Abstract
We present a method to accurately predict the Helmholtz harmonic free energies of molecular crystals in high-throughput settings. This is achieved by devising a computationally efficient ...framework that employs a Gaussian Process Regression model based on local atomic environments. The cost to train the model with ab initio potentials is reduced by starting the optimization of the framework parameters, as well as the training and validation sets, with an empirical potential. This is then transferred to train the model based on density-functional theory potentials, including dispersion-corrections. We benchmarked our framework on a set of 444 hydrocarbon crystal structures, comprising 38 polymorphs and 406 crystal structures either measured in different conditions or derived from these polymorphs. Superior performance and high prediction accuracy, with mean absolute deviation below 0.04 kJ mol
−1
per atom at 300 K is achieved by training on as little as 60 crystal structures. Furthermore, we demonstrate the predictive efficiency and accuracy of the developed framework by successfully calculating the thermal lattice expansion of aromatic hydrocarbon crystals within the quasi-harmonic approximation, and predict how lattice expansion affects the polymorph stability ranking.
Coherent and incoherent long-range atomic motions accompany singlet exciton fission in pentacene.
Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An ...aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.
The nature and origin of the glass transition is one of the great unsolved problems of condensed-matter science. With the rapid increase of viscosity upon cooling the liquid near the glass-transition ...temperature, a range of dynamical motifs are observed, revealing the sheer complexity of interactions between the amorphous units. Yet, the causal link between those motifs and the solidification process remains unclear. Here, we apply a novel approach for exploring nontrivial interactions between structural units in d-sorbitol, a canonical example of a hydrogen-bonded organic glass, by introducing a dihedral-rearrangement-indicator analysis to shed light on relaxation processes and dynamical heterogeneity, which are known for their association with the stability of a glass. We find that both α- and β-relaxation processes are governed by cooperative and heterogeneous changes in hydrogen-bond dynamics that can be described by spatial and dihedral-angle-rearrangement indicators. The methodology and findings are of general applicability to other glass-forming systems.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
5.
Cooperative excitations in superionic PbF 2 Mohn, Chris E.; Krynski, Marcin; Kob, Walter ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
11/2021, Volume:
379, Issue:
2211
Journal Article
Peer reviewed
Open access
Links between dynamical Frenkel defects and collective diffusion of fluorides in
β
-PbF
2
are explored using Born–Oppenheimer molecular dynamics. The calculated self-diffusion coefficient and ionic ...conductivity are 3.2 × 10
−5
cm
2
s
−1
and 2.4 Ω
−1
cm
−1
at 1000 K in excellent agreement with pulsed field gradient and conductivity measurements. The calculated ratio of the tracer-diffusion coefficient and the conductivity-diffusion coefficient (the Haven ratio) is slightly less than unity (about 0.85), which in previous work has been interpreted as providing evidence against collective ‘multi-ion’ diffusion. By contrast, our molecular dynamics simulations show that fluoride diffusion is highly collective. Analysis of different mechanisms shows a preference for direct collinear ‘kick-out’ chains where a fluoride enters an occupied tetrahedral hole/cavity and pushes the resident fluoride out of its cavity. Jumps into an occupied cavity leave behind a vacancy, thereby forming dynamic Frenkel defects which trigger a chain of migrating fluorides assisted by local relaxations of the lead ions to accommodate these chains. The calculated lifetime of the Frenkel defects and the collective chains is approximately 1 ps in good agreement with that found from neutron diffraction.
This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.
The aim of this paper is to examine the correlation between alcoholism and crime using the example of convicts serving prison sentences in the therapeutic system in the District Inspectorate of the ...Prison Service in Białystok. The research methods used were the following: a diagnostic survey, literature analysis and statistical method. Among the analyzed data, data obtained from the databases of the Central Board of the Prison Service and penitentiary units subordinated to the District Inspectorate of the Prison Service in Białystok were particularly valuable. The analysis of the source material collected in this way made it possible to determine the convicts opinion regarding alcoholism as a factor causing criminal behavior. The research conducted on convicts serving prison sentences in penitentiary units subordinated to the District Inspectorate of the Prison Service in Białystok shows that among convicts exposed to alcoholism, the percentage of committing crimes was significantly higher compared to those who did not abuse alcohol. Additionally, research showed that the frequency of alcohol consumption was positively correlated with crime. For this reason, the conclusions resulting from the conducted research should be thought-provoking and are important for the development of effective intervention and prevention programs aimed at reducing alcoholism and crime in society.The article concerns the correlation between alcoholism and crime on the example of convicts serving prison sentences in the therapeutic system in the District Inspectorate of the Prison Service in Białystok. This type of research has not been conducted in the presented District.
Cooperative excitations in superionic PbF2 Mohn, Chris E; Krynski, Marcin; Kob, Walter ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2021, Volume:
379, Issue:
2211
Journal Article
The classical view of the structural changes that occur at the ferroelectric transition in perovskite-structured systems, such as BaTiO3, is that polarization occurs due to the off-center ...displacement of the B-site cations. Here, we show that in the bismuth sodium titanate (BNT)-based composition 0.2(Ba0.4Sr0.6TiO3)–0.8(Bi0.5Na0.5TiO3), this model does not accurately describe the structural situation. Such BNT-based systems are of interest as lead-free alternatives to currently used materials in a variety of piezo-/ferroelectric applications. A combination of high-resolution powder neutron diffraction, impedance spectroscopy, and ab initio calculations reveals that Ti4+ contributes less than a third in magnitude to the overall polarization and that the displacements of the O2– ions and the A-site cations, particularly Bi3+, are very significant. The detailed examination of the ferroelectric transition in this system offers insights applicable to the understanding of such transitions in other ferroelectric perovskites, particularly those containing lone pair elements.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
The BIMEVOXes are among the best oxide ion conductors at low and intermediate temperatures. Their high conductivity is associated with local defect structure. In this work, the local structures of ...two BIMEVOX compositions, Bi2V0.9Ge0.1O5.45 and Bi2V0.95Sn0.05O5.475, are examined using total neutron and X-ray scattering methods, with both compositions exhibiting the ordered α-phase at 25 °C and the disordered γ-phase at 700 °C. While the diffraction data for the α-phase do not allow for the polar (C2) and nonpolar (C2/m) structures to be readily distinguished, measurements of dielectric permittivity suggest the α-phase is weakly ferroelectric in character, consistent with calculations of spontaneous polarization based on a combination of density functional calculations and machine learning methodology. Reverse Monte Carlo (RMC) analysis of total scattering data reveals Ge preferentially adopts tetrahedral geometry at both temperatures, while Sn is found to predominantly adopt octahedral coordination in the α-phase and tetrahedral coordination in the γ-phase. In all cases, V polyhedra are found to consist of tetrahedral, pentacoordinate, and octahedral geometries, as also predicted by the crystallographic analysis and confirmed by 51V solid state NMR spectroscopy. Although similar long-range structures are observed at room temperature, the oxide ion vacancy distributions were found to be quite different between the two studied compositions, with a nonrandom deficiency in vacancy pairs in the second-nearest shell along the ⟨100⟩ tetragonal direction for BIGEVOX10, compared with a long-distance (>8.0 Å) ordering of equatorial vacancies for BISNVOX05. This is attributed to the differences in the preferred coordination geometries of the substituent cations in the two systems. Impedance spectroscopy measurements reveal both compositions show high conductivity in the order of 10–1 S cm–1 at 600 °C.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
PDF
