Anion Reorientation Research Articles

Paradigms of frustration in superionic solid electrolytes.

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion–cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies.This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

First-principles study of closo-dodecaborates M2B12H12 (M = Li, Na, K) as solid-state electrolyte materials.

Closo-dodecaborates M2B12H12 are considered among the potential candidates for solid-state electrolyte materials due to their high ionic conductivities. It has been demonstrated that the reorientation of the icosahedral anion B12H122- plays a key role in high cation motion. However, this category of BnHn materials is still not well established with respect to their structural, thermodynamic and diffusion properties. In the present work, the electronic, vibrational and thermodynamic properties of M2B12H12 (M = Li, Na, K) structures are reported using first-principles calculations. The results of structural and electronic properties show that these structures have an insulator character with a large band gap of 5.75, 5.63 and 5.59 eV, respectively, for Li2B12H12, Na2B12H12 and K2B12H12. The thermodynamic stabilities of these systems are confirmed by their phonon calculation results. The primary quantities, such as heat capacity, vibrational entropy and volume variation at finite temperatures, are determined using the quasi-harmonic approximation in order to provide an input for the Gibbs free energy assessment. The calculated enthalpy of formation of the Li2B12H12 structure at 0 K and the proposed one at 300 K are found to be -127.31 and -740.44 kJ mol-1 per H2, respectively. The migration energy barrier of various cations in each system is calculated to be 0.7 (Li+), 1.16 (Na+) and 1.25 eV (K+), where the lowest energy barrier corresponds to the lithium ion migration in Li2B12H12. Additionally, the molecular dynamics simulation of M2B12H12 (M = Li, Na, K) structures demonstrated that these structures are stable above room temperature, except for the Li2B12H12 structure at 600 K, where the most stable is Na2B12H12. Finally, the temperature effect on icosahedral anion reorientation in each structure is elucidated as a function of temperature and cation type.

Differential guest location by host dynamics enhances propylene/propane separation in a metal-organic framework

Energy-efficient approaches to propylene/propane separation such as molecular sieving are of considerable importance for the petrochemical industry. The metal organic framework NbOFFIVE-1-Ni adsorbs propylene but not propane at room temperature and atmospheric pressure, whereas the isostructural SIFSIX-3-Ni does not exclude propane under the same conditions. The static dimensions of the pore openings of both materials are too small to admit either guest, signalling the importance of host dynamics for guest entrance to and transport through the channels. We use ab initio calculations together with crystallographic and adsorption data to show that the dynamics of the two framework-forming units, polyatomic anions and pyrazines, govern both diffusion and separation. The guest diffusion occurs by opening of the flexible window formed by four pyrazines. In NbOFFIVE-1-Ni, (NbOF5)2− anion reorientation locates propane away from the window, which enhances propylene/propane separation.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Effect of anion reorientation on proton mobility in the solid acids family CsHyXO4 (X = S, P, Se, y = 1, 2) from ab initio molecular dynamics simulations.

The high temperature phases of the solid acids CsHSeO4, CsHSO4 and CsH2PO4 show extraordinary high proton conductivities, while the low temperature phases do not conduct protons at all. We systematically investigate proton dynamics in the low and high temperature phases of these compounds by means of ab initio molecular dynamics simulations in order to develop a general picture of the proton transfer mechanism. For all of these compounds, proton conduction follows a Grotthuss mechanism via a combined proton transfer and subsequent structural reorientation of the environment. We demonstrate that the drastically reduced conductivity of the low temperature phases is caused by a highly ordered, rigid hydrogen bond network, while efficient long range proton transfer in the high temperature phases is enabled by the interplay of high proton transfer rates and frequent anion reorientation. Furthermore, we present a simple descriptor for the quantitative prediction of the diffusion coefficient within the solid acids family. As a side result, we show that the rate of the most elementary proton hopping reaction depends on the heavy-atom configuration of the nearest atoms in a ubiquitous manner, and is in turn almost independent from the global nature of the compound, i.e. whether it is organic or inorganic, ordered or disordered.

Phase transitions associated with conformational changes of ligand and anion reorientation trigger normal and reverse spin crossover

Phase transitions associated with conformational changes of ligand and anion reorientation trigger normal and reverse spin crossover

Carbon Incorporation and Anion Dynamics as Synergistic Drivers for Ultrafast Diffusion in Superionic LiCB11H12 and NaCB11H12

AbstractThe disordered phases of LiCB11H12 and NaCB11H12 possess superb superionic conductivities that make them suitable as solid electrolytes. In these materials, cation diffusion correlates with high orientational mobilities of the CB11H12− anions; however, the precise relationship has yet to be demonstrated. In this work, ab initio molecular dynamics and quasielastic neutron scattering are combined to probe anion reorientations and their mechanistic connection to cation mobility over a range of timescales and temperatures. It is found that anions do not rotate freely, but rather transition rapidly between orientations defined by the cation sublattice symmetry. The symmetry‐breaking carbon atom in CB11H12− also plays a critical role by perturbing the energy landscape along the instantaneous orientation of the anion dipole, which couples fluctuations in the cation probability density directly to the anion motion. Anion reorientation rates exceed 3 × 1010 s−1, suggesting the underlying energy landscape fluctuates dynamically on diffusion‐relevant timescales. Furthermore, carbon is found to modify the orientational preferences of the anions and aid rotational mobility, creating additional symmetry incompatibilities that inhibit ordering. The results suggest that synergy between the anion reorientational dynamics and the carbon‐modified cation–anion interaction accounts for the higher ionic conductivity in CB11H12− salts compared with B12H122−.

Ion transport in salts orientationally disordered in anionic sublattice

A short review of transport properties of orientationally disordered phases (ODP) of alkali nitrates and perchlorates is presented. Analysis of the data shows that the conductivity, the activation energy and the pre-exponential factor do not change trivially with the increase of the cation size. It may be explained by two mechanisms: a classic one and a cooperative mechanism involving reorientation of anions, the latter becomes dominant in systems with a large free volume. Among nitrates ODP RbNO 3-III is the most conducting and its stability range may be controlled via cationic and anionic substitutions. The mechanism of the ionic transport in RbNO 3 seems to be different from those in perchlorates. According to the results of MD simulation conductivity is limited by the defect formation rather than the cation migration which proceeds via vacancy mechanism. Correlation times for the anion reorientation process correlate with ionic conductivity values thus suggesting that the cation diffusion and anion reorientation are closely interrelated.

Crystalline Cation Conductors with Rotational Anion Disorder: Results of Quasielastic Neutron Scattering Experiments on Orthophosphates

Abstract Results and interpretations of several quasielastic neutron scattering measurements on samples of the isostructural high-temperature phases of Ag3PO4 and Na3PO4, prominent members of the group of fast cation conducting plastic phases are reported.Time-of-flight experiments reveal details on the phosphate reoriention in Ag3PO4: We find isotropic rotational diffusion. A consistent interpretation of the data suggests that silver ions are involved in the fast anion dynamics, a behaviour which is in line with the displacement of phosphorus detected in earlier neutron diffraction experiments. The reorientation activation energy of 0.17 eV is in the typical range found also in other ion conducting rotor phases.Na3PO4 was examined in time-of-flight and backscattering experiments, differing considerably in the time scale of the detected molecular dynamics. Anion reorientation, observed on the time scale of picoseconds and activated with 0.18 eV, appears as a predominantly circular diffusion with one P–O axis fixed. Our results suggest the involvement of sodium ions in the anion reorientation, i.e., a correlated motion of both types of ions in the picosecond regime.The much slower sodium jump diffusion in Na3PO4 was found to be thermally activated with 0.74 eV. Moreover, we find clear evidence for the predominance of jumps between tetrahedrally co-ordinated cation sites.Quasielastic neutron scattering reveals a wealth of dynamic details on the two high-temperature phases of Na3PO4 and Ag3PO4 which exhibit considerable differences (e.g., in the geometric details of anion dynamics) in spite of their structural similarities. Both plastic phases, however, give evidence of the involvement of cations in the anion dynamics, i.e., in a correlated reorientational motion of cations and anions on the picosecond time scale.

Conductivity of alkali perchlorates at high temperatures

Ionic conductivity of high-temperature phases of alkaline metal perchlorates is studied. It is found that the conductivity passes through a minimum with increasing radius of cation, and KClO 4 exhibits the lowest conductivity. This is explained by a decrease in the relative size of conduction channel, which hampers the cation transfer, and an increase in the relative free volume. The free-volume increase promotes the perchlorate anion reorientation and reduces the activation energy for ion transfer by the “paddle-wheel” mechanism; as a result, the conductivity increases.

Conductivity of alkaline metal perchlorates

Ionic conductivity of high-temperature phases of alkaline metal perchlorates is studied. It is found that the conductivity passes through a minimum with increasing radius of cation, and KClO4 exhibits the lowest conductivity. This is explained by a decrease in the relative size of conduction channel, which hampers the cation transfer, and an increase in the relative free volume. The free-volume increase promotes the perchlorate anion reorientation and reduces the activation energy for ion transfer by the “paddle-wheel” mechanism; as a result, the conductivity increases.

Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics

Proton dynamics in polymer electrolyte membranes are multifaceted processes, and the relative contributions of various mechanisms can be difficult to distinguish. Judicious choices of model systems can aid in understanding the critical steps. In this study, we characterize anion dynamics in a series of benzimidazole-alkyl phosphonate salts, and compare those dynamics to a membrane prototype, built on a decane backbone. The series of salts are characterized, using high resolution (1)H solid-state magic angle spinning (MAS) NMR, DQ MAS NMR, and (31)P centreband-only detection of exchange (CODEX) NMR spectroscopy, to determine the influence of the nature of the alkyl group on the rates and geometries of anion dynamics, and overall proton exchange processes. The alkyl group is shown to slow the correlation times for anion reorientation, when compared at ambient temperature. However, it is also apparent that the lowered lattice energy of the salt lowers the activation energy and allows good dynamics at intermediate temperatures in both the benzimidazolium ethylphosphonate and in the HBr adduct of 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa).

NMR studies of cation transport in the crystalline ion conductors MCF 3SO 3 (M = Li, Na) and Li 7TaO 6

In this contribution we present studies on the mechanism of ion transport in crystalline solid electrolytes employing a range of different solid state nuclear magnetic resonance (NMR) experiments. The first part is devoted to the elucidation of a possible correlation of cation transport and anion reorientation in the dynamically disordered rotor phases of alkali trifluoromethane sulfonates MCF 3SO 3 (M = Li, Na) employing 7Li, 13C, 17O and 23Na NMR line shape analysis, whereas the second part focuses on the tracking of cation diffusion pathways in the hexaoxometalate Li 7TaO 6 utilizing 6Li 1D and 2D exchange MAS NMR approaches.

Anion-Promoted Cation Motion and Conduction in Zeolites

The motion of sodium cations in sodalite and cancrinite has been investigated by force field calculations, solid-state NMR, and impedance spectroscopy. Special emphasis is dedicated to the influence of anions on sodium mobilities. Local cation motion is promoted when they interact with anions. However, not all systems with high local mobilities exhibit good ion conductivities, as cooperativity of the motion appears to be an important factor, as well. The activation barrier for local sodium motion (calculations) and long-range transport (dc conductivities) is lowered in sodalite when halogenide anions, Cl(-), Br(-), or I(-), are present. The activation barriers increase with increasing size of the anion and decreasing coordination in the transition state. On the basis of (23)Na solid-state NMR data, all the sodium ions in the dense sodalite structure are rather rigid up to 470 K. All the cations in chromate sodalite, and Na(+) in the small cancrinite epsilon-cages without anion interactions, show a restricted local motion at higher temperatures. There is a selective high local motion of Na(+) in the neighborhood of chromate anions in the more open channel system of cancrinite. These results suggest that sodium migration can be enhanced, at least locally, in open channel systems by anion interactions. A dynamics coupling between anion reorientation and cation mobility was not observed.

Sodium Ion Conduction in Plastic Phases: Dynamic Coupling of Cations and Anions in the Picosecond Range

Results of simple computer simulations and model calculations for ion conducting rotor phases are compared to quasi-elastic neutron scattering data from solid solutions of sodium orthophosphate and sodium sulphate, xNa2SO4⋅(1 − x)Na3PO4. These materials are not only sodium fast-ion conductors in their high-temperature cubic phases but also show considerable dynamic reorientation disorder of their tetrahedral anions. At an elastic energy resolution of about 100 μeV, neutron spectrometry monitored oxygen scattering due to anion reorientation which occurs on the picosecond time scale. This thermally activated process exhibits activation energies between 0.184 eV (x = 0.0) and 0.052 eV (x = 0.5). Analysis of the quasielastic intensities as a function of scattering vector Q gives clear evidence of the involvement of cations in the anion reorientation. Increasing the elastic resolution to about 1 μeV full width at half-measure (FWHM) (thereby shifting the dynamic window to the nanosecond scale) allowed examination of sodium diffusion in xNa2SO4⋅(1 − x)Na3PO4. This process consists predominantly of thermally activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64 eV for x = 0.0 to 0.30 eV for x = 0.5.

Cation Mobility and Anion Reorientation in Sodium Trifluoromethyl Sulfonate

Cation Mobility and Anion Reorientation in Sodium Trifluoromethyl Sulfonate

Correlated motion of anions and cations in fast cation conducting rotor phases

A set of quasielastic neutron scattering experiments on solid solutions of sodium orthophosphate and sodium sulfate is reported. These materials are not only fast-ion conductors in their high-temperature cubic phases but also show considerable dynamic reorientation disorder of their tetrahedral anions. High resolution backscattering experiments were used to examine sodium diffusion in xNa 2SO 4·(1− x)Na 3PO 4, which was found to be dominated by thermally activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64 eV for x=0.0 to 0.30 eV for x=0.5. Time-of-flight scattering proved to be mainly sensitive to oxygen scattering and thus to anion reorientation. This thermally activated motion exhibits activation energies between 0.184 eV ( x=0.0) and 0.052 eV ( x=0.5). Analysis of the quasielastic intensities as a function of scattering vector Q gives clear evidence of the involvement of cations in the anion motion.

Anion reorientation in anhydrous Na3PO4 during the phase transformation

The transport phenomena in alkali-metal super ionic conductors based on Na3PO4 structure are of particular interest due to their potential technological application. Differential thermal analysis (DTA), differential scanning calorimetry (DSC), Raman spectroscopy and temperature dependent electrical conductivity measurements were carried out to probe the nature of the phase transformation involved in anhydrous Na3PO4. The changes in spectral profile of the v3 mode and the line width of v1 mode of PO 4 3− observed in the temperature interval from 331 to 345 °C revealed the high degree of disorder nature during the α-γNa3PO4 phase transformation.

Sodium Ion Conduction in Plastic Phases: Dynamic Coupling of Cations and Anions in the Picosecond Range

ABSTRACTResults of simple computer simulations and model calculations for ion conducting rotor phases are compared to quasi-elastic neutron scattering data from solid solutions of sodium orthophosphate and sodium sulphate, xNa2SO4 · (1 — x)Na3PO4. These materials are not only sodium fast-ion conductors in their high-temperature cubic phases but also show considerable dynamic reorientation disorder of their tetrahedral anions.At an elastic energy resolution of about 100 μeV, neutron spectrometry monitored oxygen scattering due to anion reorientation which occurs on the picosecond time scale. This thermally activated process exhibits activation energies between 0.184 eV (x = 0. 0) and 0.052 eV (x = 0.5). Analysis of the quasielastic intensities as a function of scattering vector Q gives clear evidence of the involvement of cations in the anion reorientation.Increasing the elastic resolution to about 1 μeV FWHM (thereby shifting the dynamic window to the nanosecond scale) allowed to examine sodium diffusion in x Na2SO4 · (1 — x)Na3PO4. This process consists predominantly of thermally-activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64 eV for x = 0.0 to 0.30 eV for x = 0.5.

Ion dynamics in solid solutions of sodium phosphate and sodium sulfate

Time-of-flight quasielastic neutron scattering experiments have been performed on solid solutions of Na2SO4 and Na3PO4 in order to examine the reorientational anion motion in the picosecond range. The obtained data were analysed on the basis of the Q-dependent quasielastic intensity. Comparing the predictions of different models for the coherent quasielastic scattering of individual rotating units with the experimental results gives a clear indication that the observed motion is performed by three oxygen atoms which rotate on a circle, while the fourth oxygen atom, at least on the time scale defined by the experiment, remains fixed. The quasielastic intensity in the low-Q regime is well explained by the involvement of cations in the anion reorientation. Quantitative agreement between model predictions and experimental data was obtained under the assumption that only the phosphate (and not the sulfate) ion rotation is observed in our experiment. The results strongly support the view of a dynamic coupling between cations and anions in the Na2SO4–Na3PO4 system.