Vibrational properties of materials from first-principles calculations

Abstract

The effect of hydrostatic pressure on the structural, energetic, electronic, and vibrational properties of bulk ReS$_2$ was studied. The electronic band gap of the 1T phase is shown to undergo a nearly-direct to indirect transition at about 9 GPa, while the 1T' phase is found to remain a robust nearly-direct band gap material under pressure. The computational analysis of the vibrational properties of both ReS$_2$ phases reproduced existing experimental Raman spectroscopy data for $\omega$ \textit{vs.} $P$ trends and provided a path towards an accurate phase discrimination using infrared spectroscopy, inelastic neutron, and X-ray scattering.

The effect of silver intercalation on the low-frequency (LF) interlayer modes in few-layer MoS$_2$ was investigated. LF interlayer modes correspond to rigid vibrations of each layer as a whole unit within two-dimensional layered materials (2DMs) with restoring forces governed by the weak interlayer interactions. As such they are more sensitive changes affecting the interlayer gap than their high-frequency (HF) intralayer counterparts. A significant red-shift (frequency decrease) of LF modes was predicted upon Ag intercalation into the vdW gap of MoS$_2$ with thicknesses ranging from two to four layers. Such a frequency decrease can be used as a fingerprint of successful Ag diffusion into MoS$_2$ and as a silver-concentration measure.

A systematic study of the Raman active phonons modes of layered anisotropic WTe$_2$ systems with thickness ranging from bulk down to a single layer was conducted. Experimentally, a dramatic change in the Raman spectra occurs between the monolayer and few-layer WTe$_2$ as a vibrational mode centered at ~86.9 cm$^{-1}$ in the monolayer splits into two active modes at 82.9 and 89.6 cm$^{-1}$ in the bilayer. Davydov splitting of these two modes is found in the bilayer, as further evidenced by polarized Raman measurements. These findings were supported by the corresponding first-principles calculations of Raman spectra that allowed for unambiguous mode assignment.

Most of the $\partial \omega/ \partial P$ slopes are positive as expected, but the symmetry of the zircon lattice also results in two negative slopes for modes that involve slight shearing and rigid rotation of SiO$_4$ tetrahedra. The effect of atomic impurities on the structural and vibrational properties of zircon was investigated. Atomic impurities considered include radioactive elements U and Th, as well as Hf, Sn, and Ti, substituted on the Zr-site. Impurities were found to cause changes in the volume of the host lattice. This effect was shown to be partially equivalent to the application of a lattice strain. This quantum-based finding is in excellent agreement with the heuristic lattice-strain model traditionally employed in the geosciences to account for the compatibility of impurities in host lattices. Vibrational properties of substituted zircon were also investigated in order to provide a quantum mechanical understanding of Raman spectroscopy measurements on natural zircon. The computational analysis reproduces existing experimental data reported for uranium-substituted zircon and provides general predictive trends for other impurities including Th, Hf, Sn, and Ti.

The effects of pressure on the Raman-active modes of zircon (ZrSiO$_4$) were studied. For pressures between 0 and 7 GPa, excellent qualitative agreement of frequency-pressure slopes $\partial \omega/ \partial P$ with results of previous experimental studies. In addition, a rationalization the $\omega$ vs. $P$ behavior was provided based on details of the vibrational modes and their atomic displacements.

In this thesis, density functional theory (DFT) based first-principles calculations were used to study the properties of various materials. The specific focus was placed on vibrational properties and their link to experimental vibrational spectroscopies, particularly Raman spectroscopy. Vibrational properties are closely related to structural, energetic, and electronic features of materials that were explored as well. The studied systems can be divided into two groups, bulk, and two-dimensional layered materials. Furthermore, two main external conditions were of interest: application of pressure on the structure and presence of external impurities. These settings can modify material properties dramatically and are therefore practically relevant for materials engineering.