconductor A semiconductor that in pure form is composed of a mixture of atoms of one or more elements from Column III and an equal number of atoms of one or more elements from Column V of the Periodic Table. The Column III atoms are arranged on one sublattice and the Column V atoms are on another. Vapor-phase epitaxy (VPE) An epitaxial growth process that only uses chemical precursors that are delivered to the growing surface in the vapor-phase. 

Most transition elements are available in a pure state as metals which can be dissolved in acids. A mixture of nitrates can be evaporated to dryness and calcined to form precursor oxide mixtures for the preparation of spinel and garnet ferrites. Alternatively, mixed oxides, carbonates or oxalates can be precipitated. Microwave ferrites that are required to be of high purity can be prepared by one of these chemical routes. 

Preparative Methods the title compound can be prepared by reaction of (R)-2-[l-(dimethylamino)ethyl]phenyllithium with elemental sulfur (eq 1). A solution of pure (R)-2-[l-(dimethyl-amino)ethyl]phenyllithium in THF is slowly added at —50°C to a suspension of a stoichiometric amount of freshly sublimed sulfur. The solution is warmed to room temperature and quenched with an equimolar amount of a 10 M aqueous HCl solution. All volatiles are evaporated at reduced pressure and the residue is sublimed at 120 °C in vacuo (0.1 mmHg). The nitrogen-functionalized derivatives (R)-2-[l-(l-pyrrolidinyl) ethyljbenzenethiol and (R)-2-[l-(l-piperidinyl)ethyl]benzen-ethioP may be prepared in a similar way. It should be noted that reaction with MesSiCl instead of HCl after the sulfur insertion reaction affords the corresponding trimethylsilyl thio ether, which also is a valuable catalyst precursor. ... 

A fraction of Ce, La. Nd and Pr derived from bastnasite or monazite is a typical feedstock in the recovery process of cerium on a commercial scale. Separation of the rare-earth elements may be achieved by splitting the mixed rare-earth elements into a cerium/lanthanum and didymium (Nd/Pr) fraction first. The cerium/lanthanum fraction may be used as a further feedstock in a second extraction stage and will yield high pure cerium and lanthanum solution respectively. Cerium can then be precipitated as. for example, an oxalate or a carbonate which may be used as precursor for cerium derivatives. 

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