Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4

Mixtures of CsHSO₄ and CsH₂PO₄ were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ were formed during milling. The mechanochemically treated composite consisting of Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs₂(HSO₄)(H₂PO₄) was generated from the composite at around 100 °C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 °C on heating. The value becomes 2 × 10^− 3 S cm^− 1 at around 180 °C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs₂(HSO₄)(H₂PO₄).     

Proton conductivity and phase composition of mixed salts in the systems <Emphasis Type="Italic">M</Emphasis>H<Subscript>2</Subscript>PO<Subscript>4</Subscript>–CsHSO<Subscript>4</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Cs,K)

Phase transformations, electrical transport and thermal properties of the systems K_1?xCs_x(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.95) and Cs(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.30) have been studied in detail. It has been shown that the mixed compounds Cs(H₂PO₄)_1–x(HSO₄)_x are characterized by an increase in the low-temperature electrical conductivity by one to five orders of magnitude depending on the composition, as well as by the disappearance of the superionic phase transition at x ≥ 0.15. The partial substitution of HSO₄^- ions for the anions in CsH₂PO₄ at x = 0.01–0.10 leads to the formation of Cs(H₂PO₄)_1?x(HSO₄)_x solid solutions isostructural with the CsH₂PO₄ (P2₁/m) phase. For Cs(H₂PO₄)_1–x(HSO₄)_x with x = 0.15–0.30 at room temperature, there is a stabilization of the high-temperature cubic phase isostructural with the CsH₂PO₄ (\(Pm\overline 3 m\)) phase existing in CsH₂PO₄ at temperatures above 230°C. The stability of the \(Pm\overline 3 m\) cubic phase at room temperature has been investigated using X-ray powder diffraction, ¹H NMR spectroscopy, and impedance spectroscopy. In the K_1–xCs_x(H₂PO₄)_1–x(HSO₄) system, there are two regions of compositions with x = 0.05–0.50 and 0.60–0.95, where the proton conductivity and thermal properties are determined respectively by the formation of the CsH₅(PO₄)₂ phase, which is stoichiometrically different from the initial salts, and the potassium-containing phase, which is isostructural with the superionic salt Cs₃(HSO₄)₂(H₂PO₄).     

Structure,thermal behavior,and dielectric properties of new cesium hydrogen sulfate arsenate: Cs<Subscript>2</Subscript>(HSO<Subscript>4</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)

Synthesis and structural characterization by single-crystal X-ray diffraction method, thermal behavior, and electrical proprieties are given for a new compound with a superprotonic phase transition Cs₂(HSO₄)(H₂AsO₄). The title compound crystallizes in the monoclinic system with the P2₁/n space group. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (10ī). The existence of O–H and (S/As)–O bonds in the structure at room temperature has been confirmed by IR and Raman spectroscopy in the frequency ranges 4000–400 cm^?1and 1200–50 cm^?1, respectively. Differential scanning calorimetry analysis of the superprotonic transition in Cs₂(HSO₄)(H₂AsO₄) showed that the transformation to high temperature phase occurs at 417 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 534 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 423 K is 1.58 × 10^?4 Ω^?1 cm^?1, and the activation energy for the proton transport is 0.28 eV. The conductivity relaxation parameters associated with the high disorder protonic conduction have been examined from analysis of the M”/M”_max spectrum measured in a wide temperature range. Transport properties of this material appear to be due to the proton hopping mechanism.     

Effect of cation substitution in Cs<Subscript>1–2<Emphasis Type="Italic">x</Emphasis></Subscript>Ba<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> on structural properties and proton conductivity

We synthesized compounds with partial substitution of Cs⁺ cations in CsH₂PO₄ by Ba²⁺ cations. The structural, electron transport and thermodynamic properties of Cs_1–2x Ba _x H₂PO₄ (x = 0–0.15) were studied for the first time with the help of a set of physicochemical methods: infrared and impedance spectroscopy, X-ray diffraction and synchronous thermal analysis. The proton conductivity of Cs_1–2x Ba _x H₂PO₄ at 50–230°C was investigated in detail by impedance measurements. The formation of solid substitution solutions isostructural with CsH₂PO₄ (P2₁/m) is observed in the range of substitution degrees of x = 0–0.1, with a slight decrease in the unit cell parameters and some salt amorphization. The conductivity of disordered Cs_1–2x Ba _x H₂PO₄ in the low-temperature region increases by two orders of magnitude at x = 0.02 and increases with an increasing fraction of barium cations by three or four orders of magnitude at x = 0.05–0.1; the superionic phase transition practically disappears. At x = 0.15, heterophase systems based on salts are formed, showing high conductivity and a further decrease in the activation energy of conductivity to 0.63 eV. The conductivity of the high-temperature phase of Cs_1–2x Ba _x H₂PO₄ does not change with increasing fraction of the substituent.     

On the isostructural and superprotonic Cs<Subscript>5</Subscript>H<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript>·<Emphasis Type="Italic">x</Emphasis>H<Subscript>2</Subscript>O transformations: physical or chemical nature?

For over 20 years, researchers have agreed that when pentacesium trihydrogen tetrasulfate hydrate (Cs₅H₃(SO₄)₄·xH₂O) is heated through 141 °C, the observed conductivity increase corresponds to a physical transformation: a first-order superprotonic phase transition. A careful high-temperature phase behavior examination of this acid salt was performed by means of simultaneous thermogravimetric and differential scanning calorimetry, conventional and modulated differential scanning calorimetry, and impedance spectroscopy. The results present evidence that this transformation is of chemical, instead of physical nature. The conductivity increase is an exclusive consequence of a partial thermal decomposition, where liquid water (dissolving part of the surface salt) and hygroscopic cesium pyrosulfate (Cs₂S₂O₇), as decomposition products, behave like a polymer electrolyte membrane where the proton transport mechanism includes the vehicle type, using hydronium (H₃O⁺) as a charge carrier. Additionally, it was found that the intermediate temperature transformation (so-called isostructural phase transition) at around 87 °C is also of chemical nature.     

Features of the behavior of longitudinal static and dynamic dielectric, piezoelectric, and elastic characteristics of KH2PO4 and NH4H2PO4 crystals

Along with their electromechanical coupling coefficients, the longitudinal dielectric, piezoelectric and elastic characteristics in ferroelectric KH₂PO₄ and antiferroelectric NH₄H₂PO₄ crystals are calculated using a modified proton ordering model that considers piezoelectric coupling and the four-particle cluster approximation. The possibility of detecting piezoactivity in solid solutions of K_{1 − x} (NH₄)xH₂PO₄ is substantiated.     

Structural,vibrational study and superprotonic behavior of a new mixed dipotassium hydrogenselenate dihydrogenphosphate K2(HSeO4)1.5(H2PO4)0.5

Ongoing studies of the KHSeO₄–KH₂PO₄ system aiming at developing novel proton conducting solids resulted in the new compound K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (dipotassium hydrogenselenate dihydrogenphosphate). The crystals were prepared by a slow evaporation of an aqueous solution at room temperature. The structural properties of the crystals were characterized by single-crystal X-ray analysis: K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (denoted KHSeP) crystallizes in the space group P 1¯ with the lattice parameters: a = 7.417(3) Å, b = 7.668(2) Å, c = 7.744(5) Å, α = 71.59(3)°, β = 87.71(4)° and γ = 86.04(6)°. This structure is characterized by HSeO₄⁻ and disordered (H_xSe/P)O₄⁻ tetrahedra connected to dimers via hydrogen bridges. These dimers are linked and stabilized by additional hydrogen bonds (O–H–O) and hydrogen bridges (O–H…O) to build chains of dimers which are parallel to the [0, 1, 0] direction at the position x = 0.5.The differential scanning calorimetry diagram showed two anomalies at 493 and 563 K. These transitions were also characterized by optical birefringence, impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of the proton conductors in this compound were determined in a wide temperature range. The transport properties in this material are assumed to be due to H⁺ protons hopping mechanism.     

Heats of solution/substitution in TlNO3 and CsNO3 crystals and in RbNO3 and CsNO3 crystals from heats of transition: the complete phase diagrams of TlNO3-CsNO3 and RbNO3-CsNO3 systems

From measurements of the decrease in the heat (enthalpy) of transition in the solid phase using differential scanning calorimetry, the apparent molar heats of solution, slope ΔH_t/x, the partial molar heats of solution at infinite dilution, χ, and the heats of solution, ΔH_s^°, of Tl⁺ in CsNO₃ crystal and Cs⁺ in TlNO₃ crystal and Rb⁺ in CsNO₃ crystal and Cs⁺ in RbNO₃ crystal along with their recovered lattice energies, ΔH_L^°, are reported. ΔH_s^° of Tl⁺ and Rb⁺ in CsNO₃ crystal are each found to be negligible or zero representing an ideal solid solution, i.e. ΔH_mix=0. The complete phase diagrams of the TlNO₃-CsNO₃ and RbNO₃-CsNO₃ systems with details of the sub-solidus regions are included. The properties of Tl_(1−x)Cs_xNO₃ and Rb_(1−x)Cs_xNO₃ compositions are discussed in terms of a ‘mixed crystal’ or ‘crystalline solid solution’ in relation to parallel compositions of Tl_(1−x)Rb_xNO₃.     

Anomalous acid proton self-diffusion in N(CH3)4HSO4 — A candidate for proton superionic conductivity

The appearance of a “liquid-like” proton T₂ component above 100°C and the relatively high value of the proton self-diffusion coefficient D = (5–8) × 10^-7cm²sec^-1 between 175°C and 200°C demonstrate the onset of a super-ionic state in N(CH₃)₄HSO₄. The ratio between the “liquid” and “solid” like components shows that acid protons are responsible for the high ionic conductivity.     

Preparation of proton conductive composites with CsHSO4/CsH2PO4 and phosphosilicate gel

New composite materials were prepared using cesium hydrogen sulfate (CsHSO₄) or cesium dihydrogen phosphate (CsH₂PO₄) and phosphosilicate gel (P₂O₅–SiO₂ gel). In X-ray diffraction patterns of these composites, diffraction peaks due to Cs₂H₅(SO₄)₂(PO₄) and CsH₅(PO₄)₂ were observed for CsHSO₄–(P₂O₅–SiO₂ gel) composites and CsH₂PO₄–(P₂O₅–SiO₂ gel) composites, respectively. These composites showed high conductivities in the order of 10^− 3 S cm^− 1 at 150 °C due to melting of Cs₂H₅(SO₄)₂(PO₄) or CsH₅(PO₄)₂ in the composites. In the cooling process, the CsHSO₄–(P₂O₅–SiO₂ gel) composites kept relatively high conductivity to 110 °C where solidification of Cs₂H₅(SO₄)₂(PO₄) occurs, whereas CsH₂PO₄–(P₂O₅–SiO₂ gel) composites showed relatively high conductivity continuously to ambient temperature.     

Proton ordering and spontaneous polarization in mixed KDP-ADP crystals

Temperature dependences of the transverse permittivity of mixed K_1?x (NH₄)_xH₂PO₄ crystals (x≈0, 0.04, 0.09, and 0.19) are experimentally investigated with the aim of determining the parameter P _A that characterizes proton ordering in the region of the ferroelectric phase transition. A comparison of the temperature dependences of the P _A parameter and spontaneous polarization shows that the spontaneous polarization is preceded by a partial ordering of protons in all the compositions studied. It is found that the crystals with a high ammonium content are characterized by a weaker effect of the lattice polarization on the proton ordering.     

Hydrogenation of FeCoZr–Al2O3 nanocomposites studied by Mössbauer spectroscopy and magnetometry

Hydrogenation effects on crystalline and magnetic structure of nanocomposites (FeCoZr) _x (Al₂O₃)_100???x , 38?≤?x?≤?63 at.% are studied by ⁵⁷Fe Mössbauer spectroscopy and magnetometry. Variations of local structure, blocking temperature and mean FeCoZr nanoparticles’ volume are discussed with respect to (i) composition and (ii) two competing processes—H₂ incorporation and annealing—occurred during treatment in H₂ plasma.     

Silica–zirconium phosphate–phosphoric acid composites: preparation,proton conductivity and use in gas sensors

Mesoporous composites made of silica and α-zirconium phosphate (SiO₂·xZrP) were synthesized starting from mixtures of delaminated ZrP dispersions and tetrapropylammonium oligosilicate solutions. The surface area of the composites reaches a maximum of 700 m²/g for x≈0.02, while the average pore diameter is about 40 Å for x in the range 0.05–0.35. In order to increase proton conductivity at low relative humidity (r.h.), SiO₂·xZrP·yH₃PO₄ composites were prepared and characterised by ²⁹Si and ³¹P MAS NMR and conductivity measurements. At 100 °C and 6% r.h., the conductivity of the composites, with H₃PO₄ loadings of 80% of the pore volume, rises from 5×10⁻⁴ to 2×10⁻² S/cm for x decreasing between 0.35 and 0.05, as a consequence of the concomitant increase of pore volume. For the composite with x=0.18, the dependence of conductivity on H₃PO₄ loading was also investigated at different temperatures and r.h. values. The combined increase of humidity, temperature and H₃PO₄ loading results in an increase of conductivity from 1×10⁻⁷ S/cm (y=0.09, T=25 °C, 0% r.h.) to 4×10⁻² S/cm (y=0.61, T=100 °C, 30% r.h.). SiO₂·0.18ZrP·0.61H₃PO₄ was also tested as a proton electrolyte in an oxygen sensor consisting of a disk of the composite sandwiched between a platinum sensing electrode and a reference electrode based on Ni_1−xO. The sensor is able to detect O₂ at room temperature in a dry environment with a response time of 20–30 s.     

Hydrothermal synthesis and properties of manganese-doped LiFePO4

A series of carbon-coated LiFe_1???x Mn _x PO₄ compounds are prepared by a hydrothermal method at 170 °C for 12 h. The structure and morphology of the prepared composites are characterized to examine the effects of Mn²⁺ substitution. All LiFe_1???x Mn _x PO₄ compositions are found to have an ordered olivine-type structure with homogeneous Fe²⁺ and Mn²⁺ distributions. The substitution leads to grain refinement from ~500 to ~150 nm, as well as to increased initial capacity and improved electronic conductivity. The amount of carbon coating varies with increased doping amount. The discharge curves of the LiFe_1???x Mn _x PO₄/C materials reveal a high discharge plateau corresponding to Fe^2+/3+ and no obvious plateau assigned to Mn^2+/3+, although a slight contribution of manganese is detected. However, the electrochemical performance, including the discharge capacity and cyclic performance, deteriorates with increased Mn content in the composite.     

The Mössbauer spectrum of synthetic hureaulite: Fe5 2+(H2O)4(PO4H)2(PO4)2

The crystal structure of synthetic ferrous hureaulite, Fe₅ ²⁺ (H₂O)₄(PO₄H)₂(PO₄)₂, was refined from single-crystal X-ray data. It is monoclinic, space group C2/c, with a=17.487(4), b=9.017(2), c=9.338(2) Å, β=96.27(3)°, V=1463.6(6) Å³, Z=4 and D _calc=3.327 g/cm³. This end member of the hureaulite series was crystallized under distinctly acidic conditions, by a method that gives perfect crystals, large enough for X-ray single crystal studies. The main feature of the hureaulite structure is that it has an equal number of normal (PO₄)³⁺ and acid (PO₄H)²⁺ tetradentate groups. These are centered on Fe²⁺ atoms and share corners with edge-linked octahedra, forming pentamer units. The five Fe²⁺ atoms are distributed on three distinct sites in these units. This can be directly observed in the Mössbauer spectrum at 295 K, which contains three doublets whose relative intensities correspond to the 1:2:2 distributions of crystallographic sites.     

Synthesis,thermal and electrical behavior of the superprotonic conductor phase in Rb<Subscript>3</Subscript>(HSeO<Subscript>4</Subscript>)<Subscript>2.5</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>0.5</Subscript> compound

The Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound was prepared and its thermal behavior and electric properties were investigated. The thermogravimetry (TGA) analysis and the differential scanning calorimetric (DSC) show the presence of a structural phase transition of the title compounds at 374 K which is confirmed by the variation of fp and σ_dc as a function of temperature. The complex impedance of the Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound has been investigated in the temperature range of 295–453 K and in the frequency range 209 Hz–1 MHz. The impedance plots show semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance Rp and constant phase elements CPE₁ in series with fractal capacity CPE₂. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of modulus, conductivity data, and circuit equivalent confirm that the transport is through the ion hopping mechanism, dominated by the motion of the H⁺ proton in the structure of the investigated materials.     

Ionic conductivity and crystal structure for the Li3−2xCr2−xTax(PO4)3 system

The monoclinic phase (P2₁/n) was formed for 0≤x≤0.6 and the NASICON-type rhombohedral phase (R3̄c) was obtained for the region 0.8≤x≤1.2 in the Li_3−2xCr_2−xTa_x(PO₄)₃ system. The activation energy for Li⁺ migration was ca. 0.45 eV for the monoclinic structure and ca. 0.36 eV for the rhombohedral structure. The maximum conductivity of 8.4×10⁻⁶ S cm⁻¹ at 298 K was obtained for x=0.8 of the Li_3−2xCr_2−xTa_x(PO₄)₃ system. The conductivity of LiCrTa(PO₄)₃ was enhanced about three to five times by the addition of the lithium salt due to the improvement of the sinterablity. The maximum conductivity was 2.4×10⁻⁵ S cm⁻¹ at 298 K for LiCrTa(PO₄)₃–0.2Li₃BO₃.     

Ionic conductivity of anhydrous zirconium bis(monohydrogen orthophosphate) and its sodium ion forms

Conductivities have been measured by the vector impedance method for the layered compounds anhydrous α-zirconium phosphate, Zr(HPO₄)₂, and several of its sodium ion phases. The values of the ionic diffusion coefficients are of the order of 10^?6-10^?8 cm²/s at 200°C and activation energies range from 15.6 cal/mol for Zr(HPO₄)₂ to 18.6 cal/mol for ZrHNa(PO₄)₂. Comparisons are made with literature values of conductivity and self-diffusion coefficients for Zr(NaPO₄)₂·3H₂O and kinetically determined diffusion coefficients for the phases containing both sodium and hydrogen ions.     

The Application of Raman Spectroscopy to the Study of the Uranyl Mineral Coconinoite Fe2Al2(UO2)2(PO4)4(SO4)(OH)2 · 20H2O

ABSTRACT

Raman spectra of the uranyl-containing mineral coconinoite, Fe₂Al₂(UO₂)₂(PO₄)₄(SO₄)(OH)₂ · 20H₂O, are presented and compared with the mineral's infrared spectra. Bands connected with (UO₂)²⁺, (PO₄)^3?, (SO₄)^2?, (OH)^?, and H₂O stretching and bending vibrations are assigned. Approximate U?O bond lengths in uranyl, (UO₂)²⁺, and O?H…O hydrogen bond lengths are calculated from the wavenumbers of the U?O stretching vibrations and (OH)^? and H₂O stretching vibrations, respectively, and compared with published data for similar natural and synthetic compounds.     

A Raman and infrared spectroscopic study of Ca2+ dominant members of the mixite group from the Czech Republic

The minerals of the mixite group—zálesíite CaCu₆[(AsO₄)₂(AsO₃OH)(OH)₆]·3H₂O from abandoned uranium deposit Zálesí, Czech Republic and calciopetersite CaCu₆[(PO₄)₂(PO₃OH)(OH)₆]·3H₂O from a quarry near Domašov na Bystřicí, northern Moravia, Czech Republic—were studied by Raman and infrared spectroscopy. The observed bands were assigned to the stretching and bending vibrations of (AsO₄)³⁻ and (AsO₃OH)²⁻ ions in zálesíite, and (PO₄)³⁻ and (PO₃OH)²⁻ in calciopetersite, and to molecular water, hydroxyl ions, and Cu‐(O,OH) units in both minerals. O H···O hydrogen‐bond lengths in zálesíite and calciopetersite structures were calculated with Libowitzky's empirical relation. Copyright © 2010 John Wiley & Sons, Ltd.