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1.	Fransson, Erik, 1990, et al. (författare) Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites 2023 Ingår i: Communications Physics. - 2399-3650. ; 6:1 Tidskriftsartikel (refereegranskat)abstract The soft modes associated with continuous-order phase transitions are associated with strong anharmonicity. This leads to the overdamped limit where the phonon quasi-particle picture can break down. However, this limit is commonly restricted to a narrow temperature range, making it difficult to observe its signature feature, namely the breakdown of the inverse relationship between the relaxation time and damping. Here we present a physically intuitive picture based on the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior for the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr3 via molecular dynamics simulations, and show that the overdamped region extends almost 200 K above the transition temperature. Further, we investigate how the dynamics of these soft phonon modes change when crossing the phase transition.
2.	Fransson, Erik, 1990, et al. (författare) Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr₃ and MAPbI₃ 2023 Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 8229- Tidskriftsartikel (refereegranskat)abstract Halide perovskites have emerged as a promising class of materials for photovoltaic applications. A challenge of these applications is preventing the crystal structure from degrading to photovoltaically inactive phases, which requires an understanding of the free energy landscape of these materials. Here, we uncover the free energy landscape of two prototypical halide perovskites, CsPbBr3 and MAPbI3, via atomic-scale simulations using umbrella sampling and machine-learned potentials. For CsPbBr3, we find very small free energy differences and barriers close to the transition temperatures for both the tetragonal-to-cubic and orthorhombic-to-tetragonal transitions. For MAPbI3, however, the situation is more intricate. In particular, the orthorhombic-to-tetragonal transition exhibits a large free energy barrier, and there are several competing tetragonal phases. Using large-scale molecular dynamics simulations, we explore the character of these transitions and observe the latent heat and a discrete change in the structural parameters for the tetragonal-to-cubic phase transitions in both CsPbBr3 and MAPbI3, indicating first-order transitions. We find that in MAPbI3, the orthorhombic phase has an extended metastability range, and we identify a second metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI3 that includes potential metastable phases.
3.	Lindgren, Eric, 1997, et al. (författare) calorine: A Python package for constructing and sampling neuroevolution potential models 2024 Ingår i: Journal of Open Source Software. - 2475-9066. ; 9:95, s. 6264-6264 Tidskriftsartikel (refereegranskat)abstract Molecular dynamics (MD) simulations are a key tool in computational chemistry, physics, and materials science, aiding the understanding of microscopic processes but also guiding the development of novel materials. A MD simulation requires a model for the interatomic interactions. To this end, one traditionally often uses empirical interatomic potentials or force fields, which are fast but inaccurate, or ab-initio methods based on electronic structure theory such as density functional theory, which are accurate but computationally very expensive (Müser et al., 2023). Machine-learned interatomic potentials (MLIPs) have in recent years emerged as an alternative to these approaches, combining the speed of heuristic force fields with the accuracy of ab-initio techniques (Unke et al., 2021). Neuroevolution potentials (NEPs), implemented in the GPUMD package, in particular, are a highly accurate and efficient class of MLIPs (Fan et al., 2021, 2022; Fan, 2022). NEP models have already been used to study a variety of properties in a range of materials, with recent examples including radiation damage in tungsten (Liu et al., 2023), phase transitions (Fransson, Wiktor, et al., 2023) and dynamics of halide perovskites (Fransson, Rosander, et al., 2023) as well as thermal transport in two-dimensional materials (Sha et al., 2023). Here, we present calorine, a Python package that simplifies the construction, analysis and use of NEP models via GPUMD.
4.	Rahm, Magnus, 1990, et al. (författare) A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd–Au–H 2021 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 211 Tidskriftsartikel (refereegranskat)abstract Due to their ability to reversibly absorb/desorb hydrogen without hysteresis, Pd–Au nanoalloys have been proposed as materials for hydrogen sensing. For sensing, it is important that absorption/desorption isotherms are reproducible and stable over time. A few studies have pointed to the influence of short and long range chemical order on these isotherms, but many aspects of the impact of chemical order have remained unexplored. Here, we use alloy cluster expansions to describe the thermodynamics of hydrogen in Pd–Au in a wide concentration range. We investigate how different chemical orderings, corresponding to annealing at different temperatures as well as different external pressures of hydrogen, impact the behavior of the material with focus on its hydrogen absorption/desorption isotherms. In particular, we find that a long-range ordered L12 phase is expected to form if the H2 pressure is sufficiently high. Furthermore, we construct the phase diagram at temperatures from 250 K to 500 K, showing that if full equilibrium is reached in the presence of hydrogen, phase separation can often be expected to occur, in stark contrast to the phase diagram in para-equilibrium. Our results explain the experimental observation that absorption/desorption isotherms in Pd–Au are often stable over time, but also reveal pitfalls for when this may not be the case.
5.	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.
6.	Brorsson, Joakim, 1988, et al. (författare) Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy 2022 Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 5:2 Tidskriftsartikel (refereegranskat)abstract High-order force constant expansions can provide accurate representations of the potential energy surface relevant to vibrational motion. They can be efficiently parametrized using quantum mechanical calculations and subsequently sampled at a fraction of the cost of the underlying reference calculations. Here, force constant expansions are combined via the hiphive package with GPU-accelerated molecular dynamics simulations via the GPUMD package to obtain an accurate, transferable, and efficient approach for sampling the dynamical properties of materials. The performance of this methodology is demonstrated by applying it both to materials with very low thermal conductivity (Ba8Ga16Ge30, SnSe) and a material with a relatively high lattice thermal conductivity (monolayer-MoS2). These cases cover both situations with weak (monolayer-MoS2, SnSe) and strong (Ba8Ga16Ge30) pho renormalization. The simulations also enable to access complementary information such as the spectral thermal conductivity, which allows to discriminate the contribution by different phonon modes while accounting for scattering to all orders. The software packages described here are made available to the scientific community as free and open-source software in order to encourage the more widespread use of these techniques as well as their evolution through continuous and collaborative development.
7.	Eriksson, Fredrik, 1992, et al. (författare) The Hiphive Package for the Extraction of High-Order Force Constants by Machine Learning 2019 Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 2:5 Tidskriftsartikel (refereegranskat)abstract The efficient extraction of force constants (FCs) is crucial for the analysis of many thermodynamic materials properties. Approaches based on the systematic enumeration of finite differences scale poorly with system size and can rarely extend beyond third order when input data is obtained from first-principles calculations. Methods based on parameter fitting in the spirit of interatomic potentials, on the other hand, can extract FC parameters from semi-random configurations of high information density and advanced regularized regression methods can recover physical solutions from a limited amount of data. Here, the HIPHIVE Python package, that enables the construction of force constant models up to arbitrary order is presented. HIPHIVE exploits crystal symmetries to reduce the number of free parameters and then employs advanced machine learning algorithms to extract the force constants. Depending on the problem at hand, both over and underdetermined systems are handled efficiently. The FCs can be subsequently analyzed directly and or be used to carry out, for example, molecular dynamics simulations. The utility of this approach is demonstrated via several examples including ideal and defective monolayers of MoS2 as well as bulk nickel.
8.	Eriksson, Fredrik, 1992, et al. (författare) Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering 2023 Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 17:24, s. 25565-25574 Tidskriftsartikel (refereegranskat)abstract It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.
9.	Fransson, Erik, 1990 (författare) Atomic-scale investigation of interfacial structures in WC-Co at finite temperatures 2021 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in high-speed machining of steels and in wear resistance tools. These excellent mechanical properties are to a large extent dependent on the microstructure and thus the interfacial properties of the material. Hence, being able to predict and understand interfacial properties in this material can allow for, e.g., optimizing the manufacturing process in order to improve mechanical properties further. Electronic structure calculations allow for accurately predicting interface energies for a given structure and composition. However, finding the ground-state interfacial structure and composition is challenging as the search space is very large when considering all degrees of freedom. Furthermore, direct sampling of interfacial properties at finite temperature using density functional theory (DFT) is usually computationally impractical as hundreds, thousands or even millions of calculations may be required. Therefore, employing atomic-scale models based on DFT calculations is advantageous and allows for investigation of the interface structure, composition and free energy at finite temperatures. In this thesis computational methods for calculating temperature-dependent interfacial free energies are developed and applied to the WC-Co system. The emphasis is on understanding under which conditions cubic interfacial structures (complexions) can form on the WC basal plane in contact with Co. Configurational degrees of freedom are treated with cluster expansions and Monte Carlo simulations. Vibrational properties are mainly treated in the harmonic approximation using a regression approach to extract the harmonic force constants, which significantly reduces the number of DFT calculations. Interfacial phase diagrams are obtained for both the undoped WC-Co system and the Ti-doped system. Detailed information pertaining to structure and composition of the interfacial phases are obtained and show good agreement with experimental observations.
10.	Fransson, Erik, 1990, et al. (författare) Complexions and grain growth retardation: First-principles modeling of phase boundaries in WC-Co cemented carbides at elevated temperatures 2021 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 216 Tidskriftsartikel (refereegranskat)abstract WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in metal machining and in wear resistant tools. Controlling the WC grain size is important during sintering as grain size plays a crucial role for the mechanical properties of the material. Experimental studies have observed different growth rates and grain morphologies in W-rich and C-rich materials, but the mechanism behind this has not been clarified. Here, we consider the possibility of an interface-stabilized state, a complexion, at the WC/Co phase boundary in cemented carbides, namely thin WC films with cubic structure. An interfacial phase diagram is derived using ab-initio calculations and first-principles modeling. Cluster expansions are employed to model carbon vacancies and Monte Carlo simulations to sample the configurational entropy. Force-constant fitting is used to extract the harmonic free energy for ground-state structures and the effects from anharmonicity and electronic excitations are effectively incorporated from a companion study on WC bulk phases. We predict the stabilization of thin cubic WC films at liquid phase sintering temperatures but only at W-rich conditions. This is consistent with experimental findings where thin films with cubic stacking have been observed predominantly in W-rich materials. We use this knowledge to suggest an explanation for the observed different growth rates and grain morphologies in W-rich and C-rich cemented carbides.
11.	Fransson, Erik, 1990 (författare) Computational investigation of interface structure and composition in cemented carbides at finite temperatures 2019 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in high-speed machining of steels and in wear resistance tools. These excellent mechanical properties are to a large extent dependent on the microstructure and thus the interfacial properties of the material. Hence, being able to predict and understand interfacial properties in this material can allow for e.g. optimizing the manufacturing process in order to improve mechanical properties further. Atomic scale ab-initio calculations allow for accurately predicting interface energies for a given structure and composition. However, finding the ground-state interfacial structure and composition becomes a challenge as the search space is very large when considering all degrees of freedom. Furthermore, direct sampling of interfacial properties at finite temperature using density functional theory (DFT) often becomes computationally unfeasible as hundreds, thousands or even millions of calculations may be required. Therefore, employing atomic scale models based on DFT calculations is advantageous and allows for investigation of the interface structure, composition and free energy at finite temperatures. In this thesis the computational methods for calculating temperature dependent interfacial free energies are developed and applied to the WC-Co system. An interfacial phase diagram for cubic thin films in undoped WC-Co is constructed. Here, configurational degrees of freedom are treated using cluster expansion models and Monte Carlo sampling. Vibrations are treated in the harmonic approximation using force constant fitting to significantly reduce the number of DFT calculations. The temperature dependence of interface free energies for surfaces, grain boundaries and phase boundaries is using an analytic bond order potential. Here, multiple different free energy calculation methods are employed such as quasi-harmonic approximation, λ-integration, thermodynamic integration and surface tension calculation.
12.	Fransson, Erik, 1990, et al. (författare) Defects from phonons: Atomic transport by concerted motion in simple crystalline metals 2020 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 196, s. 770-775 Tidskriftsartikel (refereegranskat)abstract Point defects play a crucial role in crystalline materials as they do not only impact the thermodynamic properties but are also central to kinetic processes. While they are necessary in thermodynamic equilibrium spontaneous defect formation in the bulk is normally considered highly improbable except for temperatures close to the melting point. Here, we demonstrate by means of atomistic simulations that processes involving concerted atomic motion that give rise to defect formation are in fact frequent in body-centered cubic metals even down to about 50% of the melting temperature. It is shown that this behavior is intimately related to the anharmonicity of the lattice vibrations and a flat energy landscape along certain crystallographic directions, a feature that is absent in, e.g., face-centered cubic lattice structures. This insight has implications for our general understanding of these materials and furthermore provides a complementary explanation for the so-called anomalous diffusion in group 4 transition metals.
13.	Fransson, Erik, 1990, et al. (författare) Dynasor - A Tool for Extracting Dynamical Structure Factors and Current Correlation Functions from Molecular Dynamics Simulations 2021 Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 4:2 Tidskriftsartikel (refereegranskat)abstract Perturbative treatments of the lattice dynamics are widely successful for many crystalline materials; however, their applicability is limited for strongly anharmonic systems, metastable crystal structures and liquids. The full dynamics of these systems can, however, be accessed via molecular dynamics (MD) simulations using correlation functions, which includes dynamical structure factors providing a direct bridge to experiment. To simplify the analysis of correlation functions, here the dynasor package is presented as a flexible and efficient tool that enables the calculation of static and dynamical structure factors, current correlation functions as well as their partial counterparts from MD trajectories. The dynasor code can handle input from several major open source MD packages and thanks to its C/Python structure can be readily extended to support additional codes. The utility of dynasor is demonstrated via examples for both solid and liquid single and multi-component systems. In particular, the possibility to extract the full temperature dependence of phonon frequencies and lifetimes is emphasized.
14.	Fransson, Erik, 1990, et al. (författare) Efficient construction of linear models in materials modeling and applications to force constant expansions 2020 Ingår i: npj Computational Materials. - : Springer Science and Business Media LLC. - 2057-3960. ; 6:1 Tidskriftsartikel (refereegranskat)abstract Linear models, such as force constant (FC) and cluster expansions, play a key role in physics and materials science. While they can in principle be parametrized using regression and feature selection approaches, the convergence behavior of these techniques, in particular with respect to thermodynamic properties is not well understood. Here, we therefore analyze the efficacy and efficiency of several state-of-the-art regression and feature selection methods, in particular in the context of FC extraction and the prediction of different thermodynamic properties. Generic feature selection algorithms such as recursive feature elimination with ordinary least-squares (OLS), automatic relevance determination regression, and the adaptive least absolute shrinkage and selection operator can yield physically sound models for systems with a modest number of degrees of freedom. For large unit cells with low symmetry and/or high-order expansions they come, however, with a non-negligible computational cost that can be more than two orders of magnitude higher than that of OLS. In such cases, OLS with cutoff selection provides a viable route as demonstrated here for both second-order FCs in large low-symmetry unit cells and high-order FCs in low-symmetry systems. While regression techniques are thus very powerful, they require well-tuned protocols. Here, the present work establishes guidelines for the design of protocols that are readily usable, e.g., in high-throughput and materials discovery schemes. Since the underlying algorithms are not specific to FC construction, the general conclusions drawn here also have a bearing on the construction of other linear models in physics and materials science.
15.	Fransson, Erik, 1990, et al. (författare) First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures 2021 Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:9 Tidskriftsartikel (refereegranskat)abstract WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider C <-> vacancy and W <-> Ti substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Comaterials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.
16.	Fransson, Erik, 1990, et al. (författare) Impact of Organic Spacers and Dimensionality on Templating of Halide Perovskites 2024 Ingår i: ACS Energy Letters. - 2380-8195. ; 2024:9, s. 3947-3954 Tidskriftsartikel (refereegranskat)abstract Two-dimensional (2D) halide perovskites (HPs) are promising materials for various optoelectronic applications; yet, a comprehensive understanding of their dynamics is still elusive. Here, we offer insight into the dynamics of prototypical 2D HPs based on MAPbI3 as a function of linker molecule and the number of perovskite layers using atomic-scale simulations. We show that the layers closest to the linker undergo transitions that are distinct from those of the interior layers. These transitions can take place anywhere between a few tens of Kelvin degrees below and more than 100 K above the cubic-tetragonal transition of bulk MAPbI3. In combination with the thickness of the perovskite layer, this enables one to template phase transitions and tune the dynamics over a wide temperature range. Our results thereby reveal the details of an important and generalizable design mechanism for tuning the properties of these materials.
17.	Fransson, Erik, 1990, et al. (författare) Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials 2023 Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 127:28, s. 13773-13781 Tidskriftsartikel (refereegranskat)abstract The atomic scale dynamics of halide perovskites havea direct impactnot only on their thermal stability but also on their optoelectronicproperties. Progress in machine-learned potentials has only recentlyenabled modeling the finite temperature behavior of these materialsusing fully atomistic methods with near first-principles accuracy.Here, we systematically analyze the impact of heating and coolingrate, simulation size, model uncertainty, and the role of the underlyingexchange-correlation functional on the phase behavior of CsPbX3 with X = Cl, Br, and I, including both the perovskite andthe & delta;-phases. We show that rates below approximately 60 K/nsand system sizes of at least a few tens of thousands of atoms shouldbe used to achieve convergence with regard to these parameters. Bycontrolling these factors and constructing models that are specificfor different exchange-correlation functionals, we then assess thebehavior of seven widely used semilocal functionals (LDA, vdW-DF-cx,SCAN, SCAN+rVV10, PBEsol, PBE, and PBE+D3). The models based on LDA,vdW-DF-cx, and SCAN+rVV10 agree well with experimental data for thetetragonal-to-cubic-perovskite transition temperature in CsPbI3 and also achieve reasonable agreement for the perovskite-to-deltaphase transition temperature. They systematically underestimate, however,the orthorhombic-to-tetragonal transition temperature. All other models,including those for CsPbBr3 and CsPbCl3, predicttransition temperatures below the experimentally observed values forall transitions considered here. Among the considered functionals,vdW-DF-cx and SCAN+rVV10 yield the closest agreement with experiment,followed by LDA, SCAN, PBEsol, PBE, and PBE+D3. Our work providesguidelines for the systematic analysis of dynamics and phase transitionsin inorganic halide perovskites and similar systems. It also servesas a benchmark for the further development of machine-learned potentialsas well as exchange-correlation functionals.
18.	Fransson, Erik, 1990, et al. (författare) Understanding Correlations in BaZrO 3 : Structure and Dynamics on the Nanoscale 2024 Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 36:1, s. 514-523 Tidskriftsartikel (refereegranskat)abstract Barium zirconate (BaZrO3) is one of few perovskites that is claimed to retain an average cubic structure down to 0 K at ambient pressure while being energetically very close to a tetragonal phase obtained by condensation of a soft phonon mode at the R-point. Previous studies suggest, however, that the local structure of BaZrO3 may change at low temperature, forming nanodomains or a glass-like phase. Here, we investigate the global and local structure of BaZrO3 as a function of temperature and pressure via molecular dynamics simulations using machine-learned potential with near density functional theory (DFT) accuracy. We show that the softening of the octahedral tilt mode at the R-point gives rise to weak diffuse superlattice reflections at low temperatures and ambient pressure, which are also observed experimentally. However, we do not observe any static nanodomains but rather soft dynamic fluctuations of the ZrO6 octahedra with a correlation length of 2-3 nm over time scales of about 1 ps. This soft dynamic behavior is the precursor of a phase transition and explains the emergence of weak superlattice peaks in measurements. On the other hand, when increasing the pressure at 300 K, we find a phase transition from the cubic to the tetragonal phase at around 16 GPa, also in agreement with experimental studies.
19.	Gren, Martin, 1989, et al. (författare) A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides 2020 Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 87 Tidskriftsartikel (refereegranskat)abstract Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid. This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.
20.	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.
21.	Kiefer, David, 1989, et al. (författare) A Solution-Doped Polymer Semiconductor:Insulator Blend for Thermoelectrics 2017 Ingår i: Advanced Science. - : Wiley. - 2198-3844 .- 2198-3844. ; 4:1, s. 1600203- Tidskriftsartikel (refereegranskat)abstract Poly(ethylene oxide) is demonstrated to be a suitable matrix polymer for the solution-doped conjugated polymer poly(3-hexylthiophene). The polarity of the insulator combined with carefully chosen processing conditions permits the fabrication of tens of micrometer-thick films that feature a fine distribution of the F4TCNQ dopant:semiconductor complex. Changes in electrical conductivity from 0.1 to 0.3 S cm−1 and Seebeck coefficient from 100 to 60 μV K−1 upon addition of the insulator correlate with an increase in doping efficiency from 20% to 40% for heavily doped ternary blends. An invariant bulk thermal conductivity of about 0.3 W m−1 K−1 gives rise to a thermoelectric Figure of merit ZT ∼ 10−4 that remains unaltered for an insulator content of more than 60 wt%. Free-standing, mechanically robust tapes illustrate the versatility of the developed dopant:semiconductor:insulator ternary blends.
22.	Kim, Shi En, et al. (författare) Extremely anisotropic van der Waals thermal conductors 2021 Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 597:7878, s. 660-665 Tidskriftsartikel (refereegranskat)abstract The densification of integrated circuits requires thermal management strategies and high thermal conductivity materials1–3. Recent innovations include the development of materials with thermal conduction anisotropy, which can remove hotspots along the fast-axis direction and provide thermal insulation along the slow axis4,5. However, most artificially engineered thermal conductors have anisotropy ratios much smaller than those seen in naturally anisotropic materials. Here we report extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations, which produce a room-temperature thermal anisotropy ratio close to 900 in MoS2, one of the highest ever reported. This is enabled by the interlayer rotations that impede the through-plane thermal transport, while the long-range intralayer crystallinity maintains high in-plane thermal conductivity. We measure ultralow thermal conductivities in the through-plane direction for MoS2 (57 ± 3 mW m−1 K−1) and WS2 (41 ± 3 mW m−1 K−1) films, and we quantitatively explain these values using molecular dynamics simulations that reveal one-dimensional glass-like thermal transport. Conversely, the in-plane thermal conductivity in these MoS2 films is close to the single-crystal value. Covering nanofabricated gold electrodes with our anisotropic films prevents overheating of the electrodes and blocks heat from reaching the device surface. Our work establishes interlayer rotation in crystalline layered materials as a new degree of freedom for engineering-directed heat transport in solid-state systems.
23.	Lindroth, Daniel, 1978, et al. (författare) Thermal conductivity in intermetallic clathrates: A first-principles perspective 2019 Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 100:4 Tidskriftsartikel (refereegranskat)abstract Inorganic clathrates such as Ba8GaxGe46-x and Ba8AlxSi46-x commonly exhibit very low thermal conductivities. A quantitative computational description of this important property has proven difficult, in part due to the large unit cell, the role of disorder, and the fact that both electronic carriers and phonons contribute to transport. Here, we conduct a systematic analysis of the temperature and composition dependence of low-frequency modes associated with guest species in Ba8GaxGe46-x and Ba8AlxSi46-x ("rattler modes"), as well as thermal transport in stoichiometric Ba8Ga16Ge30. To this end, we account for phonon-phonon interactions by means of temperature-dependent effective interatomic force constants, which we find to be crucial in order to achieve an accurate description of the lattice part of the thermal conductivity. While the analysis of the thermal conductivity is often largely focused on the rattler modes, here it is shown that at room temperatures modes with ω 10meV account for 50% of lattice heat transport. Finally, the electronic contribution to the thermal conductivity is computed, which shows the Wiedemann-Franz law to be only approximately fulfilled. As a result, it is crucial to employ the correct prefactor when separating electronic and lattice contributions for experimental data.
24.	Overview of JET results for optimising ITER operation 2022 Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 62:4 Forskningsöversikt (refereegranskat)
25.	Rosander, Petter, 1993, et al. (författare) Anharmonicity of the antiferrodistortive soft mode in barium zirconate BaZrO3 2023 Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 108:1 Tidskriftsartikel (refereegranskat)abstract Barium zirconate (BaZrO3) is one of the very few perovskites that is claimed to retain an average cubic structure down to 0K, while being energetically very close to an antiferrodistortive phase obtained by condensation of a soft phonon mode at the R point of the Brillouin zone boundary. In this work, we report a combined experimental and theoretical study of the temperature dependence of this soft phonon mode. Inelastic neutron and x-ray scattering measurements on single crystals show that it softens substantially from 9.4meV at room temperature to 5.6meV at 2K. In contrast, the acoustic mode at the same R point is nearly temperature independent. The effect of the anharmonicity on the lattice dynamics is investigated nonperturbatively using direct dynamic simulations as well as a first-principles-based self-consistent phonon theory, including quantum fluctuations of the atomic motion. By adding cubic and quartic anharmonic force constants, quantitative agreement with the neutron data for the temperature dependence of the antiferrodistortive mode is obtained. The quantum fluctuations of the atomic motion are found to be important to obtain the proper temperature dependence at low temperatures. The mean-squared displacements of the different atoms are determined as function of temperature and are shown to be consistent with available experimental data. Adding anharmonicity to the computed fluctuations of the Ba-O distances also improves the comparison with available extended x-ray absorption fine structure data at 300K.
26.	Stukowski, A., et al. (författare) Atomicrex-a general purpose tool for the construction of atomic interaction models 2017 Ingår i: Modelling and Simulation in Materials Science and Engineering. - 1361-651X .- 0965-0393. ; 25:5, s. Article Number: 055003 - Tidskriftsartikel (refereegranskat)abstract We introduce ATOMICREX, an open-source code for constructing interatomic potentials as well as more general types of atomic-scale models. Such effective models are required to simulate extended materials structures comprising many thousands of atoms or more, because electronic structure methods become computationally too expensive at this scale. ATOMICREX covers a wide range of interatomic potential types and fulfills many needs in atomistic model development. As inputs, it supports experimental property values as well as ab initio energies and forces, to which models can be fitted using various optimization algorithms. The open architecture of ATOMICREX allows it to be used in custom model development scenarios beyond classical interatomic potentials while thanks to its Python interface it can be readily integrated e.g., with electronic structure calculations or machine learning algorithms. ELL GC, 1985, PHYSICAL REVIEW B, V31, P6184
27.	Wiktor, Julia, 1988, et al. (författare) Quantifying Dynamic Tilting in Halide Perovskites: Chemical Trends and Local Correlations 2023 Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 6737- Tidskriftsartikel (refereegranskat)abstract Halide perovskiteshave emerged as one of the most interestingmaterials for optoelectronic applications due to their favorable properties,such as defect tolerance and long charge carrier lifetimes, whichare attributed to their dynamic softness. However, this softness hasled to apparent disagreements between the local instantaneous andglobal average structures of these materials. In this study, we rationalizethis situation through an assessment of the local tilt angles of octahedrain the perovskite structure using large-scale molecular dynamics simulationsbased on machine-learned potentials trained using density functionaltheory calculations. We compare structural properties given by differentdensity functionals [local density approximation, PBE, PBE + D3, PBEsol,strongly constrained and appropriately normed (SCAN), SCAN + rVV10,and van der Waals density functional with consistent exchange] andestablish trends across a family of CsMX3 perovskites withM = Sn or Pb and X = Cl, Br or I. Notably, we demonstrate strong short-rangeordering in the cubic phase of halide perovskites. This ordering isreminiscent of the tetragonal phase and provides the bridge betweenthe disordered local structure and the global cubic arrangement. Ourresults provide a deeper understanding of the structural propertiesof halide perovskites and their local distortions, which is crucialfor further understanding their optoelectronic properties.

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