The suppression of superconductivity in layered lanthanum cuprates near x = 1/8 coincides with a structural phase transition from a low-temperature orthorhombic to a low-temperature tetragonal phase. The low-temperature phases are characterised by a static tilt of the CuO6 octahedra away from the layering axis in distinct directions. It remained an open question whether the orthorhombic-to-tetragonal phase transition would only occur in the context of competing electronic orders in the lanthanum cuprates.
This thesis proposes a novel approach to studying the orthorhombic-to-tetragonal phase transition using the novel La2MgO4 system. La2MgO4 adopts the layered Ruddlesden-Pepper structure of the lanthanum cuprates but lacks the strong electron correlations and octahedral distortions associated with the Jahn-Teller active Cu site. Combining first-principles simula- tions using density-functional theory with experimental data on the novel La2MgO4 system, the context in which these structural phases can occur is detailed, outlining the key param- eters determining the stability of the phase which suppresses bulk superconductivity. The same sequence of structural phase transitions occurs in La2MgO4 as in La1.875Ba0.125CuO4, and the tetragonal phase is stabilised via steric effects beyond a critical octahedral tilt magnitude. Larger Jahn-Teller distortions favour the orthorhombic phase.
The effect of isotropic and anisotropic pressure on La2MgO4 and La2CuO4 is explored. These form the basis for a structural mechanism to understand the experimental trends of the bulk superconducting transition temperature under uniaxial pressure. Finally, the justification for the methodology used throughout this thesis to simulate these systems is provided, highlighting that DFT+U accurately describes their atomic and electronic structure.