Superionic Phase Research Articles

Emergence of superconductivity and superionicity in the electride [formula omitted]La under extreme conditions

Superconducting electrides, a unique type of conventional superconductors, have recently attracted considerable attention. Inspired by the discovery of high critical temperature (Tc) in lanthanum hydrides and the feasibility of forming electrides in lithium-based compounds, we have conducted a systematic investigation the phase diagram and superconductivity of Li-La alloys under high pressure. Using the crystal structure prediction method and first-principles calculations, we uncovered three pressure-stabilized compounds with stoichiometries LiLa3, LiLa2 and Li6La. All three compounds exhibit metallic behavior with electrons transferred from Li to La atoms. Notably, Li6La emerges as a prototypical superconducting electride with Tc of 11.6 K at 350 GPa, which increases to 16.3 K as pressure decreases to 200 GPa. Electron–phonon analysis reveal that the enhancement of superconductivity mainly originates from the coupling between the increased interstitial quasiatoms (ISQs) and low-frequencies phonons. Additionally, akin to lanthanum hydrides, Li6La is predicted to enter a superionic phase characterized by high lithium diffusion coefficients under high pressure and temperature. Our findings indicate the coexistence of superconductivity and superionicity in the electride Li6La, which may prompt further experimental investigations.

Phase transition kinetics of superionic H2O ice phases revealed by Megahertz X-ray free-electron laser-heating experiments

H2O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H2O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H2O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers.

Superionic iron hydride shapes ultralow-velocity zones at Earth’s core–mantle boundary

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary.

Amorphous-Like Ultralow Thermal Transport in Crystalline Argyrodite Cu7PS6.

Due to their amorphous-like ultralow lattice thermal conductivity both below and above the superionic phase transition, crystalline Cu- and Ag-based superionic argyrodites have garnered widespread attention as promising thermoelectric materials. However, despite their intriguing properties, quantifying their lattice thermal conductivities and a comprehensive understanding of the microscopic dynamics that drive these extraordinary properties are still lacking. Here, an integrated experimental and theoretical approach is adopted to reveal the presence of Cu-dominated low-energy optical phonons in the Cu-based argyrodite Cu7PS6. These phonons yield strong acoustic-optical phonon scattering through avoided crossing, enabling ultralow lattice thermal conductivity. The Unified Theory of thermal transport is employed to analyze heat conduction and successfully reproduce the experimental amorphous-like ultralow lattice thermal conductivities, ranging from 0.43 to 0.58Wm-1 K-1, in the temperature range of 100-400 K. The study reveals that the amorphous-like ultralow thermal conductivity of Cu7PS6 stems from a significantly dominant wave-like conduction mechanism. Moreover, the simulations elucidate the wave-like thermal transport mainly results from the contribution of Cu-associated low-energy overlapping optical phonons. This study highlights the crucial role of low-energy and overlapping optical modes in facilitating amorphous-like ultralow thermal transport, providing a thorough understanding of the underlying complex dynamics of argyrodites.

Phase transitions, baro- and piezocaloric effects in single crystal and ceramics of ferroelectric NH4HSeO4

A study of heat capacity, thermal dilatation and sensitivity to hydrostatic and uniaxial pressure was carried out on single-crystal and ceramic samples of NH4HSeO4. The main parameters of low-temperature successive phase transitions B2 (T1) ↔ incommensurate IC (T2) ↔ ferroelectric P1 (T3) ↔ non-ferroelectric did not depend on the type of samples. The behavior of the volumetric strain and the results of direct measurements of T3(p) contributed to the resolution of the longstanding problem associated with the ambiguity of the sign of the corresponding volumetric baric coefficient. The role of thermal expansion anisotropy in the formation of the piezocaloric effect (PCE) near the ferroelectric phase transition at T3 has been studied. Due to the strong difference in the linear baric coefficients, the main contribution to the barocaloric effect (BCE) comes from the inverse intensive and extensive PCE associated with the a-axis. Compared to a single crystal, ceramics demonstrate lower BCE values, which, however, exist in a wider temperature range, which leads to close values of integral caloric parameters. The strong decrease in both BCE and PCE at low-temperature transformations in NH4HSeO4 compared to the ferroelectrics NH4HSO4 and NH4NH4SO4 is associated with a small change in entropy during three low-temperature phase transitions, ΣΔSi = 2.52 J/mol∙K, which is a consequence of a high degree of structural ordering in selenate as a result of a high-temperature transformation at T0 between the superionic and B2 phases, accompanied by a giant change in entropy, ΔS0≈Rln21.

Raman and IR spectra of water under graphene nanoconfinement at ambient and extreme pressure-temperature conditions: a first-principles study.

The nanoconfinement of water can result in dramatic differences in its physical and chemical properties compared to bulk water. However, a detailed molecular-level understanding of these properties is still lacking. Vibrational spectroscopy, such as Raman and infrared, is a popular experimental tool for studying the structure and dynamics of water, and is often complemented by atomistic simulations to interpret experimental spectra, but there have been few theoretical spectroscopy studies of nanoconfined water using first-principles methods at ambient conditions, let alone under extreme pressure-temperature conditions. Here, we compute the Raman and IR spectra of water nanoconfined by graphene at ambient and extreme pressure-temperature conditions using ab initio simulations. Our results revealed alterations in the Raman stretching and low-frequency bands due to the graphene confinement. We also found spectroscopic evidence indicating that nanoconfinement considerably changes the tetrahedral hydrogen bond network, which is typically found in bulk water. Furthermore, we observed an unusual bending band in the Raman spectrum at ∼10 GPa and 1000 K, which is attributed to the unique molecular structure of confined ionic water. Additionally, we found that at ∼20 GPa and 1000 K, confined water transformed into a superionic fluid, making it challenging to identify the IR stretching band. Finally, we computed the ionic conductivity of confined water in the ionic and superionic phases. Our results highlight the efficacy of Raman and IR spectroscopy in studying the structure and dynamics of nanoconfined water in a large pressure-temperature range. Our predicted Raman and IR spectra can serve as a valuable guide for future experiments.

Theoretical study of the structural and thermodynamic properties of U-He compounds under high pressure.

Uranium is considered as a very important nuclear energy material because of the huge amount of energy it releases. As the main product of the spontaneous decay of uranium, it is difficult for helium to react with uranium because of its chemical inertness. Therefore, bubbles will be formed inside uranium, which could greatly reduce the performance of uranium or cause safety problems. Additionally, nuclear materials are usually operated in an environment of high-temperature and high-pressure, so it is necessary to figure out the exact state of helium inside uranium under extreme conditions. Here, we explored the structural stability of the U-He system under high pressure and high temperature by using density functional theory calculations. Two metastable phases are found between 50 and 400 GPa: U4He with space group Fmmm and U6He with space group P1̄. Both are metallic and adopt layered structures. Electron localization function calculation combined with charge density difference analysis indicates that there are covalent bonds between U and U atoms in both Fmmm-U4He and P1̄-U6He. Regarding the elastic modulus of α-U, the addition of helium has certain influence on the mechanical properties of uranium. Besides, first-principles molecular dynamics simulations were carried out to study the dynamical behavior of Fmmm-U4He and P1̄-U6He at high-temperature. It was found that Fmmm-U4He and P1̄-U6He undergo one-dimensional superionic phase transitions at 150 GPa. Our study revealed the exotic structure of U-He compounds beyond the formation of bubbles under high-pressure and high-temperature, which might be relevant to the performance and safety issues of nuclear materials under extreme conditions.

AC Conductivity and Dielectric Constant of Fast Ion-Electronic Conductor AgI-C

The conductivity, dielectric constant and crystal structure of the superionic-electronic conductor AgI-porous carbon composites have been investigated at various temperatures and carbon concentrations (ϕ ≤ 10%). At a fixed temperature, both conductivity and dielectric constant exhibit anomalous variation with ϕ showing maxima of the values. We observed significant differences in the behavior between the superionic (α) phase and insulating (β) phase. By incorporating carbon, the conductivity increases by four times in the β phase and six times in the α phase whereas the dielectric constant increases by five times and eight times respectively. Unlike in the β phase, the conductivity of the α phase exhibits two distinct maxima whereas the dielectric constant exhibits different features of the profiles in different frequency regions in the α phase. Also, the peak value of these properties exhibits power-law dependence on the frequency in both phases but with different values for the exponents.

Superionic Phase Transition of Copper(I) Sulfide and Its Implication for Purported Superconductivity of LK-99

Superionic Phase Transition of Copper(I) Sulfide and Its Implication for Purported Superconductivity of LK-99

Prediction of superionic state in LiH2 at conditions enroute to nuclear fusion

Hydrogen and lithium, along with their compounds, are crucial materials for nuclear fusion research. High-pressure studies have revealed intricate structural transitions in all these materials. However, research on lithium hydrides beyond LiH has mostly focused on the low-temperature regime. Here, we use density functional theory and ab initio molecular dynamics simulations to investigate the behavior of LiH2, a hydrogen-rich compound, near its melting point. Our study is particularly relevant to the low-pressure region of the compression pathway of lithium hydrides toward fusion. We discovered a premelting superionic phase transition in LiH2 that has significant implications for its mass transportation, elastic properties, and sound velocity. The theoretical boundary for the superionic transition and melting temperature was then determined. In contrast, we also found that the primary compound of lithium hydrides, LiH, does not exhibit a superionic transition. These findings have important implications for optimizing the compression path to achieve the ignition condition in inertial confinement fusion research, especially when lithium tritium-deuteride (LiTD) is used as the fuel.

Nature of the Superionic Phase Transition of Lithium Nitride from Machine Learning Force Fields

Nature of the Superionic Phase Transition of Lithium Nitride from Machine Learning Force Fields

Structural, morphological and photocatalytic properties of nanostructured TiO2/AgI photocatalyst

Nanostructured TiO2/AgI photocatalyst under the action of ultraviolet or visible electromagnetic radiation effectively neutralizes organic pollutants in the aqueous environment. It is a nanostructure in which micro- and small mesopores of anatase TiO2 are filled with silver iodide in the superionic state. The content of the α-AgI ion-conducting phase in the volume of ТіО2 pores can be ~20 wt %.
 To obtain a photocatalyst, titanium dioxide is synthesized by the sol-gel method, using a titanium aquacomplex solution [Ti(OH2)6]3+•3Cl- and a Na2CО3 modifier additive as a precursor. The modifying additive during synthesis ensures the fixation of =О2СО carbonate groups on the surface of oxide material particles. The presence of these groups leads to an increase in both the pore volume and the specific surface area of ТіО2. The specific surface area of carbonized titanium dioxide is 368 m2•g-1, the pore volume is 0.28 cm3•g-1, and their size is 0.9-4.5 nm.
 To fill the micro- and small mesopores of TiO2 with the superionic α-AgI phase, Ag+ cations are first adsorbed from the AgNO3 solution on the titanium dioxide surface, and then the oxide material is contacted with the KI solution.
 Compared to the Evonik P25-TiO2 photocatalyst, the nanostructured TiO2/AgI photocatalyst demonstrates a significantly higher efficiency of photodegradation of organic dyes Congo Red and Methyl Orange in visible and ultraviolet radiation. The most active ТіО2/40AgI sample achieved complete degradation of the CR dye in 6 minutes of UV irradiation, while the efficiency of commercial Р25-TiO2 over the same time was only 42%.

Revisiting the melting curve of H2O by Brillouin spectroscopy to 54 GPa.

The melting curve of H2O was investigated up to 54GPa and ∼2000K by Brillouin scattering spectroscopy in a diamond anvil cell. A CO2 laser was used for heating the H2O sample directly in order to reduce the risk of chemical reactions. The melting was identified based on the appearance of the Brillouin peaks derived from the liquid phase. The longitudinal wave velocity (Vp) of the liquid phase along the melting curve exhibits a smooth increase with pressure. The melting temperature of H2O shows no kink previously reported but a monotonic increase between 26 and 54GPa. Present melting data suggest that the melting occurs from body-centered-cubic superionic phase in the pressure-temperature range investigated.

Superionicity of Hδ− in LaH10 superhydride

Recent computational studies have successfully predicted the dramatic uptake of hydrogen by metals under pressure leading to the formation of superhydrides, now ubiquitously observed. ${\mathrm{LaH}}_{10}$ exemplifies the properties of these novel H-rich compounds, some of which form a novel class of superconducting materials. We show here another remarkable property for superhydrides, namely, ${\text{H}}^{\ensuremath{\delta}\ensuremath{-}}$ superionicity. By means of ab initio molecular dynamics simulations in ${\mathrm{LaH}}_{10}$, an exceptionally high hydride $({\mathrm{H}}^{\ensuremath{\delta}\ensuremath{-}})$ diffusion coefficient is calculated at high temperature, with $D=1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.28em}{0ex}}\mathrm{cm} {}^{2}$/s at 170 GPa and 1500 K, corresponding to an ionic conductivity of $\ensuremath{\sigma}=0.9\phantom{\rule{0.16em}{0ex}}{(\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm})}^{\ensuremath{-}1}$ and so indicating a superionic phase. The superionic phase is surprisingly stable up to 2500 K and its melting temperature is remarkably high, similar to that of pure La. The connected path for the hydride ionic diffusion is disclosed, with the H sublattice keeping its clathrate structure. The conductivity properties of ${\mathrm{LaH}}_{10}$ are discussed in relation to the recently discovered family of compounds showing fast pure hydride ions transport.

Built-in superionic conductive phases enabling dendrite-free, long lifespan and high specific capacity composite lithium for stable solid-state lithium batteries

A composite lithium anode with built-in superionic conductive Li3N and LiNxOy phases can greatly promote the ionic diffusion capability of bulk lithium, simultaneously improve the wettability, and construct a robust ionic conductive interface.

Synergistic Approach Toward a Reproducible High zT in n-Type and p-Type Superionic Thermoelectric Ag2Te.

Recently, superionic thermoelectrics have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000 K in Cu2X (X: chalcogen atom) family despite a relatively low superionic transition temperature of ∼400 K. At such high temperatures, the liquid-like flow of the metal ions results in material's degradation. Here, we present thermoelectric properties of superionic Ag2Te synthesized by various methods. The sintered Ag2Te samples are shown to exhibit an unpredictable behavior with respect to the sign of thermopower (S) in the superionic phase and the magnitude of electrical conductivity (σ). We overcome this issue using an all-room-temperature fabrication technique leading to an excellent reproducibility from one sample to another. To improve the zT of Ag2Te beyond the phonon-liquid electron-crystal limit (∼0.64 at 575 K in the ingot samples), we adopted a heirarchical nanostructuring technique, which effectively suppressed the thermal conductivity, leading to a significant improvement in the zT values for both n-type and p-type samples. We obtained zT of 1.2 in the n-type and 0.64 in the p-type Ag2Te at 570 K. These values supersede the zT of any Ag2Te previously reported. At 570 K, for our ball-milled/cold-pressed samples, the critical current density for metal-ion migration exceeds 15 A cm-2, which further confirms that Ag2Te is a promising thermoelectric material. Our results are supported by first-principles density functional theory calculations of the electronic and thermal properties.

Plastic deformation of superionic water ices

Due to their potential role in the peculiar geophysical properties of the ice giants Neptune and Uranus, there has been a growing interest in superionic (SI) phases of water ice. So far, however, little attention has been given to their mechanical properties, even though plastic deformation processes in the interiors of planets are known to affect long-term processes, such as plate tectonics and mantle convection. Here, using density functional theory calculations and machine learning techniques, we assess the mechanical response of high-pressure/temperature solid phases of water in terms of their ideal shear strength (ISS) and dislocation behavior. The ISS results are well described by the renormalized Frenkel model of ideal strength and indicate that the SI ices are expected to be highly ductile. This is further supported by deep neural network molecular dynamics simulations for the behavior of lattice dislocations for the SI face-centered cubic (fcc) phase. Dislocation velocity data indicate effective shear viscosities that are orders of magnitude smaller than that of Earth's lower mantle, suggesting that the plastic flow of the internal icy layers in Neptune and Uranus may be significantly faster than previously foreseen.

Neutron scattering studies on ionic diffusion behaviors of superionic α-Cu2−δ Se

We present studies on crystal structure and ionic diffusion behaviors of superionic Cu2−δ Se (δ = 0, 0.04, and 0.2) by utilizing neutron powder diffraction and quasi-elastic neutron scattering. In the superionic phase, the structural model with Cu ions occupying the Wyckoff sites of 8c and 32f provides the best description of the structure. As the content of Cu increasing in Cu2−δ Se, the Cu occupancy increases on the 32f site but decreases on the 8c site. Fitting to the quasi-elastic neutron scattering spectra reveals two diffusion modes: the localized diffusion between the 8c and 32f sites and the long-range diffusion between the adjacent 8c sites using the 32f site as a bypass. Between 430 and 650 K, we measured that the compound with more Cu content exhibits a larger long-range diffusion coefficient. Temperature in this range does not affect the long-range diffusion process obviously. Our results suggest the two diffusion modes cooperative and, thus, provide a microscopic understanding of the ionic diffusion of the Cu ions in superionic Cu2−δ Se.

The first-principles phase diagram of monolayer nanoconfined water.

Water in nanoscale cavities is ubiquitous and of central importance to everyday phenomena in geology and biology. However, the properties of nanoscale water can be substantially different fromthose of bulk water, as shown, for example, by the anomalously low dielectric constant of water in nanochannels1, near frictionless water flow2 or the possible existence of a square ice phase3. Such properties suggest that nanoconfined water could be engineered for technological applications in nanofluidics4, electrolyte materials5 and water desalination6. Unfortunately, challenges in experimentally characterizing water at the nanoscale and the high cost of first-principles simulations have prevented the molecular-level understanding required to control the behaviour of water. Here we combine a range of computational approaches to enable a first-principles-level investigation of a single layer of water within a graphene-like channel. We find that monolayer water exhibits surprisinglyrich and diverse phase behaviour that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials. Notably, this suggests that nanoconfinement could be a promising route towards superionic behaviour under easily accessible conditions.

Thermal properties of nanocrystalline copper sulfides KxCu1.85S (0 < x < 0.05)

This paper presents the comparative data on thermal properties of five compounds of nanocrystalline copper sulfides KxCu1.85S (0 < x < 0.05) at the temperature range from 300 to 700 K. X-ray phase analysis shows that Cu31S16 (or Cu1.9375S) monoclinic djurleite phase are found as prevailing in all KxCu1.85S samples (0.01< x < 0.05). Also in all samples there are oxygen oxide Cu2O and orthorhombic anilite Cu7S4 (Cu1.75S). Variable components of the alloys are monoclinic and hexagonal chalcocite, triclinic roxbyite. The sizes of crystallites in the alloys estimated from half-width of X-ray lines are in the range from 11 to 265 nm at room temperature. Differential scanning calorimetry revealed considerable endothermic thermal effect in the temperature range from 370 to 375 K for all compounds, obviously caused by the superionic phase transition from monoclinic djurleite to hexagonal chalcocite. The thermal conductivity of alloys decreases to 0.16 ÷ 0.25 W K−1m−1 in the temperature range of 450 – 600 K due to an increase in the amount of potassium (for x = 0.04, 0.05). This favors the achievement of high thermoelectric figure of merit of the material. The obtained results are important for semiconductor materials used in the field of thermoelectric devices.