| 1. |
- Gren, Martin, 1989, et al.
(författare)
-
Modeling of vibrational and configurational degrees of freedom in hexagonal and cubic tungsten carbide at high temperatures
- 2021
-
Ingår i: Physical Review Materials. - 2475-9953. ; 5:3
-
Tidskriftsartikel (refereegranskat)abstract
- Transition metal carbide is a class of materials characterized by high hardness, high melting points, and low chemical reactivity. It is widely used in industrial applications involving exposure to elevated temperatures, aggressive media, and heavy loads, and is thus of technological and industrial importance. In this paper the high-temperature thermodynamic properties of tungsten carbide, WC, is studied. At most temperatures below melting, WC assumes a hexagonal structure with essentially no vacancies (δ-WC). Only at very high temperatures (around 3000 K), WC crystallizes in the cubic rocksalt structure (γ-WC), which is more common for the transition metal carbides and in the case for WC can contain up to 40% carbon vacancies. At lower temperatures, γ-WC can, however, form as thin interfacial structures or nanoparticles. Hence, the thermodynamic properties of both γ-WC and δ-WC are of relevance. Here, we conduct a first-principles density-functional theory based computational investigation of the γ-WC and δ-WC phases, which requires modeling of high carbon vacancy concentrations at high temperatures. The configurational degrees of freedom are modeled with an alloy cluster expansion and sampled through Monte Carlo simulations. To account for the dynamic instability of the cubic γ-WC phase at low temperatures, the vibrational degrees of freedom are treated using effective harmonic models constructed from ab initio molecular dynamics simulations. Finally, we obtain a part of the W-C phase diagram in reasonably quantitative agreement with experimental data.
|
|
| 2. |
- Fant, Magnus, et al.
(författare)
-
To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule
- 2021
-
Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 60:10, s. 5132-5135
-
Tidskriftsartikel (refereegranskat)abstract
- Loewenstein's rule, which states that Al−O−Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.
|
|
| 3. |
- Rahm, Martin, 1982, et al.
(författare)
-
Non-Bonded Radii of the Atoms Under Compression
- 2020
-
Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 21:21, s. 2441-2453
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract: We present quantum mechanical estimates for non-bonded, van der Waals-like, radii of 93 atoms in a pressure range from 0 to 300 gigapascal. Trends in radii are largely maintained under pressure, but atoms also change place in their relative size ordering. Multiple isobaric contractions of radii are predicted and are explained by pressure-induced changes to the electronic ground state configurations of the atoms. The presented radii are predictive of drastically different chemistry under high pressure and permit an extension of chemical thinking to different thermodynamic regimes. For example, they can aid in assignment of bonded and non-bonded contacts, for distinguishing molecular entities, and for estimating available space inside compressed materials. All data has been made available in an interactive web application.
|
|
| 4. |
- Ångqvist, Mattias, 1989
(författare)
-
Atomic scale modeling of ordering phenomena
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ordering phenomena in materials often have a crucial impact on materials properties. They are governed by the competition between entropy and energy. Accordingly simulating these aspects requires the construction of models that enable a computationally efficient exploration of the relevant configuration space. The alloy cluster expansion technique is particular well suited for this task as they can be trained to reach high accuracy while being computationally suitable for rapid sampling via Monte Carlo simulations. In paper I we present the icet software for the construction and sampling of alloy cluster expansions. In this thesis the alloy cluster expansion method is applied to study several different materials. The first group of materials studied are inorganic clathrates. In paper II and III we studied the ordering behavior and related properties as a function of composition and temperature for the clathrates Ba8AlxSi46-x, Ba8AlxGe46-x, Ba8GaxGe46-x, and Ba8GaxSi46-x. We achieved very good agreement with the available experimental data for the site occupancy factors (SOFs). In paper IV and V we constructed the phase diagram for the W-Ti and W-C system respectively. A cluster expansion for each system was constructed and the configurational free energy was calculated. By also including other contributions to the free energy, most notably the vibrational free energy, the phase diagrams for these systems could be constructed. In paper VI we studied the SSZ-13 zeolite and showed both that the Löwenstein rule is not respected with hydrogen as counterion and provided a rationale for this behavior.
|
|
| 5. |
- Ångqvist, Mattias, 1989
(författare)
-
Atomic scale modeling of ordering phenomena in inorganic clathrates
- 2018
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ordering phenomena in materials often have a crucial impact on materials properties. They are governed by the competition between entropy and energy. Accordingly simulating these aspects requires the construction of models that enable an computationally efficient exploration of the relevant configuration space. Alloy 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 Ba8Al?Si46−?, Ba8Al?Ge46−?, Ba8Ga?Ge46−?, and Ba8Ga?Si46−? 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.
|
|
| 6. |
- Ångqvist, Mattias, 1989, et al.
(författare)
-
ICET - A Python Library for Constructing and Sampling Alloy Cluster Expansions
- 2019
-
Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 2:7
-
Tidskriftsartikel (refereegranskat)abstract
- Alloy cluster expansions (CEs) provide an accurate and computationally efficient mapping of the potential energy surface of multi-component systems that enables comprehensive sampling of the many-dimensional configuration space. Here, integrated cluster expansion toolkit (ICET), a flexible, extensible, and computationally efficient software package, is introduced for the construction and sampling of CEs. ICET is largely written in Python for easy integration in comprehensive workflows, including first-principles calculations for the generation of reference data and machine learning libraries for training and validation. The package enables training using a variety of linear regression algorithms with and without regularization, Bayesian regression, feature selection, and cross-validation. It also provides complementary functionality for structure enumeration and mapping as well as data management and analysis. Potential applications are illustrated by two examples, including the computation of the phase diagram of a prototypical metallic alloy and the analysis of chemical ordering in an inorganic semiconductor.
|
|
| 7. |
- Ångqvist, Mattias, 1989, et al.
(författare)
-
Optimization of the Thermoelectric Power Factor: Coupling between Chemical Order and Transport Properties
- 2016
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 28:19, s. 6877-6885
-
Tidskriftsartikel (refereegranskat)abstract
- Many thermoelectric materials are multicomponent systems that exhibit chemical ordering, which can affect both thermodynamic and transport properties. Here, we address the coupling between order and thermoelectric performance in the case of a prototypical inorganic clathrate (Ba8Ga16Ge30) using a combination of density functional and Boltzmann transport theory as well as alloy cluster expansions and Monte Carlo simulations. The calculations describe the experimentally observed site occupancy factors and reproduce experimental data for the transport coefficients. By inverting the cluster expansion, we demonstrate that the power factor can be increased by more than 60% for certain chemical ordering patterns that involve reducing the number of the trivalent species on the 6c Wyckoff site. This enhancement is traced to specific features of the electronic band structure. The approach taken in the present work can be readily adapted to other materials and enables a very general form of band structure engineering. In this fashion, it can guide the computational design of compounds with optimal transport properties.
|
|
| 8. |
- Ångqvist, Mattias, 1989, et al.
(författare)
-
Structurally driven asymmetric miscibility in the phase diagram of W-Ti
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:7
-
Tidskriftsartikel (refereegranskat)abstract
- © 2019 American Physical Society. Phase diagrams for multicomponent systems represent crucial information for understanding and designing materials but are very time consuming to assess experimentally. Computational modeling plays an increasingly important role in this regard but has been largely focused on systems with matching lattice structures and/or stable boundary phases. Here, using a combination of density functional theory calculations, alloy cluster expansions, free energy integration, and Monte Carlo simulations, we obtain the phase diagram of W-Ti, a system that features metastable boundary phases on both sides of the phase diagram. We find that the mixing energy on the body-centered cubic (BCC) lattice is asymmetric and negative with a minimum of about -120 meV/atom, whereas for the hexagonal closed packed (HCP) lattice the mixing energy is positive. By combining these data with a model for the vibrational free energy, we propose a revision of the W-rich end of the phase diagram with a much larger solubility of Ti in BCC-W than previous assessments. Finally, by comparison with the W-V and W-Re systems, we demonstrate how strongly asymmetric phase diagrams can arise from a subtle energy balance of stable and metastable lattice structures.
|
|
| 9. |
- Ångqvist, Mattias, 1989, et al.
(författare)
-
Understanding Chemical Ordering in Intermetallic Clathrates from Atomic Scale Simulations
- 2017
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 29:17, s. 7554-7562
-
Tidskriftsartikel (refereegranskat)abstract
- Intermetallic clathrates exhibit great variability with respect to elemental composition and distribution. While this provides a lot of flexibility for tuning properties, it also poses a challenge with regard to developing a comprehensive understanding of these systems. Here, we employ a combination of alloy cluster expansions and density functional theory calculations to exhaustively sample the compositional space with ab initio accuracy. We apply this methodology to study chemical ordering and related properties in the clathrate systems Ba8GaxGe46-x, Ba8GaxSi46-x, Ba8AlxGe46-x, and Ba8AlxSi46-x as a function of composition and temperature. We achieve very good agreement with the available experimental data for the site occupancy factors (SOFs) even for stoichiometries outside the composition range considered during construction of the cluster expansions. This validation enables us to reconcile the variations in the experimental data and explain nonmonotonic variations of the SOFs. In particular, we provide a rationale for the extreme SOF behavior with varying composition observed in Al-based clathrates. Furthermore, we quantify the effect of chemical ordering on both heat capacity and lattice expansion. Finally, we determine the effect of chemical disorder on the displacements of the guest species (Ba), which enables us to at least partially explain experimental observations of the nuclear density of Ba in different clathrates.
|
|
