Solid-State Metathesis Materials Synthesis

In the last two decades, a new method termed solid-state metathesis (SSM) has been developed to synthesize compounds that are often difficult to produce conventionally. Alkali or alkaline-earth metal main group compounds are combined with halides to form oxides,^1–7 sulfides,^{3, 8} selenides,^{3, 8} tellurides,³ nitrides,^9–15 phosphides,^16–18 arsenides,¹⁷ antimonides,¹⁷ carbides,^{19, 20} silicides,²¹ borides,^{15, 22, 23} and aluminides.²⁴ Additionally, nanostructured materials such as nanotubes,²⁵ nanocrystals,^{26, 27} and high-surface-area materials²⁸can be produced. These reactions provide a novel tool for preparing difficult-to-synthesize materials. Although the precursor is usually a halide, an oxide can be used as well. For example, we have shown that MgSiN₂ can be synthesized by reacting SiO₂ with Mg₃N₂.¹³Oxides have also been successfully used as precursors for hydroxyapatite²⁹ and silicides.²¹

The driving force behind solid-state metathesis reactions is the formation of stable byproducts. For example, combining GaI₃ with Li₃N to produce GaN, as given in Scheme 1, has a ΔH_rxn of –515 kJ.

This is more than four times as energetic as the elemental reaction Ga + 0.5N2 > GaN (ΔH = –110 kJ).

SSM reactions can be initiated by flame, furnace, heated filament, ball mill, or microwave. The phase produced is often determined by the rate of the reaction.²⁴ Similarly, the reaction rate can be tuned to control the crystallite size of the products formed.⁹ Diluents or fast reaction times can be used to produce nanoscale powders or materials with high surface areas.⁹

In a typical SSM reaction, an alkali or alkaline-earth metal compound with the desired anion is reacted with a metal halide or oxide. Reaction temperatures are controlled through the choice of halide, alkali or alkaline-earth metal,³⁰ dilution, and vessel design. It is important to note that the driving force for these reactions is the heat released by the formation of alkali or alkaline-earth halides.

Alkali halides usually have lower heats of formation than alkalineearth halides, so reactions utilizing alkaline-earth metals will generally result in higher reaction temperatures. The upper limit of the reaction temperature is usually governed by the boiling point of the salt produced. Alkaline-earth salts have higher melting and boiling points, thereby increasing the upper limit of the reaction temperature.

The heavier the halogen used, generally the lower the reaction temperature. This can be useful for the synthesis of compounds that are unstable at high temperatures. For example, it is easier to obtain GaN from GaI₃ than GaCl₃ in a reaction with Li₃N,¹⁰ due to the differences in heats of formation of LiI (ΔH = –270.4 kJ/mol) vs LiCl (ΔH = –408.5). Conversely, the synthesis of some materials is favored by high temperatures. The use of AlCl3 is a better choice than AlI₃ when preparing AlN from the reaction with Ca₃N₂.⁹

Another effective method for lowering a reaction temperature is by using a diluent.³¹ This can be realized by adding inert salts, such as the byproduct salt itself, to the reaction mixture. This process can be improved for nitrides by adding low-melting or -vaporizing nitrogen containing compounds (like NH₄Cl or LiNH₂).¹⁰ Not only do these compounds act as a heat sink, but they can supply nitrogen to the reaction as well.

A final method for reducing a reaction temperature is by controlling the heat loss. In this way, these reactions can be effectively scaled up without producing destructive amounts of heat. This can be achieved by specifically designing the reaction vessel¹⁰ or by controlling the amount of heat produced by slow addition of precursors into the reactor. If a hotter reaction is desired, choose the starting materials carefully or begin these reactions at an elevated temperature.

In summary, solid-state metathesis (SSM) reactions have developed over the past two decades into an effective method for synthesizing materials that are difficult to make by conventional methods.³¹