Superionic Compounds Research Articles

Remarkable effects of shear-exfoliation and restacking on microstructural texturing and thermoelectric properties of AgCrSe2

Layer-structured AgCrSe2 has attracted much attention for thermoelectrics for its low lattice thermal conductivity, however, the intrinsic electrical transport properties are poor. Here, remarkable effects of an exfoliation-restacking process were demonstrated for a series of Ag-deficient Ag1-xCrSe2 bulk ceramics by comparison. An effectively increased crystallographic orientation together with a reduction of grain size was obtained in such processed samples compared to that for the simply Ag-deficient ones. Electrical conductivity was significantly improved due to the increase in carrier mobility, while Seebeck coefficient was affected little with the unchanged carrier concentration and effective mass, leading to an optimal power factor of 7.3 μWcm−1K−2 at 773 K with x = 0.06; Meanwhile, lattice thermal conductivity was further reduced due to the enhanced phonon scattering by grain boundaries. Ultimately, a maximal ZT of ∼0.79 was achieved with x = 0.06 at 773 K, which stands among the tops so far for AgCrSe2-based thermoelectric materials. These results demonstrated a remarkable effectiveness of shear-exfoliation and restacking on tuning the thermoelectric performance of AgCrSe2-like layered superionic compounds.

Na superionic conductor-type compounds as protective layers for dendrites-free aqueous Zn-ion batteries

Aqueous rechargeable Zn-ion batteries (ARZIBs) have attracted much attention owing to their safety, high energy density and environmental friendliness. However, dendrite formation and corrosive reactions on Zn anode surface limit the development of ARZIBs. Here, Ga3+-doped NaV2(PO4)3 with Na superionic conductor (NASICON) structure [NVP-Ga(x), x = 0, 0.25, 0.5, 0.75] have been exploited as the high-efficiency artificial layer to stabilize Zn anode. The optimal NVP-Ga(0.5) layer can homogenize ion flux and promote uniform deposition of zinc, the dendrite growth and the parasitic reactions can be greatly inhibited. The symmetric cell based on this unique protection layer can stably operate over 1,300 h at 0.5 mA cm−2 with 0.5 mAh cm−2. Benefitting from the high-performance Zn metal anode, the full batteries paired with MnO2 cathode deliver a high discharge capacity of 106 mAh/g with the capacity retention rate of 85 % after 8,000 cycles. This work provides an advanced strategy to stabilize Zn anode for the industrialization of ARZIBs in the near future.

Sublattice melting in binary superionic colloidal crystals.

In superionic compounds one component premelts, providing high ionic conductivity to solid-state electrolytes. Here we find sublattice melting in colloidal crystals of oppositely charged particles that are highly asymmetric in size and charge in salt solutions. The small particles in ionic compounds melt when the temperature increases, forming a superionic phase. These delocalized small particles in a crystal of large oppositely charged particles, in contrast to superionic phases in atomic systems, form crystals with nonelectroneutral stoichiometric ratios. This generates structures with multiple domains of ionic crystals in percolated superionic phases with adjustable stoichiometries.

Mechanical properties of composites based on (Cu<sub>1−x</sub>Ag<sub>x</sub>)<sub>7</sub>GeSe<sub>5</sub>I mixed crystals

Purpose. The purpouse of the present work is to investigate a mechanical properties of composites based on polymer matrix and superionic compounds (Cu1−xAgx)7GeSe5I, to perform a comparative analysis with similar properties of 81 Uzhhorod University Scientific Herald. Series Physics. Issue 46. — 2019 a single crystals and to study changes of structural rigidity, which depend on concetration ratio of polymer and microcrystalline components. Methods. The сomposites were prepared by pressing a mixture of micropowder and polymer. Investigations of their mechanical properties were performed using microindentation test. An indentation was performed by Vickers indenter. For imprints analysis interferential microscope MII–4 was used. Results. The dependence of microhardness versus concetration ratio of micropowder and polymer has been analyzed. The relative contributions of elastic and plastic deformation components for Cu7GeSe5I single crystal, composite based on it, and ethylyne-vinyl acetate polymer have been calculated and compared. The decreasing of composites’ microhardness at Cu+→Ag+ isovalent cation substitution has been revealed. Conclusions. The microhardness numerical values of composites based on polymer and superionic micropowder were calculated. It has been founded, that at concentration of microcrystals increase up to 90 wt.%, the mechanical rigidity crossover ”soft-rigid structure” is observed. It has been shown, that with different component concentration the deformation mechanism changes: elastic in polymer, plastic in composites and elasto-plastic in superionic crystals. The microhardness of composites at Cu+→Ag+ cation substitution, decreases.

Aguilarite Ag4SSe Thermoelectric Material: Natural Mineral with Low Lattice Thermal Conductivity.

Ag4SSe, named as aguilarite in mineralogy, belongs to the family of liquid-like superionic compounds at middle temperature, but its thermoelectric (TE) properties are rarely investigated so far. In this study, a series of Ag-deficient Ag4SSe samples have been synthesized by the melting-annealing method. Their crystal structures, phase transition features, and electrical and thermal transport properties have been systematically investigated. It is found that Ag4SSe allows a small amount of Ag deficiency inside the lattice without altering the initial crystal structure and phase transition feature. The stoichiometric Ag4SSe has a low TE figure of merit ( zT) of 0.24 at 613 K because of the overhigh carrier concentration. Introducing Ag deficiency in Ag4SSe significantly lowers the carrier concentration, yielding a greatly enhanced Seebeck coefficient and suppressed carrier thermal conductivity throughout the entire measured temperature range. A maximum TE zT of 0.53 is achieved at 493 K for Ag3.96SSe, about two times that for the stoichiometric Ag4SSe. The present study suggests that aguilarite Ag4SSe is a promising TE material applied at the middle temperature range.

Comment on “High-pressure phases of group-II difluorides: Polymorphism and superionicity”

Nelson et al. [Phys. Rev. B 95, 054118 (2017)] recently have reported first-principles calculations on the behaviour of group-II difluorides (BeF$_{2}$, MgF$_{2}$, and CaF$_{2}$) under high-pressure and low- and high-temperature conditions. The calculations were based on ab initio random structure searching and the quasi-harmonic approximation (QHA). Here, we point out that, despite the of inestimable value of such calculations at high-pressure and low-temperature conditions, the high-$P$ high-$T$ phase diagram proposed by Nelson et al. for CaF$_{2}$ neither is in qualitative agreement with the results of previous ab initio molecular dynamics simulations nor with the existing corps of experimental data. Therefore, we conclude that the QHA-based approach employed by Nelson et al. cannot be applied reliably to the study of phase boundaries involving superionic phases. This conclusion is further corroborated by additional ab initio calculations performed in the superionic compounds SrF$_{2}$, BaF$_{2}$, Li$_{3}$OCl, and AgI.

Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations.

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag1- xCuS and AgCu1- xS samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag+/Cu+) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag1- xCuS and AgCu1- xS samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.

Synthesis and Electrochemical Property of I-4 Type Li1+2xZn1-XPS4 Solid Electrolyte

Accompanying the recent technological advance, sulfide-based all-solid-state Li ion battery (ASSB) is anticipated to reach to the stage of commercialization within 10 years. In particular, significant progress has been made within the last five years with the discoveries of several sulfide-based compounds whose conductivity is greater than 10 mS/cm, such as Li10GeP2S12 [1], Li7P3S11 [2], and Li9.54Si1.74P1.44S11.7Cl0.3[3]. Usually such new Li-ion conductive materials have been discovered through engineering manners, such as changing the composition of a known superionic compound, or adjusting the synthesis conditions. Recently, however, there have been other attempts to predict new SE materials by means of a computational approach using first-principles calculations. W. Richards et al. [4] predicted that Na10SnP2S12, in which Li is substituted to Na in Li10SnP2S12, would be a good Na ion conductor. They also synthesized Na10SnP2S12 in practice and confirmed that the experimentally measured ionic conductivity and the activation energy of Na10SnP2S12 agreed well with those of the predicted values. In this study, we examined I-4 type Li1+2xZn1-xPS4 — a new Li ionic conductor computationally predicted by Richards et al. [5-6]. They predicted that the stoichiometric LiZnPS4 is almost insulative, but non-stoichiometric compound of Li1+2xZn1-xPS4 can show superionic conductivity with the predicted room temperature ionic conductivity (σ25 ℃) as large as 50 mS/cm at x=0.5. Li1+2xZn1-xPS4 was synthesized by using solid state reaction. We ball-milled the starting materials and heated it at elevated temperature under vacuum. We measured the XRD patterns and the Raman spectra for Li1+2xZn1-xPS4 at various x and confirmed that the samples with x ≤ 0.625 have I-4 structure with little impurities. Figure 1 shows the ionic conductivity at 25℃ (σ25 ℃) and the activation energy (Ea) for Li1+2xZn1-xPS4 at various x. Li1+2xZn1-xPS4 shows negligible conductivity when x ≤ 0.2. At x = 0.375, σ25 ℃ starts increasing and Ea starts decreasing. The highest σ25 ℃ achieved was 5.7 x 10-4 S/cm at x = 0.625. This tendency qualitatively agrees with the theoretical predictions [6], but there remains a considerable amount of discrepancy. When x ≥ 0.75, σ25 ℃ decreases, which likely corresponds to low phase purity and poor crystallinity of the material. We also examined the feasibility of Li2.25Zn0.375PS4(x=0.625) in a practical ASSB with NCM cathode and various anodes (graphite, indium, LTO), and found that this material was not stable with graphite and indium, but can be paired with NCM and LTO.

CuxS superionic compounds: Electronic structure and thermoelectric performance enhancement

In the present work, we have investigated the effect of Cu content (x) on the phase structure and thermoelectric transport properties of CuxS (x = 1.92, 1.94, 1.98) bulks fabricated by combining mechanical alloying and spark plasma sintering. From density functional theory calculations of the valence band energy levels, we hypothesized that favourable valence band alignments accelerate electrical transport and can be achieved by optimizing the molar ratio of Cu1.96S to Cu2S. Experimentally, the sample with x = 1.92 showed an improved electrical conductivity 538.7 Scm−1 at 623 K and retained a good Seebeck coefficient, synergistically leading to a broad, high power factor ranging from 105.9 to 900.3 μWm−1K−2. The highly-disturbed Cu ions give CuxS compounds an intrinsically low lattice thermal conductivity (∼0.5 Wm−1K−1), which endows a high ZT of 1.23 as x = 1.94 to this otherwise simple semiconductor. The results indicate that low-cost and non-toxic CuxS superionic compounds are promising thermoelectric materials.

Thermoelectric transport in Cu7PSe6 with high copper ionic mobility.

Building on the good thermoelectric performances of binary superionic compounds like Cu2Se, Ag2Se and Cu2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu7PSe6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure-property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu7PSe6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes.

Neutron scattering study of ionic diffusion in Cu–Se superionic compounds

Paper reports the results of the neutron scattering study of crystal structure and diffusion of Cu2−δSe compounds in superionic α-phase. We found that the crystallographic model with Cu atoms in the 8c (0.25, 0.25, 0.25) and 32f (x, x, x) (x=0.33–0.39) sites in fluorite lattice provides the best description of the average structure. The quasi-elastic neutron scattering data reveal the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu atoms between diffusion hops. Combination of neutron diffraction, quasi-elastic scattering experimental data with the Bond-Valence Method simulations strongly suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.

Neutron diffraction study of diffuse scattering in Cu 2− δSe superionic compounds

Crystal structure and short-range order in Cu 2− δ Se compounds were studied in superionic and non-superionic phases using high-resolution neutron diffractometer Echidna at ANSTO. In diffraction patterns of β-Cu 1.98Se (ordered phase at ambient T), both Bragg peaks and diffuse background change sharply through the β → α structural phase transition at T = 414 K during heating. In case of α-Cu 1.75Se (disordered superionic phase at ambient T) the changes are monotonic, showing gradual shifts of Bragg peaks and increased intensity of the diffuse background as a function of temperature. On cooling, both compounds undergo a β → β′ transformation. Diffuse scattering in the α-phase shows an oscillating dependence on wavevector, with broad peaks centred at Q ∼ 3, 5.5 and 8 Å −1. The measurements taken in energy dispersive mode show that the oscillating diffuse background arises from correlated thermal displacements of the ions. Diffuse scattering is higher for compositions close to stoichiometry and increases with temperature. Theoretical calculations show that the increase in diffuse intensity both with temperature and Cu content is related to correlated thermal vibrations of Se and Cu atoms, with Se–Cu( 8c, 32f) and Cu( 8c)–Cu( 8c) correlations being the most important.

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of $$ {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} $$ superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.

Synthesis and electrical conductivity of a new Ag6SnS4Br2 superionic compound

The conditions of synthesizing a new Ag6SnS4Br2 compound were studied. The crystallographic parameters of the unit cell were determined as follows: space group Pnma, a=6.67050(10) A, b=7.82095(9) A, c=23.1404(3) A, and Z=4. The total electrical conductivity and its ionic component were measured by a dc probe method in the temperature range 210–380 K. Kinks in the conductivity curve and the differential thermogram of heating the alloy were revealed at 235 K. It was concluded that the mass and charge transfers in the compacted Ag6SnS4Br2 alloy powder have an intragrain character.

Anion diffusion within highly conducting layered fluorides

Neutron diffraction has played an important role in the study of solids which possess high values of ionic conductivity, ranging from ‘model’ superionic compounds such as AgI and β-PbF 2 to more technologically relevant materials for use within lightweight Li + batteries and oxide fuel cells. An understanding of the structural properties of these systems is essential to identify those factors which promote high ionic conductivity and underpin the quest for new materials. In recent years, attention has focussed on the role of cations characterized by lone-pair electrons (such as Bi 3+, Pb 2+, Sn 2+ and Tl +) within highly conducting oxides and halides, with differing views concerning their importance in promoting extensive anion disorder. In this context, the results of powder neutron diffraction studies of two layered compounds within the MSnF 4 (M=Pb, Sr and Ba) and MSn 2F 5 (M=K, Rb, Cs and Tl) families are presented, to illustrate the structural role of Sn 2+ ions within these anion conductors.

High-temperature behaviour of average structure and vibrational density of states in the ternary superionic compound AgCuSe

Results of simultaneous thermal analysis (STA), synchrotron powder diffraction (in the range 300–973 K) and inelastic neutron scattering (at 285 and 505 K) on non-superionic β- and superionic α-AgCuSe are reported. The sample is stable in argon on heating. The volume change at the superionic phase transition is about 5%. A model for the average structure of α-AgCuSe is proposed. No anomalies in the temperature dependence of the parameters of the average structure were revealed. Ionic conductivity in α-AgCuSe can originate from cation jumps in “skewed” 〈100 〉 directions between nearest-neighbour tetrahedral sites via the peripheries of the octahedral cavities. A correlation between the temperature dependence of the cation redistribution in α-AgCuSe and the temperature dependence of the ionic conductivity is supposed. Various contributions (anharmonic effects, time-average static disorder and phonon-phonon scattering) to the widths of individual phonons upon temperature increase lead to pronounced changes in the neutron-weighted densities of states of β- and α-AgCuSe and accompany the superionic phase transition as well.

Band Energy Structure and Optical Properties of Ag2CdI4 Superionic Compound

This work deals with theoretical and experimental investigations of electronic structure of Ag2CdI4 superionic compound. The band energy structure was calculated and optical absorption and luminescence spectra were studied in temperature region 77–300 K. It was shown that the conduction band is formed by 5p-, 5s-orbitals of Cd and 5s(Ag)-orbitals, while the valence band consists of 4d(Ag)- and 5p-, 5s-orbitals of the iodine. The interpretation of the bands observed in luminescence and absorption spectra is proposed within the framework of the results calculated.

Phase transitions and transport phenomena in Li0.25Cu1.75Se superionic compound

AbstractPhase transformation points in Li0.25Cu1.75Se mixed electronic–ionic conductor have been determined by calorimetric, conductometric and thermoelectric measurements. The phase transformation (PT) from triclinic to monoclinic occurs at 403–413 K. At 503–515 K the monoclinic phase is followed by a rhombohedral modification. Both of these PTs are accompanied by drops on the calorimetric curve. At about 653 K observed anomalies in the temperature dependencies of the ionic conductivity, of the chemical diffusion coefficient and the jump of the ionic Seebeck coefficient have been induced by the PT to hexagonal phase. Neutron diffraction studies reveal the cubic structure of Li0.25Cu1.75Se compound (with space group Fm3m) at 773 K. The corresponding PT causes anomalies in the electrical and diffusion properties at 703–713 K. Cu ions are statistically distributed over tetrahedral and trigonal voids in an Fm3m cage; lithium ions randomly occupy 32(f) positions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phonons in mixed superionic fluorites (BaF2)1-x(LaF3)x

In recent years, the fluorite-structured solid solutions with the general formula, (MF2)1-x(RF3)x (M = Ca, Sr, Ba, Pb and R is a rare-earth element or Y), have been the subject of numerous experimental studies focussed on their superionic properties. The overall cubic crystal symmetry (space group Fm3m) is conserved up to x ≶ xmax, where xmax ⊁ 0.4-0.5 depending on M and R. The zone centre phonons and phonon dispersion along three symmetry directions of the mixed superionic compound (BaF2)1-x(LaF3)x have been investigated by applying de Launey angular force model for x ≶ xmax. The calculated results are compared and explained with available experimental results.

A model for the composition dependence of sound velocity in liquid silver chalcogenides

It is known that superionic conductors such as silver chalcogenides exhibit an intermediate behavior between solids and liquids. This observation has lead to the concept of sublattice melting and to a model for superionic conductors such as a ‘crystalline cage immersed in a viscous liquid’. On the other hand, it has been suggested that in the liquid phase of a superionic compound, there is a tendency towards sublattice ordering. In the present contribution, the two fluid model of superionic melt is presented. The model is applied to study the composition dependence of the sound velocity in liquid silver chalcogenides. It is shown that the model reproduces quite well the behavior observed experimentally.