Atomic scale modeling of ordering phenomena in inorganic clathrates
Licentiate thesis, 2018
cluster expansions are a technique that is particular suitable for this task as they can be trained to reach high accuracy while being computationally suitable for rapid sampling via Monte Carlo techniques.
In this thesis alloy cluster expansions have been applied in combination with Monte Carlo simulations to study the ordering behavior in various inorganic clathrates. Inorganic clathrates constitute a class of systems with a cage-like framework that can trap loosely bound atoms or even small molecules. These systems are small
band gap semiconductors and have a very low lattice thermal conductivity, which gives rise to very good thermoelectric properties. Additionally the host atoms and cage framework can be occupied by a wide range of elements which provides extensive opportunities for property optimization. Inorganic clathrates are thus
good examples for systems with a high degree of variability in composition, for which ordering phenomena play a crucial role.
In paper I we studied the ordering behaviour of Ba8Ga16Ge30. Configurations representative for different annealing temperatures were extracted from Monte Carlo simulations and further analyzed to obtain the temperature dependency of the thermoelectric power factor. These data was subsequently used to construct a cluster expansion for the power factor itself, which enabled us to optimize the chemical ordering that maximizes this property. The approach developed in this work is generalizable and can be adapted to other materials.
In paper II we studied the ordering behavior and related properties in the clathrate systems Ba8Al𝑥Si46−𝑥, Ba8Al𝑥Ge46−𝑥, Ba8Ga𝑥Ge46−𝑥, and Ba8Ga𝑥Si46−𝑥 as a function of composition and temperature. We achieved very good agreement with the available experimental data for the site occupancy factors (SOFs). This enabled us to reconcile experimental data from different sources and explain the nonmonotonic variations of the SOFs. In particular, we provided a rationale for the extreme SOF behavior with varying composition observed in Al based clathrates.
Chalmers, Physics, Materials and Surface Theory
Understanding Chemical Ordering in Intermetallic Clathrates from Atomic Scale Simulations
Chemistry of Materials,; Vol. 29(2017)p. 7554-7562
Optimization of the Thermoelectric Power Factor: Coupling between Chemical Order and Transport Properties
Chemistry of Materials,; Vol. 28(2016)p. 6877-6885
C3SE (Chalmers Centre for Computational Science and Engineering)
Thesis for the degree of licentiate of engineering / Department of Materials Science and Engineering, Chalmers University of Technology
Opponent: Mogens Christensen, Aarhus university, Denmark