2
COMMON REACTIONS EMPLOYED IN SYNTHESIS

Various types of chemical reactions are used in the synthesis of inorganic materials [1, 2]. Corbett [1] has written a fine article on the subject. Some of the common reactions employed for the synthesis of inorganic materials are described as follows:

1. Decomposition
2. Addition
3. Metathetic reaction (which combines 1 and 2)
4. Other exchange reactions

Typical examples of these reactions are as follows:

Complex reactions involving more than one type of reaction are employed in solid-state synthesis. For example, in the preparation of complex oxides, it is common to carry out thermal decomposition of a compound followed by oxidation (in air or O2) essentially in one step.

Vapour phase reactions and liquid-gas reactions yield solid products in many instances. For example, the reaction of TiCl4 and H2S gives solid TiS2 and HCl gas. Reaction of metal halides with NH3 to yield nitrides is another example.

In chemical vapour transport reactions, a gaseous reagent acts as a carrier to transport a solid by transforming it into the vapour state. For example, MgCr2O4 cannot be readily formed by the reaction of MgO and Cr2O3. However, Cr2O3 (s) reacts with O2 giving CrO3 (g), which then reacts with MgO giving the chromate. The overall reaction is

Some of the typical transport reaction equilibria are

Transport of two substances in opposite directions is possible if the reactions have opposite heats of reaction. For example, Cu2O and Cu can be separated by using HCl as the transporting agent.

Another example of this kind is the separation of WO2 and W by using I2 (g), involving the formation of WO2I2 (g). Volatility of the product also allows its separation from other species. Thus, the reaction of Cl2 gas with a solid mixture of Al2O3 and carbon yields AlCl3 and CO gas.

Vapour transport methods are used in the synthesis of materials as exemplified by the reaction of MgO and Cr2O3; another example is the formation of NiCr2O4 involving the CrO3 (g) species:

The formation of Ca2SnO4 by the reaction of CaO and SnO2 is facilitated by CO via the formation of gaseous SnO, which then reacts with CaO. ZnWO4 is made by heating ZnO and WO3 at 1330 K in the presence of Cl2 gas (volatile chlorides being the intermediates). In the reaction of Al and sulfur to form Al2S3 by using I2, the sulfide is transported through the formation of AlI3.

Cu3TaSe4 is formed by the reaction of Cu, Ta and Se in the presence of gaseous I2. In Table 2.1, we list a few examples of the chemical transport system. Table 2.2 lists some crystals grown by the chemical vapour transport method.

Table 2.1 Examples of chemical transport

Table 2.2 Examples of crystals grown by chemical transport

Oxidation of many metals occurs slowly. Thus, oxidation of Cu stops at the stage of Cu2O at 1270 K in oxygen. In order to promote further oxidation (e.g. to CuO in the case of Cu), an easily oxidizable salt is used (e.g. CuI CuO at 620 K). Similarly, fluorination of a compound may be easier than that of the native metal (e.g. CuCl2 CuF2 in the presence of F2, instead of Cu + F2).

Reduction of oxides is carried out in an atmosphere of (flowing) pure or dilute hydrogen (e.g. N2-H2 mixtures) or sometimes in an atmosphere of CO or CO-CO2 mixtures. Reduction of oxides for the purpose of lowering the oxygen content is also achieved by heating oxides in argon or nitrogen or by using other metals as getters (e.g. Ti or Zr sponge, molten Na) to remove some of the oxygen. Thus, the oxygen content of YBa2Cu3O7-d can be varied by heating in N2 or in the presence of hot Ti sponge. Application of vacuum at an appropriate temperature (vacuum annealing or decomposition at low pressures) is also used. Exact control of oxygen stoichiometry in oxides such as Fe3O4 or V2O3 is accomplished by annealing the oxide in CO-CO2 mixtures of known oxygen fugacity at an appropriate temperature. In preparing oxides of exact stoichiometry, it is necessary to have the fugacity diagrams of the type shown in Figure 2.1. The obvious means of reducing solid compounds is by hydrogen. Hydrogen reduction is employed for reducing not only oxides, but also halides and other compounds. Thermal decomposition of metal halides often yields lower halides.

Figure 2.1 Stability diagrams for (a) Co1-xO and (b) Fe1-xO in long f (O2)-temperature representation. Upper solid line gives the oxidation limit and lower solid line the reduction limit. Dashed lines, CO/CO2 gas mixtures with percentage of CO2 shown in number (i.e., 100CO2/C) + CO2).

Reduction of oxides can be accomplished by reacting with elemental carbon or with a metal. Reduction of halides is also carried out by metals.

Metals such as aluminium are used as reducing agents for other metal halides.

Metal oxychlorides are obtained by heating oxides with Cl2 (LaOCl from La2O3). Fluorination is generally carried out by using elemental fluorine, HF or other fluorine compounds (see Section 14.3 for details). There are examples where oxides are reacted with a fluoride such as BaF2 to attain partial fluorination. Sulfidation is generally carried out by heating the metal and sulfur together in a sealed tube (see Section 14.2). Oxides can be sulfided by heating them in a stream of H2S or CS2.

Plasma or electrical discharge reactions have been employed for material synthesis. Amorphous silicon is produced by the decomposition of SiH4 under discharge. Unusual compounds such as ZrCl3 are obtained by rapid quenching of the plasma out of the discharge region. Plasma spray techniques are employed to prepare films of materials. In the presence of oxygen, the plasma technique is useful in preparing certain oxides as exemplified by oxygen-excess La2CuO4.

Substitution of one metal ion by another is often carried out to attain new structures and properties. For example, partial substitution of Ni in metallic LaNiO3 by Mn makes it non-metallic. On the other hand, partial substitution of Ln3+ by Sr2+ in insulating LnCoO3 (Ln = La, Pr, Nd etc.) makes the d-electron itinerant and the material becomes ferromagnetic. Thus La0.5 Sr0.5CoO3 is a ferromagnetic metal [3]. Similar changes are brought about by the substitution of La3+ by Sr2+ or Ca2+ in LaMnO3 [4]. Partial substitution of V by Ti in V2O3 wipes out the metal insulator transition and makes the material metallic. In the non-linear optical material, KTiOPO4, tetravalent Ti can be usefully replaced partly by pentavelent Nb, provided P is proportionately replaced by Si as in KTi0.5Nb0.5OP0.5 Si0.5O4 [5]. Relative ionic size and charge neutrality govern these substitutions.

2.1 SOFT-CHEMISTRY ROUTES

It was pointed out earlier that soft-chemistry routes have been receiving considerable attention recently. It would be instructive to examine a few typical examples of soft-chemical methods of material synthesis (chimie douce). Marchand et al. [6] obtained a new form of TiO2 by the dehydration of H2TiO9·xH2O, which in turn was prepared by the exchange of K + with H + in K2Ti4O9. The mechanism of this transformation has been described recently by Fiest and Davis [7] and we show this schematically in Figure 2.2. Rebbah et al. [8] prepared Ti2Nb2O9 by the dehydration of HTiNbO5, the latter having been prepared from KTiNbO5 by the cation exchange (Fig. 2.3). A fine example that typifies an entire class of reactions yielding novel, metastable materials is the oxidative deintercalation of LiVS2 to give VS2, which...