Sohan Ahmed’s Dissertation Defense
About the event
Speaker: Sohan Ahmed
Group: Dr. Choong-Shik Yoo
Title: REVISITING THE PHASE DIAGRAM OF CO2: NEW CONSTRAINTS AT EXTREME CONDITIONS
Abstract
Carbon dioxide is one of the most thoroughly studied — and, paradoxically, one of the most controversial — molecular solids in high-pressure science. Despite three decades of effort, several regions of its phase diagram remain poorly constrained, the mechanisms that link its molecular, intermediate, and extended-bonded phases are incompletely understood, and the structural identity of certain phases is still contested. This dissertation revisits the phase diagram of CO₂ over the pressure–temperature range 0–70 GPa and 300–750 K using in-situ Raman spectroscopy and synchrotron X-ray diffraction in laser-heated and externally heated diamond-anvil cells, and reports several findings that substantially revise the current picture.
First, a tetragonal-to-orthorhombic distortion of CO₂-II (P4₂/mnm → Pnnm) is identified above 27 GPa by X-ray diffraction and above ~34 GPa by Raman spectroscopy, persisting to at least 720 K. The transition is marked by softening of the B₁g shear mode and lifting of the Eg degeneracy, and is directly analogous to the rutile-to-CaCl₂-type distortion universal to the group-14 dioxides SiO₂, GeO₂, and SnO₂. Landau analysis of the spontaneous strain and the Eg splitting places the critical pressure at 39.6 ± 0.5 GPa (XRD) and 37.8 ± 1 GPa (Raman), consistent with second-order character over the 38–52 GPa window.
Second, isothermal compression of this distorted CO₂-II near 60 GPa is shown to yield either crystalline CO₂-VI or a fully disordered extended solid with C–O bond length ≈ 1.41 Å, depending on the structural state and compression history of the precursor — the first systematic demonstration of path-dependence in this transformation. Third, laser heating of distorted CO₂-II at 51 GPa produces cristobalite-type CO₂-V through a previously unreported transient sixfold-coordinated CO₂-VI intermediate, clarifying the hierarchy of molecular-to-extended bonding in CO₂.
Fourth, two closely spaced triple points are resolved near the CO₂-I, II, III, IV, at 12.54 ± 0.05 GPa / 437 ± 1 K (I–III–IV) and 14.1 ± 0.05 GPa / 446 ± 1 K (III–II–IV), replacing the long-speculated single quadruple point with a topologically consistent pair. Fifth, evidence is presented that CO₂-VII forms exclusively by compression of CO₂-I near the melt line, is never recovered on decompression or isobaric heating, and is spectroscopically very similar to a strained variant of CO₂-III — behavior inconsistent with a thermodynamically stable molecular phase. Finally, laser heating of CO₂ with gold above 50 GPa and 3000 K produces cuprite-type Au₂O (Pn-3m), demonstrating that even the most noble metal can be oxidized by CO₂-derived atomic oxygen at extreme conditions.
A temperature-dependent vibron-pressure calibration for phases I, II, III, IV, and VII is also developed to support in-situ kinetics measurements. Collectively, these results resolve several long-standing ambiguities in the CO₂ phase diagram, establish CO₂ as a full structural analog of the group-14 dioxides, and expose a high-pressure, high-temperature chemistry of CO₂ that extends to its interaction with supposedly inert metals.