Boucher, Giulia; (2019) First-principles study of the phase diagram of Potassium. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

This thesis is a computer simulation project to calculate the phase diagram of potassium as a function of pressure from 0 to 100 GPa and temperature from 0 to 1200 K. The project required the application of both electronic structure techniques and classical molecular dynamics simulations to compute the free energies of solid and liquid phases. To compute the melting properties, the coexistence was used, either with a classical potential fitted to the ab initio data or directly with the ab initio potential. Tests were also carried out using the Z method, however, this method cannot compute melting properties but can provide an estimate of the upper bound of the melting temperature. The phase transitions of the solid structures of potassium were determined from 0 to 100 GPa at temperatures ranging from 0 to 450K using first-principles calculations of the free energies. The calculations were carried out within the density functional theory framework with the generalised gradient approximation and the projector augmented wave method as implemented in VASP. The quasi-harmonic contribution to the free energy of the solids was calculated using the small displacement method. To determine the melting temperature of each solid phase of potassium, the coexistence method with a potential was used but proved unsuccessful for most phases. Therefore, the direct coexistence was used but was deemed computationally too expensive. As aforementionned, the Z method was tested in order to obtain an approximation of the melting temperature and assess the performance of the method.

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