From Phonons to Phase Stability

Licentiatavhandling, 2025

The transition toward carbon-neutral energy systems requires efficient fuel conversion and carbon capture technologies. Chemical-looping combustion (CLC) enables inherent CO2 separation by using solid oxygen carriers that cyclically transfer oxygen between air and fuel reactors. The performance of these materials depends on their thermodynamic and electronic properties, which can be predicted through first-principles calculations.

This thesis presents a density functional theory (DFT)–based framework for investigating the perovskite oxide CaMnO3-δ (0 ≤ δ ≤ 0.5), a promising oxygen carrier for CLC applications. Using total energy and phonon calculations, thermodynamic quantities such as heat capacities, formation enthalpies, and Gibbs free energies were estimated and related to oxygen vacancy formation. The results reveal how increasing oxygen deficiency affects phase stability and electronic structure, including a transition from semiconducting to metallic behaviour and a reduction of Mn oxidation states.

The computed formation enthalpies were combined with experimental thermodynamic data to construct a phase diagram of the Ca-Mn-O system, providing insight into redox stability under CLC conditions. The developed computational framework links atomic-scale modelling to macroscopic material behaviour and offers a foundation for the predictive design of doped or related perovskite oxygen carriers.