= Solution
\b[A suitable representative structure is a three-dimensional network of corner-sharing CO$_4$ <tetrahedra>, with C–O single bonds.] Each <carbon> is four-coordinate; every bridging <oxygen> is bonded to two carbons. Sharing four oxygens between two <tetrahedra> gives $4/2=2$ oxygens per <carbon>, so the network retains empirical formula CO$_2$ without containing separate O=C=O <molecules>.
High <pressure> favours a compact, highly coordinated network through its smaller molar volume: at fixed <temperature>, increasing <pressure> changes relative Gibbs energies according to $\mathrm d(\Delta G)=\Delta V\,\mathrm dp$. Heating helps overcome the large reconstructive barrier to breaking and redistributing molecular pi bonding and establishing an extended single-bond network. It should not be argued that high <temperature> by itself favours the lower-entropy network. Nor is merely freezing molecular CO$_2$ sufficient: dry ice remains a molecular solid.
The sketch shows connectivity and local tetrahedral coordination, not a unique predicted <crystal structure>. Real <amorphous carbonia> need not have perfect periodic order or exclusively one coordination environment. The original <carbonia> experiments combined high-pressure preparation with infrared, Raman and diffraction evidence for loss of molecular character and formation of an amorphous network.<Amorphous silica-like carbon dioxide by Mario Santoro and others>[Abstract: spectroscopy and diffraction evidence for amorphous nonmolecular CO2]{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2023/paper-1-carbonia-network.png]
{title=Original representative corner-sharing tetrahedral CO2 network fragment; terminal bonds continue into the solid}
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