Cation Dynamics Research Articles

Thermal stability, cation dynamics, and ferroelastic domain walls in the α→β→γ phase transitions of perovskite (C2H5NH3)2MnCl4 crystals

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) confirmed that the (C2H5NH3)2MnCl4 crystal undergoes α-β-γ phase transitions at approximately TC2 = 225 K and TC1 = 424 K, remains stable until 470 K, and displays a weight loss of 25.24% at around 579 K due to partial thermal decomposition. Cation dynamics near these temperatures was investigated by MAS 1H NMR and MAS 13C NMR experiments. The small changes in the 1H chemical shift for C2H5 near TC2 are consistent with the transition from the orthorhombic γ-phase to the orthorhombic β-phase. Also, the abrupt changes in NMR signals near TC1 correspond to structural change from the orthorhombic β-phase to the tetragonal α-phase. The 13C spin-lattice relaxation time in the rotating frame (T1ρ) below TC2 is shorter for CH3 than that for CH2, due to the greater mobility at the free end of the alkyl chain. In addition, the temperature-dependent ferroelastic domain structure was studied near the α-β phase transition. There was no noticeable temperature dependence in 1H and 13C T1ρ values at TC1 and TC2, whereas the change in the domain wall was prominent near TC1.

The quantum-Ehrenfest method with the inclusion of an IR pulse: Application to electron dynamics of the allene radical cation.

We describe the implementation of a laser control pulse in the quantum-Ehrenfest method, a molecular quantum dynamics method that solves the time-dependent Schrödinger equation for both electrons and nuclei. The oscillating electric field-dipole interaction is incorporated directly in the one-electron Hamiltonian of the electronic structure part of the algorithm. We then use the coupled electron-nuclear dynamics of the π-system in the allene radical cation (•CH2=C=CH2)+ as a simple model of a pump-control experiment. We start (pump) with a two-state superposition of two cationic states. The resulting electron dynamics corresponds to the rapid oscillation of the unpaired electron between the two terminal methylenes. This electron dynamics is, in turn, coupled to the torsional motion of the terminal methylenes. There is a conical intersection at 90° twist, where the electron dynamics collapses because the adiabatic states become degenerate. After passing the conical intersection, the electron dynamics revives. The IR pulse (control) in our simulations is timed to have its maximum at the conical intersection. Our simulations show that the effect of the (control) pulse is to change the electron dynamics at the conical intersection and, as a consequence, the concomitant nuclear dynamics, which is dominated by the change in the torsional angle.

Anion and Cation Dynamics in Polyhydroborate Salts: NMR Studies.

Polyhydroborate salts represent the important class of energy materials attracting significant recent attention. Some of these salts exhibit promising hydrogen storage properties and/or high ionic conductivities favorable for applications as solid electrolytes in batteries. Two basic types of thermally activated atomic jump motion are known to exist in these materials: the reorientational (rotational) motion of complex anions and the translational diffusion of cations or complex anions. The present paper reviews recent progress in nuclear magnetic resonance (NMR) studies of both reorientational and diffusive jump motion in polyhydroborate salts. The emphasis is put on sodium and lithium closo-borates exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures. For these systems, we discuss the effects of order–disorder phase transitions on the parameters of reorientations and diffusive jumps, as well as the mechanism of low-temperature rotational tunneling.

Soil aggregation and aggregate-related exchangeable base cations under different aged tea (Camellia sinensis L.) plantations in the hilly regions of southern Guangxi, China

ABSTRACT As the secondary nutrients in soils, exchangeable base cations can form cationic bridges with clay particles and organic matters to promote the formation of soil aggregates, and to protect soil organic carbon (Corg) and nutrients from loss. To have a comprehensive understanding toward the dynamics of soil exchangeable base cations in the tea planting process, it requires to evaluate these cations’ distribution in soil aggregates at various sizes. In this study, soil samples (0–20 cm) from the tea plantations at various ages (8, 17, 25, and 43 yr.) in the hilly regions of southern Guangxi, China were divided into >2, 1–2, 0.25–1, and <0.25 mm aggregate fractions through a dry-sieving procedure. Then, the cation exchange capacity (CEC), total exchangeable bases (TEB), and different individual cations, such as exchangeable potassium (K+), sodium (Na+), calcium (Ca2+), and magnesium (Mg2+), in soil aggregates were detected. Our results suggested that the mean weight diameter (MWD), that is an indicator evaluating the stability of soil aggregates, in the 17-yr. tea plantations appeared to be the highest among all the tea plantations, indicating that more stable soil aggregates could be formed in the 17-yr. tea plantations in comparison with the rest plantations. During tea planting of 35 yr. (from 8 to 43 yr.), the increasing rate of soil CEC stock was 33.42 cmol m−2 yr.−1, while the loss rates of soil exchangeable K+, Na+, Ca2+, Mg2+, and TEB were 0.89, 0.52, 3.86, 1.23, and 6.51 cmol m−2 yr.−1, respectively. Interestingly, the loss rates of soil exchangeable base cations in the middle phase (from 17 to 25 yr.) were greater than those in other phases. Therefore, it is of critical importance to implement a sustainable management scheme to maintain soil aggregate stability and to minimize the loss of soil exchangeable base cations in the tea planting process, especially after 17 yr., in the hilly regions of southern Guangxi, China.

Solvation structure and dynamics of the dimethylammonium cation diluted in liquid water: A molecular dynamics approach.

Classical molecular dynamics simulation techniques were employed to investigate the local solvation structure and related dynamics of the dimethylammonium cation diluted in liquid water at ambient conditions. The translational and orientational order around the dimethylammonium cation was investigated in terms of the corresponding radial and angular distribution functions. The results obtained revealed that the first solvation shell of the dimethylammonium consists mainly of two and, less frequently, three water molecules. The two nearest water neighbors form hydrogen bonds with the ammonium hydrogen atoms of the cation, whereas the third neighbor interacts with the methyl hydrogen atoms as well. The distribution of the trigonal order parameter exhibits a bimodal behavior, signifying the existence of local orientational heterogeneities in the solvation shell of the dimethylammonium cation. The calculated continuous and intermittent residence and hydrogen bond lifetimes for the cation-water pairs have also been found to be longer in comparison with the water-water ones. The very similar self-diffusion coefficients of the dimethylammonium cation and the water molecules in the bulk dilute solution indicate that the translational motions of the cation are mainly controlled by the translational mobility of the surrounding water molecules.

Ionic liquid dynamics in nanoporous carbon: A pore-size- and temperature-dependent neutron spectroscopy study on supercapacitor materials

The influence of spatial confinement on the thermally excited stochastic cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium bis-((trifluoromethyl)sulfonyl)imide ([BuPy][Tf_2N]) inside porous carbide-derived carbons with various pore sizes in the sub- to a few nanometer range are investigated by quasi-elastic neutron spectroscopy. Using the potential of fixed window scans, i.e. scanning a sample parameter, while observing solely one specific energy transfer value, an overview of the dynamic landscape within a wide temperature range is obtained. It is shown that already these data provide a quite comprehensive understanding of the confinement-induced alteration of the molecular mobility in comparison to the bulk. A complementary, more detailed analysis of full energy transfer spectra at selected temperatures reveals two translational diffusive processes on different time scales. Both are considerably slower than in the bulk liquid and show a decrease of the respective self-diffusion coefficients with decreasing nanopore size. Different thermal activation energies for molecular self-diffusion in nanoporous carbons with similar pore size indicate the importance of pore morphology on the molecular mobility, beyond the pure degree of confinement. In spite of the dynamic slowing down we can show that the temperature range of the liquid state upon nanoconfinement is remarkably extended to much lower temperatures, which is beneficial for potential technical applications of such systems.

Improvement on soil structure and water retention after application of industrial organic waste as a crop fertilizer

Industrial organic waste (IOW) from slaughter and processing of poultry and swine might be potential crop fertilizer. The use of IOW is known to alter the dynamics of soil organic carbon (SOC) and exchangeable cations. However, its effects on soil physical properties and processes are still unknown. The aim of this study was to evaluate the effects of the isolated and combined application of IOW and mineral fertilizers (MF) on SOC pools, structural physical properties, water retention, and some chemical properties under long-term no-tillage system. Treatments applied during 6 years were as follows: control (no fertilizers application) and crop nutrients through MF, IOW, and MF + IOW. We evaluated SOC pools (total, permanganate (POx-C), and hot water extractable (HWE-C)), structural soil physical properties (i.e., porosity and water retention), and exchangeable cation contents at 0–10 and 10–20 cm layers. The HWE-C increased in response to IOW application in relation to the control treatment at the 0–10 cm layer. POx-C with MF was closer to IOW application. However, this SOC pool was more affected at 10–20 cm layer, in which the IOW and MF promoted increases of POx-C in relation to control. Both forms of fertilization increased total porosity and water retention and the highest values were observed for IOW. We observed a high correlation between the increases of labile SOC and the improvement of soil physical conditions. The long-term exclusive MF application induced damages on soil, such as decreases in water retention and availability of nutrients. The results of MF + IOW were closer to those of MF treatment than IOW treatment. The IOW evaluated is a suitable fertilization strategy, providing soil quality. But their use must be carried out in isolation or in combinations with mineral fertilizers greater than 50% of the blend. Also, this IOW is harmless to health and hygiene due to its treatment.

Dissociation dynamics of carbon dioxide cation (CO2 +) in the C2Σg + state via [1+1] two-photon excitation.

The dissociation dynamics of CO2 + in the C2Σg + state has been studied in the 8.14-8.68 eV region by [1+1] two-photon excitation via vibronically selected intermediate A2Πu and B2Σu + states using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces an internally cold mass selected ion sample of CO2 +. Total translational energy release (TER) and two-dimensional recoiling velocity distributions of fragmented CO+ ions are measured by time-sliced velocity map imaging. High resolution TER spectra allow us to identify and assign three dissociation channels of CO2 + (C2Σg +) in the studied energy region: (1) production of CO+(X2Σ+) + O(3P) by predissociation via spin-orbit coupling with the repulsive 14Πu state; (2) production of CO+(X2Σ+) + O(1D) by predissociation via bending and/or anti-symmetric stretching mediated conical intersection crossing with A2Πu or B2Σu +, where the C2Σg +/A2Πu crossing is considered to be more likely; (3) direct dissociation to CO+(A2Π) + O(3P) on the C2Σg + state surface, which exhibits a competitive intensity above its dissociation limit (8.20 eV). For the first dissociation channel, the fragmented CO+(X2Σ+) ions are found to have widely spread populations of both rotational and vibrational levels, indicating that bending of the parent CO2 + over a broad range is involved upon dissociation, while for the latter two channels, the produced CO+(X2Σ+) and CO+(A2Π) ions have relatively narrow rotational populations. The anisotropy parameters β are also measured for all three channels and are found to be nearly independent of the vibronically selected intermediate states, likely due to complicated intramolecular interactions in the studied energy region.

Ultrafast Proton Transfer Dynamics on the Repulsive Potential of the Ethanol Dication: Roaming-Mediated Isomerization versus Coulomb Explosion.

If a molecular dication is produced on a repulsive potential energy surface (PES), it normally dissociates. Before that, however, ultrafast nuclear dynamics can change the PES and significantly influence the fragmentation pathway. Here, we investigate the electron-impact-induced double ionization and subsequent fragmentation processes of the ethanol molecule using multiparticle coincident momentum spectroscopy and ab initio dynamical simulations. For the electronic ground state of the ethanol dication, we observe several fragmentation channels that cannot be reached by direct Coulomb explosion (CE) but require preceding isomerization. Our simulations show that ultrafast hydrogen or proton transfer (PT) can stabilize the repulsive PES of the dication before the direct CE and form intermediate H2 or H2O. These neutrals stay in the vicinity of the precursor, and roaming mechanisms lead to isomerization and finally PT resulting in emission of H3+ or H3O+. The present findings can help to understand the complex fragmentation dynamics of molecular cations.

A Nuclear Magnetic Resonance Study of Cation and Anion Dynamics in Polymer–Ceramic Composite Solid Electrolytes

Polymer–ceramic composite electrolytes are promising for the application of all-solid-state lithium-ion batteries with high energy density and improved safety. In this work, we employ temperature-d...

Investigation of the valence ionization spectrum of chromium carbonyl using an ab initio quantum dynamical approach.

The nuclear dynamics of the chromium carbonyl cation following an ionization process corresponding to the 2T2g ← 1A1g transition is studied theoretically, for the first time, using a fully quantal approach as well as high levels of the ab initio electronic structure and semiempirical density functional theory (DFT) methods. The photoelectron spectrum is calculated by the construction of a Hamiltonian model, in which the two totally symmetric modes ν19 (the Cr-C stretching mode) and ν39 (the C-O stretching mode) together with the spin-orbit (SO) coupling up to the zeroth-order SO splitting are treated. The potential energy curves along these two vibrational modes are computed by using the DFT. The simulated photoelectron spectrum is found to be in good agreement with the corresponding experimental one, leading to the conclusion that the potential energy surfaces and the diabatic population analysis are accurately determined. Our calculation confirms that the vibrational modes ν19 and ν39 are the vibrational progression of the valence ionization spectrum of the chromium carbonyl cation.

Crystal structures and rotational dynamics of a two-dimensional metal halide perovskite (OA)2PbI4.

The extended charge carrier lifetime in metal halide perovskites is responsible for their excellent optoelectronic properties. Recent studies indicate that the superb device performance in these materials is intimately related to the organic cation dynamics. Here, we focus on the investigation of the two-dimensional hybrid perovskite, (C8H17NH3)2PbI4 (henceforth, OA+ = C8H17NH3 +). Using elastic and quasielastic neutron scattering techniques and group theoretical analysis, we studied the structural phase transitions and rotational modes of the C8H17NH3 + cation in (OA)2PbI4. Our results show that, in the high-temperature orthorhombic (T > 310 K) phase, the OA+ cation exhibits a combination of a twofold rotation of the NH3-CH2 head group about the crystal c-axis with a characteristic relaxation time of ∼6.2 ps, threefold rotations (C3) of NH3 and CH3 terminal groups, and slow librations of the other atoms. Contrastingly, only the C3 rotation is present in the intermediate-temperature orthorhombic (238 K < T < 310 K) and low-temperature monoclinic (T < 238 K) phases.

L-DOPA modulates the kinetics but not the thermodynamic equilibrium of TTA+ amphiphiles forming lyotropic nematic liquid crystals.

Lyotropic liquid crystals (LLCs) are mixtures of amphiphile molecules usually studied as mimetic of biological membrane. The equilibrium dynamics of tetradecyltrimethyl ammonium cation (TTA+) molecules forming nematic LLCs (LNLCs) is guided by a dive-in mechanism where TTA+ molecules spontaneously leave and re-enter the bicelle. Of note, this dynamic behavior could be exploited to produce drug nano-delivery systems based on LNLCs. Therefore, the understanding of the effect of pharmaceutically interesting molecules in the dynamics of the dive-in mechanism should be crucial for drug delivery applications. In this work, we studied the effects of l-DOPA in the equilibrium dynamics of TTA+ bicelles forming LNLCs, employing a transdisciplinary approach based on 2H-NMR together with molecular modeling and molecular dynamics simulations. Our data suggest that l-DOPA perturbs the kinetic of the dive-in mechanism but not the thermodynamics of this process. As whole, our results provide fundamental insights on the mechanisms by which l-DOPA govern the equilibrium of LNLCs bicelles.

Radiative cooling dynamics of anthracene cations stored in DESIREE studied via the time evolution of 2-photon-absorption induced dissociation rate

SynopsisAnthracene cations (C14H10 +) have been stored in the cryogenic (13 K) electrostatic storage ring DESIREE in Stockholm to study their radiative cooling dynamics on a long time scale up to 2 s. By monitoring the laser-induced delayed dissociation curves during the storage, the evolution of the dissociation rate was obtained. Its decrease as a function of time is closely related to the time evolution of the internal energy distribution of the stored anthracene cations and consequently to the energy loss due to radiative cooling, including both Poincaré and infrared fluorescences.

Ferroelectricity in Ethylammonium Bismuth-Based Organic-Inorganic Hybrid: (C2H5NH3)2[BiBr5

The (C2H5NH3)2[BiBr5] (EBB) crystals adopt the one-dimensional (1D) polymeric anionic form [BiBr5]∞2-, which is preferred by halobismuthates(III) exhibiting polar properties and realized in R2MX5 stoichiometry. Differential scanning calorimetry and dilatometric measurements reveal reversible structural phase transitions: at 160 K (phase I → phase II) and 120 K (phase II → phase III). The resolved crystal structures of EBB show the centrosymmetric space group in phase I (Aeam), polar (Pca21) in phase II, and polar (Aea2) in phase III. The presence of dielectric hysteresis loops in phases II and III evidence ferroelectric properties. The dielectric response [ε*(ω,T)] of EBB close to 160 K is characteristic of ferroelectrics with a critical slowing down process. The molecular mechanism of a paraelectric-ferroelectric phase transition at 160 K is explained as "order-disorder" (assigned to the dynamics of the ethylammonium cations) and dominating "displacive" (related to strong distortion of the 1D anionic network). The optical band gap obtained from UV-vis measurements is about 2.6 eV. The conduction band minimum is formed by the hybridized Bi 6p and Br 4p states. An analysis of the CSD results for haloantimonates(III) and halobismuthates(III) ferroelectrics characterized by [MX4]-, [M2X9]3-, [MX5]2-, and [M2X11]5- anions is given.

Thermal decomposition and structural dynamics in perovskite (C2H5NH3)2CdCl4 crystals

In this study, (C2H5NH3)2CdCl4 single crystals were grown, and their thermal properties were studied by the thermogravimetric analysis and differential scanning calorimetry. Phase transition temperature above room temperature was observed at 470 K. In addition, the cation dynamics in this crystal were investigated by 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) and 13C cross-polarization (CP)/MAS NMR experiments. The Bloembergen–Purcell–Pound curves for 1H T1ρ and 13C T1ρ in C2H5NH3 cation exhibited minimum values, and these curves represent C2H5NH3 and C2H5 rotational motions. The activation energy for 1H in the C2H5NH3 cation is Ea = 22.63 kJ mol−1, whereas those for 13C in CH3 and CH2 are Ea = 18.05 and 19.14 kJ mol−1, respectively. Furthermore, the proton dynamics of C2H5NH3 undergo faster rotation than carbons. This implies that the molecular motion for 1H is enhanced at the C-end and N-end of an organic cation, whereas the molecular motion is not free at the main chain carbons of the organic cation.

Effect of CdSe/ZnS quantum dots on temperature-dependent luminescence properties in mixed halide perovskites

The temperature-dependent dynamics of organic cations are known to influence the optical properties of lead halide perovskites (CH3NH3PbX3) significantly. The Rashba splitting effect and internal electric field induced due to the strong spin-orbit coupling and absence of inversion symmetry in the lead halide perovskites (PS) lead to a decrease in the radiative recombination rates. This study has investigated the luminescence properties of the mixed halide PS (CH3NH3PbI2Br) covered with CdSe/ZnS quantum dots (QDs) (QD/PS hybrid structure) using temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. A shorter radiative lifetime obtained in the QD/PS hybrid structures as compared to the bare PS film was attributed to the screening effects of the Rashba splitting and internal electric field due to the charge carrier transfer from QDs to PS. The PL intensity and the PL decay times of the QD/PS hybrid structures increased when compared to those of the bare PS film because of the surface-passivation and charge transfer (CT) effects. The CT from the QDs to PS leads to an enhancement in the radiative recombination due to the enhanced oscillator strength by the screened internal electric field and Rashba splitting, which is induced by an increase in the charge carriers.

On the relation between reorientation and diffusion in glass-forming ionic liquids with micro-heterogeneous structures.

We investigate complex structure-dynamics relations in glass-forming ionic liquids comprising 1-alkyl-3-methylimidazolium cations and bis(trifluoromethylsulfonyl)imide anions. In doing so, we exploit the microheterogeneous structures emerging when the alkyl length is increased in the range n = 1-12 and use that 1H and 2H NMR give information about cation dynamics, while 19F NMR reports on anion motions. Furthermore, we combine spin-lattice relaxation analysis, including field-cycling relaxometry, with stimulated-echo experiments to follow reorientation dynamics related to structural relaxation in wide dynamic ranges and we apply static field gradients to probe translational diffusion. The resulting correlation times τ and diffusion coefficients D show Vogel-Fulcher-Tammann temperature dependence. Moreover, they indicate a moderate slowdown of both cation and anion dynamics with increasing alkyl length n. However, the relative diffusivities of the ionic species depend on the cation size, where cations are more mobile for n < 6 and anions for n > 6. Finally, we relate rotational and translational motions in the framework of the Stokes-Einstein-Debye (SED) approach. We find that the SED relation is obeyed for anion dynamics in all samples, while it breaks down for cation dynamics when n is increased. The origin of this SED breakdown is shown to differ fundamentally from that reported previously for conventional glass formers. We argue that an emergence of cation clusters causes a retardation of cation diffusion relative to cation reorientation upon cooling, i.e., the studied ionic liquids show a complex interplay of structural and dynamical properties.

From Local to Diffusive Dynamics in Polymer Electrolytes: NMR Studies on Coupling of Polymer and Ion Dynamics across Length and Time Scales

We combine 1H, 7Li, and 19F NMR methods to selectively investigate polymer, cation, and anion dynamics in polymer electrolytes on various length and time scales and over broad temperature ranges. B...

Unique Freezing Dynamics of Flexible Guest Cations in the First Molecular Postperovskite Ferroelectric: (C 5 H 13 NBr)[Mn(N(CN) 2 ) 3

Ferroelectricity is usually found in cubic and hexagonal perovskites consisting of corner- and faced-shared BX6 octahedra, respectively. In contrast, another important branch of perovskites, postpe...