Superionic State Research Articles

Theoretical Evaluation of the Persistence of Transverse Phonons across a Liquid-like Transition in Superionic Conductor KAg3Se2

A fundamental understanding of unusual atomic dynamics and collective modes in superionic conductors has been of great importance in improving renewable energy technologies such as thermoelectric energy conversion and electrolytes for rechargeable batteries. In this work, we have performed an in-depth theoretical study of the evolution of lattice dynamics across the superionic transition of KAg3Se2 by analyzing the thermally driven current fluctuations. We show that the diffuson-like phonons induced by the strong anharmonicity of the Ag sublattice dominate thermal transport below the superionic transition, resulting in nearly temperature-independent ultralow thermal conductivity in the normal ordered state. We find that the contributions of convection and conduction–convection interactions to thermal conductivity increase significantly due to the liquid-like flow of Ag atoms. We further demonstrate that the dynamic disorder is too slow to completely suppress the propagation of long-wavelength transverse phonons in the superionic state of KAg3Se2, while the propagation of Ag-dominated short-wavelength transverse phonons near the Brillouin zone boundary breaks down. These results provide new physical insights into the complex atomic dynamics of superionic conductors.

Ion dynamics into different pore size distributions in supercapacitors under compression

Compressing supercapacitor (SCs) electrode is essential for improving the energy storage characteristics and minimizing ions’ distance travel, faradaic reactions, and overall ohmic resistance. Studies comprising the ion dynamics in SC electrodes under compression are still rare. So, the ionic dynamics of five aqueous electrolytes in electrodes under compression were studied in this work for tracking electrochemical and structural changes under mechanical stress. A superionic state is formed when the electrode is compressed until the micropores match the dimensions with the electrolyte’s hydrated ion sizes, which increases the capacitance. If excessive compression is applied, the accessible pore regions decrease, and the capacitance drops. Hence, as the studied hydrated ions have different dimensions, the match between ion/pore sizes differs. To the LiOH and NaClO4 electrolytes, increasing the pressure from 60 to 120 and 100 PSI raised the capacitance from 13.5 to 35.2 F g−1 and 30.9 to 39.0 F g−1, respectively. So, the KOH electrolyte with the lowest and LiCl with the biggest combination of hydrated ion size have their point of maximum capacitance (39.5 and 36.7F g−1) achieved at 140 and 80 PSI, respectively. To LiCl and KCl electrolytes, overcompression causes a drop in capacitance higher than 23%.

Pressure Stabilized Lithium-Aluminum Compounds with Both Superconducting and Superionic Behaviors

Superconducting and superionic behaviors have physically intriguing dynamic properties of electrons and ions, respectively, both of which are conceptually important and have great potential for practical applications. Whether these two phenomena can appear in the same system is an interesting and important question. Here, using crystal structure predictions and first-principle calculations combined with machine learning, we identify several stable Li-Al compounds with electride behavior under high pressure, and we find that the electronic density of states of some of the compounds has characteristics of the two-dimensional electron gas. Among them, we estimate that Li_{6}Al at 150GPa has a superconducting transition temperature of around 29K and enters a superionic state at a high temperature and wide pressure range. The diffusion in Li_{6}Al is found to be affected by an electride and attributed to the atomic collective motion. Our results indicate that alkali metal alloys can be effective platforms to study the abundant physical properties and their manipulation with pressure and temperature.

Iron Hydride in the Earth's Inner Core and Its Geophysical Implications

AbstractHydrogen is potentially a key light element in the Earth's core. Determining the stability of iron hydride is essential for Earth's core mineralogy applications. We investigated the thermal stabilities of a range of Fe‐H binaries at core P‐T conditions. It is concluded that face‐centered cubic phase FeH is stable in the Earth's inner core. The high mobility of hydrogen in the Fe lattice suggests that hydrogen is transferred to a superionic state under the inner core conditions, where the superionic state transfer temperature of H in Fe fcc lattice is ∼500 K higher than that in hcp Fe system. The H concentration in the inner core is estimated to be ∼0.92 wt% to explain its density deficit, this value was further constrained to ∼0.21 wt% by matching the density jump at the inner‐core boundary. H alongside other light elements are required to account for the geophysical observations of the Earth's inner core.

Ionic conductivity of the Ag8GeSe6 compound

The temperature dependences of the conductivity and permittivity of the Ag8GeSe6 compound have been studied in direct and alternating electric fields. This phenomenon is associated with the process of charge accumulation at the boundary of the sample with ionic conductivity and the blocking electrode, and the formation of a double electric layer. The observed dielectric relaxation is associated with the transition of the crystal to the superionic state. It has been established that hopping conduction with a variable hopping length over localized states near the Fermi level takes place in an Ag8GeSe6 crystal.

Predicted superconductivity and superionic state in the electride Li5N under high pressure

Recently, electrides have received increasing attention due to their multifunctional properties as superconducting, catalytic, insulating, and electrode materials, with potential to offer other performance and possess novel physical states. This work uncovers that Li5N as an electride possess four novel physical states simultaneously: electride state, super-coordinated state, superconducting state, and superionic state. By obtaining high-pressure phase diagrams of the Li–N system at 150–350 GPa using a crystal structure search algorithm, we find that Li5N can remain stable as P6/mmm structure and has a 14-fold super-coordination number, as verified by Bader charge and electron localization function analysis. Its superconducting transition temperature reaches the highest at 150 GPa (T c = 48.97 K). Besides, Li5N exhibits the superionic state at 3000 K, in which N atoms act like solid, while some Li atoms flow like liquid. The above results are further verified at a macroscopic level by using deep learning potential molecular dynamics simulations.

Molecular Dynamics Study of Structural and Transport Properties of Silver Iodide Using Effective Charges.

The superionic conductor, solid state, and body-centered cubic structure, silver iodide at room temperature, has been studied via molecular dynamics simulations. The calculated results using pairwise Coulomb-Buckingham potential, zero pressure on the sample, a semi-rigid model system of 1000 Ag and 1000 I ions, () as a statistical ensemble, and an effective charge of for the pairs Ag-Ag and I-I, were found to be consistent with experimental data and one study using , different potential, and simulation software. For the pair Ag-I, there is a discrepancy due to the high silver ion diffusion. The calculated value of the diffusion constant of the silver ion is greater than iodide ion. The dynamic transport properties (mean square displacement, velocity autocorrelation function) results indicated typical behavior reported by other authors, using different potentials in their DM simulations for iodine and silver ions.

As-Li electrides under high pressure: Superconductivity, plastic, and superionic states

Inorganic electrides are a new class of compounds catering to the interest of scientists due to the multiple usages exhibited by interstitial electrons in the lattice. However, the influence of the shape and distribution of interstitial electrons on physical properties and new forms of physical states is still unknown. In this work, crystal structure search algorithms are employed to explore the possibility of forming unique electrides in the As-Li system, where interstitial electrons behave as one-dimensional (1D) electron chains (1D electride) in the $Pmmm$ phase of $\mathrm{As}{\mathrm{Li}}_{7}$ and transform into zero-dimensional (0D) electron clusters (0D electride) in the $P6/mmm$ phase at 80 GPa. The $P6/mmm$ phase has relatively high superconductivity at 150 GPa $({T}_{c}=38.4\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ compared to classical electrides, even at moderate pressure with ${T}_{c}=16.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, a Dirac cone in the band has been observed, expanding the sources of Dirac materials. The survival of $\mathrm{As}{\mathrm{Li}}_{7}$ at room temperature is confirmed by molecular dynamics simulation at 300 K. At 1000 K, the As atoms in the system act like a solid, while a portion of the Li atoms cycle around the As atoms, and another portion of the Li atoms flow freely like a liquid, showing the unusual physical phenomenon of the coexistence of the plastic and superionic states. This suggests that the superionic and plastic states cannot only be found in hydrides but also in the electride. Our results indicate that superconducting electride $\mathrm{As}{\mathrm{Li}}_{7}$ with superionic and plastic states can exist in Earth's interior.

Extending the temperature range of high thermoelectric properties of cuprous sulfide materials

Cu-rich cuprous sulfide materials enter the superionic state after high-temperature phase transition and obtain high ZT. In order to improve the average ZT of cuprous sulfide materials, it is necessary to reduce the high-temperature phase transition temperature and extend the temperature range of high ZT. In this study, the sintered Se-alloyed cuprous sulfide blocks were heat treated to vaporize anions, so as to control the relative Cu content of the material. It is found that reducing the Cu content can make the cuprous sulfide material undergo superionic phase transition at lower temperature, so as to extend the high ZT temperature range of the material to the low temperature region and improve the average ZT. Paradoxically, the increase of Cu vacancies will inevitably increase the carrier concentration and carrier thermal conductivity, thereby increasing the total thermal conductivity of the material and reducing ZT. Therefore, only by reasonably controlling the Cu content can we achieve the coordination of low carrier concentration and low superionic phase transition temperature, and improve the average ZT of materials. Based on the experimental results, it is proposed that the effect of increasing the average ZT of cuprous sulfide only by controlling the Cu content is limited. The potential strategies of simultaneously reducing the superionic phase transition temperature and carrier concentration of Cu-rich cuprous sulfide materials to improve their average ZT are prospected.

Ultralow Melting Temperature of High-Pressure Face-Centered Cubic Superionic Ice.

Superionic ice with oxygen in a face-centered cubic (fcc) sublattice is ascribed to the origin of magnetic fields of Uranus and Neptune, since the melting temperature (Tm) of fcc-superionic ice is believed to be higher than the isentropes of ice giants. However, precisely measuring the fcc-superionic phase experimentally remains a difficult task. The majority of the systematic investigations of its Tm were performed using perfect oxygen fcc-sublattice computations, which could result in superheating and overestimation of Tm. On the basis of the ab initio molecular dynamics method and the model with H2O vacancy, we avoid superheating and obtain a much lower Tm than previous reports, indicating that fcc-superionic ice cannot exist in the interiors of Uranus and Neptune. Further simulations with the two-phase method justify the conclusion. The results suggest that superheating should be seriously treated when simulating the phase diagram of other hydrogen-related superionic states, which are widely used to understand the properties of ice giants, Earth, and Venus.

Melting curve and transport properties of ammonia ice up to the deep mantle conditions of Uranus and Neptune

Motivated by the poor knowledge of the implication of planetary ice on the internal convection and anomalous magnetic field of ice giants, we perform extensive first-principles molecular dynamics simulation on ammonia ice to investigate its thermophysical behaviors at high temperature ($T$) and pressure ($P$) conditions relevant to the deep mantle of Neptune and Uranus. The melting curve, transport properties, and sound velocity of ammonia up to 350 GPa and 5500 K, spanning from fluid mixtures to a highly compressed superionic regime, are determined. A first-order phase transition of ammonia from the plastic state to the superionic state is observed explicitly, which is associated with the reduction of entropy. In contrast to previous predictions, the melting curve elucidates the existence of superionic ammonia in deep planetary interiors. Inspection of the reduction for transport properties and sound velocity along the isentrope of ice giants further evidences that superionic ammonia can sufficiently sustain the planetary dynamo mechanism and may contribute to the internal stratification which is responsible for the generation of the multipolar magnetic field. Finally, a comprehensive phase diagram of ammonia extended to a higher T-P regime is constructed, which provides a clear picture for studying the fundamental behavior and phase transition of ammonia in deep planets, and enhances our understanding of the interior structure and thermal convection of these planetary systems.

Superionic states formation in group III oxides irradiated with ultrafast lasers

After ultrafast laser irradiation, a target enters a poorly explored regime where physics of a solid state overlaps with plasma physics and chemistry, creating an unusual synergy—a warm dense matter state (WDM). We study theoretically the WDM kinetics and chemistry in a number of group III-metal oxides with highly excited electronic system. We employ density functional theory to investigate a possibility of nonthermal transition of the materials into a superionic state under these conditions. Atomic and electronic properties of the materials are analyzed during the transitions to acquire insights into physical mechanisms guiding such transformations.

Temperature-dependent potential for the molecular dynamics of the superionic conductor β-PbF2

Molecular dynamic (MD) calculations were performed to investigate the thermodynamic and structural properties of lead fluoride (PbF2) by using a proposed inter-ionic temperature-dependent potential. This potential allows calculating with high precision the linear thermal expansivity and the lattice parameter as a temperature function. In addition, the potential can be represented as a sum of two contributions, a temperature-independent potential added to another temperature-dependent potential, considered last as a correction justified by the one-dimensional Newtonian quantum equation. Two fitting regions were considered, the first region from 300 to 700 K and the other one from 700 to 900 K. These regions arise naturally due to the smooth and continuous transition that PbF2 undergoes until it reaches the superionic state and, allows us to model with high precision the anomaly in the dependence of the lattice parameter with the temperature of this material, a feature that until now under the molecular dynamic method has not been studied. These results are all in good agreement with the experimental measurements.

Superionic iron alloys and their seismic velocities in Earth’s inner core

Earth's inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition4-19. Light elements have a substantial influence on the seismic velocities4-13, the melting temperatures14-17 and the thermal conductivities18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be ina superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC21. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.

Effect of iso- and heterovalent cation substitutions on the superionic Faraday transition in fluorite-type modification β-PbF-=SUB=-2-=/SUB=-

The influence of isovalent substitutions of Pb2+-> Cd2+ and heterovalent substitutions of Pb2+-> Sc3+ on a superionic Faraday transition in Pb1-xCdxF2 (x=0.33) and Pb1-xScxF2+x (x=0.1) solid solutions based on the fluorite-type modification β-PbF2 with sp. gr. Fm 3 m has been studied. The Faraday phase transition can be characterized by the temperature Ttrλ corresponding to maximum on the heat capacity curve and temperature Ttrα corresponding to the beginning of the structural disorder anion sublattice. Both of these temperatures are found on the temperature conductivity dependence sigmadc(T) of β-PbF2, Pb0.67Cd0.33F2 and Pb0.9Sc0.1F2.1 crystals. The values of Ttrλ and Ttrα in solid solutions compared with β-PbF2 (Ttrλ=715±10 K, Ttrα = 597±12 K) decrease by 100-110 and 30-45 K for Pb0.67Cd0.33F2 and Pb0.9Sc0.1F2.1, respectively. Decreasing of temperature Ttrλ leads to an increase in temperature interval of existence of the superionic state. For T>Ttrλ the anionic conductivity of fluorite-type Pb0.67Cd0.33F2, Pb0.9Sc0.1F2.1, and β-PbF2 crystals reaches anomalously high values of sigmadc = 1-2 S/cm (873 K) at an ion transfer activation enthalpy equals to Hsigma~0.3 eV. Keywords: phase transitions, fluorides, fluorite structure, ionic conductivity, solid solutions.

Влияние изо- и гетеровалентных замещений катионов на суперионный фарадеевский переход во флюоритовой модификации β-PbF-=SUB=-2-=/SUB=-

The influence of isovalent substitutions of Pb2+ by Cd2+ and heterovalent substitutions of Pb2+ by Sc3+ on a superionic Faraday transition in Pb1 – xCdxF2 (x = 0.33) and Pb1 – x ScxF2+x (x = 0.1) solid solutions based on the fluorite-type modification β-PbF2 with sp. gr. Fm¯3m has been studied. The Faraday phase transition can be characterized by the temperature Tλ corresponding to maximum on the heat capacity curve and temperature Tα corresponding to the beginning of the structural disorder anion sublattice. Both of these temperatures are found on the temperature conductivity dependence σdc(T ) of β-PbF2, Pb0.67Cd0.33F2 and Pb0.9Sc0.1F2.1 crystals. The values of Tλ and Tα in solid solutions compared with β-PbF2 matrix (Tλ = 715 ± 10K, Tα = 597 ± 12 K) decrease by 100–110 and 30–45 K for Pb0.67Cd0.33F2 and Pb0.9Sc0.1F2.1, respectively. Decreasing of temperature Tλ leads to an increase in temperature interval of existence of the superionic state. For T > Tλ the anionic conductivity of fluorite-type Pb0.67Cd0.33F2, Pb0.9Sc0.1F2.1, and β-PbF2 crystals reaches anomalously high values of σdc = 1–2 S/cm (873 K) at an ion transfer activation enthalpy equals to Hσ = 0.3 eV.

Radiation-stimulated phase transition to superionic state of the crystal TlSe

The temperature-frequency dependences of: electrical conductivity (sigma(μ,T)), the real and imaginary parts of the complex permittivity (ε'(ν,T)) and (ε''(ν,T)), the complex impedance of the TlSe compound when exposed to various doses of gamma radiation. The process of the occurrence of concentration polarization at the boundary of the ion conductor blocking contact is investigated. The temperature-frequency-dose interval of the occurrence of ionic conductivity and the Warburg impedance has been established and a radiationally stimulated phase transition to the superionic state of the TlSe crystal has been detected. Keywords: complex impedance, blocking contact, of ionic conductivity.

Поляризационные процессы в пленках йодида серебра вблизи температуры суперионного фазового перехода

The article reports the results of dielectric spectroscopy analysis exploring polarization processes in silver iodide nanocrystalline films in the region of thermal superionic phase transition. It also provides calculations of the system’s relaxation and energy parameters. The abrupt change of parameters at the transition point is demonstrated by activation energy of semiconductor conduction and commit phases, indicator of the degree of frequency dependence of conductivity, and the maximum value of the function of the tangent of the dielectric loss angle. The article proposes a model of the micro-mechanism of the thermal phase transition from a semiconductor to a superionic state. It is shown that, along with the superionic phase transition, films undergo a percolation phase transition with the formation of a two-dimensional percolation cluster.

Metal-Ion Oligomerization Inside Electrified Carbon Micropores and its Effect on Capacitive Charge Storage.

Ion adsorption inside electrified carbon micropores is pivotal for the operation of supercapacitors. Depending on the electrolyte, two main mechanisms have been identified so far, the desolvation of ions in solvents and the formation of superionic states in ionic liquids. Here, it is shown that upon confinement inside negatively charged micropores, transition-metal cations dissolved in water associate to form oligomer species. They are identified using in situ X-ray absorption spectroscopy. The cations associate one with each other via hydroxo bridging, forming ionic oligomers under the synergic effect of spatial confinement and Coulombic screening. The oligomers display sluggish dissociation kinetics and accumulate upon cycling, which leads to supercapacitor capacitance fading. They may be dissolved by applying a positive potential, so an intermittent reverse cycling strategy is proposed to periodically evacuate micropores and revivify the capacitance. These results reveal new insights into ion adsorption and structural evolution with their effects on the electrochemical performance, providing guidelines for designing advanced supercapacitors.

Structural Aspects of the Superionic Transition in AX2 Compounds With the Fluorite Structure.

Some AX 2 binary compounds with the fluorite structure (space group ) are well-known examples of materials exhibiting transitions to ionic superconducting phases at high temperatures below their melting points. Such superionic states have been described as either highly defective crystals or part-crystal, part-liquid states where the A ions retain their crystalline order whilst the X ions undergo partial melting. However, no detailed description of the structure of these phases exists. We present here the results of our investigation of the structural changes that occur during these transitions and the structural characteristics of the resulting superionic materials. This work is based on atomic-scale molecular dynamics modelling methods as well as computational diffraction techniques. We employed a set of empirical potentials representing several compounds with the fluorite structure to investigate any potential-dependent effect. We show the importance of small-scale structure changes, with some local environments showing a hexagonal symmetry similar to what is seen in the scrutinyite structure that has been documented for example in UO2.