When 10:30 AM - 12:30 PM Aug 28, 2015
Where 3158 HH Dow
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Dynamical Instabilities and High Temperature Phase Stability in Ionic Crystals


Min-Hua Ivy Chen
Thesis Defense

Co-Chairs: Anton Van der Ven and Katsuyo S. Thornton

 

The presence of dynamical instabilities in a material suggest that perturbations to the system can trigger phase transformations into a lower energy and symmetry phase. Understanding the energetics of these transformation mechanisms and vibrational thermodynamic properties of these materials can provide further insight into their stability.

 

Using first-principles phonon calculations, we examine the dynamical stability and vibrational properties of Li3OCl, a solid electrolyte material. We show that it is dynamically unstable with respect to octhedral rotations. Further examination of the anharmonic energy landscapes resulting from these rotations revealed that while the rotations can lead to lower symmetry structures, the energy gained by these rotations are small. At low temperatures, the cubic form should persist due to anharmonic vibrational excitations. We also find that Li3OCl is entropically stabilized with respect to LiCl and Li2O at temperatures above 480 K.

 

Zirconium alloys used in nuclear fuel rod cladding materials experience corrosive environ- ments that result in their oxidation. Understanding the phase stability of these oxide phases at high temperatures is crucial to designing corrosion-resistant materials. Vibrational free energies for several Zr-O compounds were calculated and incorporated into a previously calculated temperature composition phase diagram primarily to identify the temperature stability limit of a recently identified line compound δ'-ZrO. We show that this phase is stable well beyond typical nuclear reactor temperatures, consistent with experimental observations of the phase.

 

The instabilities observed in the cubic, tetragonal, and monoclinic ZrO2 polymorphs are also studied. The cubic phase instability leads to a cubic to tetragonal transformation, where three different tetragonal variants can form. We discovered an instability in the tetragonal phase activated by volume increases that leads to a transformation into a new orthorhombic ZrO2 phase. This instability has important implications for the finite temperature stability of the tetragonal phase and the role of anharmonicity in high temperature materials. We also show that strain can affect the stabilities of the three tetragonal variants. These results suggest that strain can be used to stabilize the high temperature tetragonal phase, which is preferable for epitaxial thin films used in high k dielectrics and for ferroelastic toughening in thermal barrier coatings.