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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Simon Redfern, Earth Sciences  Co-Lead Supervisor:  Oliver Shorttlle, Institute of Astronomy

Importance of the area of research concerned: 
It is becoming apparent the familiar rules of chemistry learned from 1 atm experiments and the periodic table break down deep inside planets and "new chemistry" takes over. Developments in "ab initio" models of the atomic structures of minerals now enable "structure prediction" methods to be used to suggest likely mineral structures in the deep Earth, and planetary interiors more generally. The existence of post-perovskite was, for example, predicted by ab initio computational methods before it was experimentally verified. Similarly, the existence of high-pressure carbonate minerals in which carbon is tetrahedrally coordinated by oxygen was first predicted by computational structure search methods before being verified by experiment. The combination of computational structure prediction and experimental verification now allows us to predict the mineralogical composition and alternative pictures of chemistry for deep planetary interiors under the constraint of chemical composition. We will use this approach to predict mineralogical properties for key planetary compositions, applicable to planets ranging from Earth-like bodies to exoplanetary super-Earths
Project summary : 
Ab initio structure prediction methods have now reached a maturity that allow them to be used to model the enthalpic stabilities of phases across pseudo-binary composition sections, The project uses this development to search for new structures in key silicates, oxides and carbonates at high pressures, where "unusual" chemical configurations may be stabilised. Such computational predictions demand experimental verification using high-pressure structural techniques such as vibrational spectroscopy and X-ray diffraction of samples pressurised in the diamond anvil cell. This project seeks to identify such unexpected phases by first adopting particle swarm structure prediction methods based on quantum mechanical computational results, combined with experimental studies of these structures for key candidate silicate, oxide and carbonate chemistries.
What will the student do?: 
The student will carry out quantum mechanical structure prediction calculations of key silicate, carbonate and oxide systems across selected pseudo-binary sections in composition space. The results of such structure prediction calculations will be tested against experimental observation of the same chemical systems using high-pressure structural studies. Synchrotron X-ray powder diffraction will be used together with laboratory based Raman spectroscopy of samples held at extreme conditions in the diamond anvil cell. The results will be used to suggest the likely mineralogical structures present the deep Earth, for previously unexplored compositions, as well as in even more extreme conditions with super-Earth planets in which unusual structures may in fact be ubiquitous. Further calculations on key compositions and structures will allow the prediction of seismological and mineral physics properties of these phases at the conditions of their dominance.
Pickard, C. J., & Needs, R. J. (2015). Structures and stability of calcium and magnesium carbonates at mantle pressures. Physical Review B, 91 (10. 104101)
Merlini, M, et al. (2017) Dolomite-IV: Candidate structure for a carbonate in the Earth's lower mantle. Am Mineral vol. 102 no. 8 1763-1766
You can find out about applying for this project on the Department of Earth Sciences page.