| 1. |
- Fransson, Erik, 1990, et al.
(författare)
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Impact of Organic Spacers and Dimensionality on Templating of Halide Perovskites
- 2024
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Ingår i: ACS Energy Letters. - 2380-8195. ; 9:8, s. 3947-3954
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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.
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| 2. |
- Fransson, Erik, 1990, et al.
(författare)
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Understanding Correlations in BaZrO 3 : Structure and Dynamics on the Nanoscale
- 2024
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Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 36:1, s. 514-523
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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.
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| 3. |
- Lindgren, Eric, 1997, et al.
(författare)
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calorine: A Python package for constructing and sampling neuroevolution potential models
- 2024
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Ingår i: Journal of Open Source Software. - 2475-9066. ; 9:95, s. 6264-6264
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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.
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| 4. |
- Eriksson, Fredrik, 1992, et al.
(författare)
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Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering
- 2023
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Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 17:24, s. 25565-25574
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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.
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| 5. |
- Fransson, Erik, 1990, et al.
(författare)
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Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites
- 2023
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Ingår i: Communications Physics. - 2399-3650. ; 6:1
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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.
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| 6. |
- Fransson, Erik, 1990, et al.
(författare)
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Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials
- 2023
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Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 127:28, s. 13773-13781
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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.
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| 7. |
- Fransson, Erik, 1990, et al.
(författare)
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Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr₃ and MAPbI₃
- 2023
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Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 8229-
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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.
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| 8. |
- Rosander, Petter, 1993, et al.
(författare)
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Anharmonicity of the antiferrodistortive soft mode in barium zirconate BaZrO3
- 2023
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 108:1
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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.
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| 9. |
- Wiktor, Julia, 1988, et al.
(författare)
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Quantifying Dynamic Tilting in Halide Perovskites: Chemical Trends and Local Correlations
- 2023
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Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 6737-
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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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| 10. |
- Brorsson, Joakim, 1988, et al.
(författare)
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Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy
- 2022
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Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 5:2
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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.
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