On our exams we are asked to determine which synthesis method is best suited for a given compound. We have discussed direct synthesis methods such as high temp. and ampull, and indirect synthesis methods such as solvothermal, sol-gel, and ion exchange.

I thought that the direct high temperature synthesis was mostly used for compounds with three or more elements such as $\ce{BaTiO3}$. However, in our exam the compound $\ce{Fe3O4}$ is said to be synthesized by the direct high temperature method.

For example, one question states:

Propose and motivate two methods (each) of synthesis of $\ce{NaAlSiO4}$ and $\ce{Cu(CN)(pyr)2}$

I believe one method for $\ce{NaAlSiO4}$ should be direct high temp. because that method is applicable for aluminosilicates but have no idea what the second one could be. Is there a specific way to determine the synthesis methods of different compounds just by looking at them?

All help is truly appreciated!

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    $\begingroup$ I'd say, no, it's impossible to say which method exactly would work the best, you can only make general assumptions such as organic ligands or hydroxo-salts won't survive a high-temperature synthesis. Metal oxides, on the other hand, can be synthesized by a variety of low- and high-temp methods and their combinations (see e.g. Pechini method). $\endgroup$ – andselisk Dec 27 '19 at 12:26
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    $\begingroup$ The synthesis method depends on your preferred starting material. What do you propose for both of them? $\endgroup$ – Karl Dec 27 '19 at 12:27
  • $\begingroup$ @Karl For Cu(CN)(pyr)$_2$ I don't really have any idea on what the methods could be. For NaAlSiO$_4$ the starting materials would be NaAlO$_2$ and SiO$_2$. How does that help determine the method? $\endgroup$ – katara Dec 27 '19 at 12:36
  • $\begingroup$ Your list of methods has a few missing, e.g. precipitation from solution and transport reaction in a temperature gradient. $\endgroup$ – Karl Dec 27 '19 at 12:38
  • $\begingroup$ Yup. Both NaAlO2 and SiO2 have very high melting points. Do you have an oven and crucible that goes that high? I say no. So you need to get the necessary temperature down. Ideas? And the starting materials for the other one are rather obvious, think simple! $\endgroup$ – Karl Dec 27 '19 at 12:48

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