Research master thesis: First-principles simulations of layered-perovskite dion-Jacobson phases

Abstract

In this study, I investigate layered oxide perovskites, specifically n=2 Dion-Jacobson phases with the formula AA′B2O7, using first-principles Density Functional Theory simulations. These materials are of interest due to their potential ferroelectric properties but are often under-explored and poorly characterized. I carefully calculate the force constants associated with phonon modes by utilizing the frozen-phonon technique in the symmetry-adapted mode basis. The coupling of different phonon modes happens only if it is symmetry-allowed, giving a natural tool to study phase transitions. Unstable modes are identified by a negative force constant; the eigenvalues of these modes can be frozen into the structure, which, after relaxation, results in a more stable material. The materials studied in this work are compounds with the formula ANdNb2O7 where A is Rb, Na, or NH4 cation. The ammonia molecule-containing phase is the primary compound of this study. The presence of various unstable phonon modes complicates the prediction of a definitive ground state; thus, I present several potential lower symmetry structures derived from the aristotype. These models serve as a basis for further experimental characterization and enhance our understanding of structure-property relationships, particularly the influence of aspherical cations on the material’s ground state structure. This research provides foundational insights into the dynamic properties of layered perovskites, paving the way for future studies on their practical applications in ferroelectric and other functional materials.