Light elements under compression and at interfaces

Density functional theory within the local density approximation (LDA) and generalized gradient approximation (GGA) is used to investigate static ground state structures and electronic properties of the following: solid lithium, sodium, and oxygen at high pressures; the Si/SiO₂ interface; and the Cu/SiO₂ and Cr/oxide interfaces. In both lithium (Chapter 2) and sodium (Chapter 3), large deviations from free electron-like behavior are predicted under compression because of core overlap. The conventional close-packed atomic structures they assume under atmospheric conditions are found to be unstable to lower-coordinated crystals with increasing density, culminating in a predicted paired ground state for both solids. Instead of becoming even more free electron-like at higher densities, lithium and sodium are predicted to be nearly insulating at densities achievable with diamond-anvil cells. In Chapter 4, new insulating ground states of solid oxygen (having space groups P2/m and P 2mm) are predicted using the local spin-density approximation and gradient corrections. Remarkably, the P2/ m and P2mm phases are both antiferromagnetic and antiferroelectric; in particular the antiferroelectric ground state would explain the significant infrared activity observed in the ε-phase. In Chapter 5 oxygen p-projected densities of states, calculated from first principles in a model Si/SiO ₂ interface, are found to reproduce trends in electron energy-loss spectra (EELS). Both the shape of the unoccupied states and the magnitude of the local energy gap are explicitly related to the number of O second neighbors of a given oxygen atom. The calculated local energy gaps of the oxide become considerably smaller within 0.5 nm of the interface, suggesting that the electronic properties do not change abruptly at the interface. Finally in Chapter 6 supercells consisting of copper and chromium layers on α-cristobalite surfaces are fully relaxed. It is argued that Cu layers adhere weakly through examination of the resulting interfacial bonding arrangement. Bonding properties and local electronic structure are then investigated for an actual Cr/oxide interface by comparing densities of states to measured EELS spectra.