New Phase Transition Mechanisms in Compressed Silica
Date
2015-05-19
Authors
Hu, Qingyang
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Abstract
Silicon dioxide, also known as silica, is a fundamental constituent of the Earth interior. Silica exists in many crystal structures, but with the same chemical composition. These crystal forms are termed as silica polymorphs. Phase transitions among silica polymorphs have long been a focus of theoretical and experimental pursuits, for their great significance in geophysics and materials science. Pressure alters the atomic arrangements through phase transformations and establishes a new dimension in the phase diagrams of many compounds. Pressurizing silica to extreme conditions, e.g., up to several tens of gigapascals (GPa), not only helps discover new silica phases that are stable in deep Earth, but also sheds new light on the elusive densification mechanism of this rudimentary oxide material. This thesis focuses on pressure-induced behavior of compressed silica at room temperature (i.e., 300 K), with an emphasis on two silica polymorphs: coesite and -quartz. To this end, extensive experimental and computational efforts have been undertaken in this thesis, aiming to tackle two major unsolved issues in this area: the nature of the phase transition between different silica polymorphs and the general densification mechanism of silica-like materials. The theoretical research in this thesis employs state-of-the-art first-principles computational calculations, combining in situ synchrotron radiation experiments, to characterize silica solids under high pressure. The combined theoretic and experimental treatment has been tested on a variety of compressed solid systems previously and is found powerful in solving a number of puzzles pertaining to the phase behavior of silica. The main research results are summarized as follows: a) Compressing single-crystal coesite SiO2 under hydrostatic pressures of 26~53 gigapascal (GPa) at room temperature, a new polymorphic phase transition mechanism is discovered by means of single-crystal synchrotron X-ray diffraction (XRD) experiment and first-principles computational modeling. The transition features the formation of multiple previously unknown triclinic phases of SiO2 on the transition pathway as structural intermediates, which eventually transforms into the monoclinic post-stishovite phase. The metastable phases are similar in volume and degenerate in free-energy, but distinct in structures and X-ray diffraction patterns. Coexistence of the low-symmetry phases results in extensive splitting of the original coesite x-ray diffraction peaks that appear with dramatic peak broadening and weakening, thus resembling an amorphous material. Also discovered is the long-sought but never confirmed Si in five-coordination populated in the metastable phases. This work provides new insights into the structural transition of SiO2 crystal under high pressures, and clarifies the issue of the pressure-induced amorphization (PIA) of coesite, which has often been cited as an archetypal example of the PIA phenomena in general. b) The second part of my thesis is centered on the phase transition of compressed -quartz. Two competing transition pathways of -quartz under high pressure are uncovered, being reconstructive vis-à-vis displacive in their respective nature, toward different new phases. By means of in situ single-crystal X-ray diffraction experiment (0-60 GPa) in conjunction with advanced ab initio modeling, I demonstrate that, under quasi-static compression conditions at room temperature, compressed -quartz transits via an intermediate metastable phase (quartz II) emerging at 26 GPa en route to a new monoclinic-type post-stishovite structure. Under conditions where this thermally activated transition is kinetically frustrated, it is found that the ultimate stability of -quartz is controlled by its phonon instability, constituting alternatively a displacive transition mechanism of -quartz into a new post-quartz phase. The discovery of the two competing transition pathways, achievable at the same pressure range but under different kinetic conditions, puts into perspective of previously seemingly discordant results of compressed -quartz, and helps clarify the role of phonon softening played in phase transition, paving way to understanding the complex phase behavior of a vast silicate family in geologically important conditions. This thesis makes contributions to the fundamental understanding of the phase transition mechanisms of silica. Since silica is considered as an archetypal compound in studying tetrahedrally bonded materials such as silicates as well as other silica-like materials, this thesis work has far-reaching implications to many branches of materials science and geophysics.
Description
This work was embargoed by the author and will not be publicly available until January 2016.
Keywords
Computational science, Geophysics, High pressure, Materials science, Phase transition, Silica