The rational planning of solid‐state synthesis is hampered by the fact that in general little is known on reaction processes and thus they cannot be utilized in an efficient way. This is in stark contrast to molecular chemistry, where retrosynthesis is a very powerful tool for the development of new compounds. Time‐dependent in situ studies of solid‐state synthesis reactions by thermal analysis, diffraction and spectroscopy techniques can unravel reaction pathways. Examples for the synthesis of metal hydrides, nitrides, oxides, by solid gas, electrochemical and ball‐milling methods show that the identification and characterization of reaction intermediates is often key to reaction control. Knowledge of their behavior helps avoiding dead ends in synthesis reactions, minimize unwanted side products and optimize synthesis protocols. This is especially true for metastable compounds, where in situ studies can be of particular impact. Additionally, in situ studies on chemical reactions provide a wealth of information on the structure and dynamics of solids under varying external conditions like temperature, gas atmosphere, and many other external parameters. This knowledge also fosters understanding of function and failure of functional materials. The progress in the area of rational solid‐state synthesis planning will probably gain even more importance in the near future due to the high potential of in situ studies on chemical reactions in the solid.

The crystal structure of ${\mathrm{CO}}_2-\mathrm{II}$ is determined to be tetragonal ${P4\mathrm{}}_2/mnm$ with $z=2,$ with evidence of some tetragonal-to-orthorhombic (Pnnm) disorder in the ab plane. In this structure, carbon atoms are pseudo-sixfold-coordinated by oxygens; two oxygens are at an elongated intramolecular $\mathrm{C}=\mathrm{O}$ bond distance, 1.331(3) Å, and four are at a collapsed intermolecular distance, 2.377(2) Å, at the apices of highly distorted octahedral. Strong intermolecular association results in a large splitting of the symmetric stretching ${\nu }_1$ mode and in a high bulk modulus 131.5 GPa. At 11 GPa, ${\mathrm{CO}}_2-\mathrm{II}$ is about 6.8% denser than ${\mathrm{CO}}_2-\mathrm{III}$ (Cmca).