Studying the effects of bond character in metal halides on diffusivity using maximally localized Wannier functions
By: Salma Ahmed
Department: Chemistry
Faculty Advisor: Dr. Nicole Adelstein
Solid-state batteries have been a focus of much research because they are safer than batteries with organic electrolytes that are used commercially, but they are less conductive. This computational study focuses on the binary metal (Ag/Cu) halides to find the link between bonding character of the diffusive Ag or Cu with the halide static lattice and Ag/Cu conductivity. Maximally Localized Wannier Functions (MLWFs) are used to analyze the bond character (either polar covalent or ionic) of the system to see whether or not there are bond fluctuations between the mobile cation and the anion lattice. The bond character and diffusivity of the metal halides were analyzed in different crystal structures (alpha, beta, zinc-blende, rock-salt) and at different temperatures. The diffusion coefficient for the metal at each temperature was calculated using the mean squared displacement of the metal, and the diffusion coefficient at three temperatures was used to calculate the activation energy barrier of diffusion. Our calculated values followed experimental trends. The polarization for the halides and bond angle was determined using the MLWFs in order to analyze the abundance of polar covalent interactions and how it correlates to diffusivity. The angle and polarization distributions varied as the crystal structure, metal, or halide changed. These results point towards our hypothesis that bond character affects diffusivity. By systematically varying crystal structure and bonding interactions in superionic conductors, we can learn more about the interaction between the diffusion species and the anion lattice, and therefore better understand solid-state electrolytes.