Solid-hydroxide protonic conductors: Superionic conductivity,phase transitions,isotopic effect,and self-organized microheterogeneity

New members of the family of inorganic solid-state protonic conductors are synthesized on the basis of KOH, NaOH, and H₂O. The equimolar eutectics KOH/NaOH and KOH/KOH · H₂O and the monohydrate KOH · H₂O are superprotonic conductors in the temperature ranges 360–458, 360–370, and 320–420 K, respectively, with the conductivities of above 1 mS/cm and activation energies below 0.4 eV. A characteristic feature of the eutectics is the presence of anomalies in the temperature dependences of the conductivity and heat capacity in the range 360 ± 1 K. The isotopic effect of the protonic conductivity for KOH · H₂O and for the high-temperature form of KOH/NaOH is found to be 1.40 ± 0.15. The role played by the self-organized microheterogeneity of the KOH/NaOH and KOH/KOH · H₂O solid eutectics in the substantial increase in the conductivity and in the change of its pattern as compared to the individual KOH and NaOH is discussed.     

Structure of ZnZrF<Subscript>6</Subscript> · <Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O (<Emphasis Type="Italic">n</Emphasis> = 6–2) and ZnZrF<Subscript>6</Subscript> crystallohydrates according to vibrational spectroscopy data

Fluoridezirconate crystallohydrates ZnZrF₆ · nH₂O (n = 6–2) and anhydrous ZnZrF₆ are investigated by vibrational spectroscopy and thermography. The influence of the hydrate number on the structure of the cationic and anionic sublattices of the crystallohydrates is studied. The changes in the strength of HOH···F and HOH···O hydrogen bonds of coordinated and outer-sphere water molecules occurring with variations in the hydrate number are determined by changes in the IR spectra. The IR spectra of ZnZrF₆ · nH₂O (n =6, 4) compounds, which have isolated complex anions [ZrF₆]^2– in their structure, revealed a band with two peaks in the range of 3470–3430 cm^–1, which corresponds to stretching vibrations of coordinated water molecules. The spectra of ZnZrF₆ · nH₂O (n = 5, 3, 2, 1) crystallohydrates with a polymeric structure show a high-frequency shift of this band, which corresponds to weakening of hydrogen bonds. The vibrations of crystallization water molecules involved in the network of strong O–H···F and O–H···O hydrogen bonds manifest themselves in the spectra of ZnZrF₆ · nH₂O (n =5, 3) crystallohydrates by broad structureless bands in the region of stretching, bending, and libration vibrations.     

Protonic conductivity of β″ and ion-rich β-alumina. II: Ammonium compounds

The conductivity and thermal stability of NH⁺₄, H⁺(H₂O)_nβ″ and ion-rich β-alumina single crystals have been measured by the complex impedance method in the 25–700°C temperature range. Both structures have similar properties, but ion-rich β-alumina shows a higher stability and a lower activation energy (β: 0.18 eV, β″ 0.24 eV below 400°C and 250°C respectively). The room temperature conductivity is about 3×10^-5ω^-1cm^-1. The conducting properties and mechanisms are discussed and compared to other protonic or ionic conductors.     

Structural and thermoelectric power properties of Na-doped V2O5·nH2O nanocrystalline thin films

X-ray diffraction (XRD), thermoelectric power (S) and at room temperature electrical conductivity (σ) of Na⁺¹-doped V₂O₅·nH₂O nanocrystalline thin films fabricated by sol gel technique (colloid route) were studied. XRD showed that the Na₂O–V₂O₅·nH₂O thin films are highly oriented nanocrystals. The average value of particle size was found to be about 7.5 nm. The thermoelectric power showed that the thermoelectric power for all present nanocrystalline thin films samples decreased with increasing Na⁺¹ content. However, the electrical conductivity increased with increasing Na⁺¹ content. There is evidence that small polarons are responsible for determining the transport properties of the Na⁺¹ doped V₂O₅·nH₂O nanocrystalline thin films samples. The high value of electrical conductivity and small value of thermoelectric power is ideal for device applications, where device to device variation of the thermoelectric power must be small. This preparation technique was demonstrated to fabricate high quality Na₂O–V₂O₅·nH₂O nanocrystalline thin films for thermoelectric device applications. However, this may be further used for deposition with an ink-jet printer.     

Effect of vanadium-containing activating additives on the oxidation of aluminum powders

The effect of vanadium-containing activating additives on the oxidation of an ASD-4 aluminum powder subjected to low-rate heating in the air is studied. The tests are conducted for a number of activators introduced by impregnating the metal powder with vanadium-containing gels with the following compositions: Li₂V₁₂O₃₁ · nH₂O, Na₂V₁₂O₃₁ · nH₂O, Na₂MoV₁₁O₃₁ · nH₂O, V₂O₅ · nH₂O, and 6V₂O₅ · B₂O₃ · nH₂O. A gel-preparation method based on the instantaneous cooling of melts of the respective reagents in cold water is proposed. It is found that the oxidation of the activated powders is shifted to a low-temperature region.     

Structural studies of some new lamellar magnesium,manganese and calcium phosphonates

Layered phosphonate salts of divalent metal ions (Mg, Ca and Mn) are prepared by combining solutions of soluble metal salts and alkyl- or arylphosphonic acids. In this way the compounds Mg(O₃PC_{nH2n+1)·H2O (n=1−12), Mg(O3PC6H5)·H2O, Mg(HO3PCH(C6H5)2)2·8 H2O, Mn(O3PCH3)·H2O, Mn(O3PC6H5)·H2O, Ca(O3PCnH2n+1)·H2O (n⩽5), Ca(HO3PC6H5)2 and Ca(HO3PCnH2n+1)2 (n⩾6) were prepared. The M(O3PC6H5)·H2O compounds show good thermal stability, losing lattice water at 250–300°C without further decomposition below 550°C. Compounds derived from alkylphosphonic acids decompose at lower temperatures. The Mg(O3PCnH2n+1)·H2O series, Mg(O3PC6H5)·H2O, and Mn(O3PC6H5)·H2O group Pmn21; for the latter compound unit cell dimensions (Å) are a=5.733, b=14.298, c=4.931. The structure consists of roughly coplanar layers of metal atoms coordinated by phenylphosphonate groups above and below. Each metal atom is coordinated by five phosphonate oxygens and one lattice water molecule. Mg(O3PCnH2n+1}·H₂O adopts a similar structure; infrared spectra indicate all-trans alkyl chains. In Mg(HO₃PCH(C₆H₅)₂)₂·8 H₂O, Mg(H₂O)²⁺₆ ions and lattice water lie in hydrogen-bonded sheets; the benzhydryl groups lie above and below and make van-der-Waals contacts between layers.     

Relative humidity influence on the water content and on the protonic conductivity of the phosphatoantimonic acids HnSbnP2O3n+5, xH2O (n = 1, 3, 5)

The phosphatoantimonic acids H_nSb_nP₂O_3n+5, xH₂O (n = 1, 3, 5) have been prepared from the corresponding potassium compounds by ion-exchange in acidic medium. For n = 1 and 3 they are layered materials. When n = 5 the covalent framework is three dimensional with large interconnected channels. The title acids are all hydrated and their water content, lattice parameters and protonic conductivity have been studied at 20°C as a function of the relative humidity. When n = 1 a great part of the water content is physisorbed and the electrical behavior is that of a particle hydrate. For n = 3, the compound is a true lattice hydrate and the protonic conductivity is closely related to the water content. This is also the case when n = 5; however the contribution of surface water to the proton diffusion is clearly evidenced.     

The hydrates and deuterates of ferrous sulfate (FeSO4): a Raman spectroscopic study

A comparative analysis has been carried out on the Raman spectra of FeSO₄·nH₂O (n = 1, 4, 7) including the ²D‐analogs. The effects of changing the degrees of hydration have been found from the lattice, SO₄²⁻ internal, and H₂O internal modes. Increasing degrees of hydration shift the intense ν₁(SO₄) peak to lower wavenumbers and reduce the amount of splitting on the ν₃(SO₄) peaks. Some of the water librational bands cause the broadening of the ν₄(SO₄) peaks in FeSO₄·7H₂O and the ν₂(SO₄) peaks in FeSO₄·7D₂O. The ν₂(H₂O) band in FeSO₄·H₂O is red‐shifted in excess of 100 cm⁻¹ relative to the unperturbed H₂O band. Between 240 and 190 K and between 140 and 90 K in the spectra of FeSO₄.4H₂O, two potential phase transitions have been identified from the changes in the lattice and water‐stretching regions. The resolution of the ν₁(H₂O) and ν₃(H₂O) bands in FeSO₄·4H₂O and FeSO₄·H₂O also improved sharply at low temperatures. The capability of distinguishing various forms of FeSO₄ hydrates unambiguously makes the Raman technique a potential analytical tool for the identification of sulfate minerals on planetary surfaces. Copyright © 2006 John Wiley & Sons, Ltd.     

Field dependence of the thermal conductivity of K2CuCl4 · 2H2O near its ferromagnetic Curie temperature

The thermal conductivity of the ferromagnetic insulator K₂CuCl₄ · 2H₂O has been measured near its Curie temperature T_c. The measurements were made as a function of temperature in constant external magnetic field and as a function of field along isotherms. The results indicate a relaxation rate for magnetic critical scattering of phonons varies as H^?1/2.     

The bonded water molecule3ĪII: 180° Flips and diffusion

The proton spin-lattice relaxation time, T₁, is measured as a function of temperature in α -(COOH)₂·-2H₂O, K₂HgCl₄· H₂O and LiCHO·H₂O. The relaxation is caused by 180° flips of the water molecules about their 2-fold axes and good agreement is obtained between calculated and observed values of T₁. Empiricly the flip rate follows a classical Arrhenius equation: P· exp $\text{(? ΔH}\text{(RT))}$. A literature survey of values of P and ΔH obtained from similar investigations on other hydrates is given. The survey shows that the preexponential factor, P, is a function of the activation enthalpy, ΔH. P increases from 10¹² to 10¹⁷ Hz when ΔH changes from 2 to 17 $\text{kcal}\text{mole}$. Using a dynamical rate theory as formulated by Feit, we find the flip rate is given by: K₂· √(ΔH)· exp (K₁ΔH)· exp $\text{(?ΔH}\text{(RT>))}$. This expression can be fitted to the observed data using K₁ = 0.69 $\text{mole}\text{kcal}$ and K₂ = 2 × 10¹¹ Hz · $\text{(kcal}\text{mole)}$$\text{?1}\text{2}$. Thus both the frequency factor, K₂√ (ΔH), and the entropic factor, exp (K₁ΔH), have been obtained for flipping water molecules in hydrates. The values of K₁ and K₂ are shown to be physically reasonable.     

NMR study of proton transport in the inorganic ion-exchange compounds SnO2·nH2O and TiO2·nH2O

Proton NMR relaxation times (T₂, T₁, T_1? and T₁D) are reported for hydrous tin oxide (SnO₂·nH₂O) and hydrous titania (TiO₂·nH₂O) in the temperature range 135 K<T<336 K at 60 and 20 MHz. The data show proton transport including exchange between three environments: (1) surface hydroxyl groups, (2) “acid solution” in micropores (diameter <100 Å) and (3) “acid solution” in macropores (diameter>1000 Å). The NMR behaviour has many features in common with that of adsorbed water systems. A consistent interpretation of both NMR and conductivity data is presented.     

Vibrational spectroscopic investigation of the hydrates of manganese(II) oxalate

The three known hydrates of manganese(II) oxalate, α‐MnC₂O₄ · 2H₂O, γ‐MnC₂O₄ · 2H₂O and MnC₂O₄ · 3H₂O were synthesized by known procedures and characterized by X‐ray powder diffractometry. Their infrared (IR) and Raman spectra were recorded and discussed on the basis of its structural peculiarities allowing to establish some interesting relations between them and with other, previously investigated, oxalate complexes. The IR spectra of partially deuterated samples of α‐MnC₂O₄ · 2H₂O were also discussed, reinforcing some of the performed assignments. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigations on field desorption products from silver surfaces by mass spectrometry

Field evaporation of silver and field desorption of silver surface compounds were investigated by analysing positive ions with a mass spectrometer. In particular, the well known adsorption states of oxygen, and further the interactions of H₂O, NH₃, H₂, CO and CH₄ were measured in the field ion mass spectrometer under steady state fields of > 0.1 V/Å with a sensitivity of < 0.1 ions s^?1 and at temperatures between 80 °K and 425 °K. Although oxygen is usually chemisorbed at Ag surfaces, no AgO⁺, AgO⁺₂ or other Ag-O compounds could be detected as positive ions, Ag⁺ and O₂⁺ are the only observed ions at best image fields in oxygen up to fields of field evaporation of Ag⁺(≈ 2.2 V/Å). Even after the actual adsorption of oxygen with zero-field (6 × 10⁵ Langmuir at 10^?3 Torr) at 323 °K and 473 °K and subsequent application of the desorption field at 210°K no silver-oxygen compounds were found in positive ionic form. Small quantities of AgO⁺ and AgO⁺₂ were only formed — besides Ag(H₂O)_x⁺ complexes — if atomic oxygen was supplied by the field induced dissociation of water.Gases which do not adsorb on silver under zero-field conditions (H₂, CO, CH₄, N₂) yield the ions Ag(H₂)_n, Ag(CO)_n⁺, n=1, 2; AgCH₄⁺, AgN₂⁺. The situation with H₂O and NH₃ is more complicated: Molecular ions [Ag(H₂O)_n]⁺·mH₂O, n=1,…, 4, m=1,…, 8 and [Ag(NH₃)_n]⁺·mNH₃, n=1, 2, m=1,…, 6 are found besides Ag⁺.From the temperature and field dependence conclusions are drawn about the mechanisms of evaporation and formation of ionic surface complexes. The activation energies of evaporation of Ag⁺ are found to depend on the square root of the field strength. In general, the generation of surface compounds can be described by field induced reactions rather than usual gas adsorption.     

Conductivity of β″ and ion rich β alumina.I.H.+(H2O)n compounds

The conductivity and thermal stability of H⁺(H₂O)_n β″ and ion rich β alumina single crystals have been measured by the complex impedance method in the 25–700°C temperature range. Two mechanisms of conductivity were assumed: proton transfer at lower temperatures and H₃O⁺ diffusion in the high-temperature range. Both structures have similar properties, but ion rich β alumina possesses the best stability and the lowest activation energy (β: 0.15 eV, β″: 0.20 eV below 400 and 300°C respectively). The room-temperature conductivity is ≈5×10^?6 Ω^?1 cm^?1. The conducting properties and mechanisms are discussed and compared to other protonic or ionic conductors.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Indication from NMR of Grotthuss mechanism for proton conduction in H2Sb4O11·nH2O

We have measured the second moment, the linewidth and the relaxation times T₁ and T₂ of the ¹H magnetic resonance signal from 4.2 to 380 K in the fact proton conductors H₂Sb₄O₁₁·nH₂O. Our results reveal that the high ionic conductivity of these materials is due to a Grotthuss-type proton diffusion mechanism with succession of molecular reorientations of H₃O⁺ ions or H₂O molecules and of proton jumps from H₃O⁺ to H₂O.     

Phase transitions,ferroelectric behavior and second-order nonlinear optics of a new mixed sodium dihydrogen phosphate–arsenate

Aiming at the development of new proton conducting solids, recent studies of the NaH₂PO₄·H₂O–NaH₂AsO₄·H₂O system have lead to the synthesis of a new compound NaH₂(PO₄)_0.48(AsO₄)_0.52·H₂O (NDAP). Calorimetric studies have confirmed the presence of four reversible phase transitions (abbreviated by PhT), at 257/270 (PhT, IV), 261/290 (PhT, III), 267/301 (PhT, II) and 317/317.5 K (PhT, I) (for cooling/heating processes, respectively). It is shown that the III and IV phase transitions are of a first order type, with a “order-disorder and displacive” character, accompanied by specific dielectric anomalies. The behavior of the dielectric constant ε′_r and of tan δ shows that, at 272 K, the (PhT, IV) could be ferroelectric–paraelectric. As for the (PhT, III) at 296 K, it leads to a superionic–protonic phase; a jump in the conductivity is associated to this transition with an unusual high value of conductivity 1.07×10^?4 Ω^?1 cm^?1 and a low activation energy 0.39 eV (Kh. Jarraya et al.). Quandratic nonlinear (NLO) properties of NDAP powder was confirmed efficiency of the grown crystal by the Kurtz and Perry second harmonic generation (SHG) technique.     

Vibrational study of H3OUO2PO4·3 H2O (HUP) and related compounds. Phase transitions and conductivity mechanism: Part II,H3OUO2PO4·3 H2O

Infrared and Raman spectra of polycrystalline H₃OUO₂PO₄·3 H₂O (HUP) and its D and P¹⁸O₄ derivatives, in the form of dense transparent disks and wet powder, have been investigated at various temperatures in the 100–300 K region. The bands due to framework vibrations are similar to those of KUP, whereas those for the protonic species are different. OH stretching and bending bands of the oxonium ion have been identified at 2920, 1740 and 1160 cm^?1 in the low-temperature spectrum of HUP. Differential scanning calorimetry (DSC) and infrared (IR) intensity investigations show a phase transition between 274 and 260 K. The mechanism of the phase transition consists, as in the case of KUP, of ordering of the protonic species, which induces ordering of PO₄ tetrahedra. The ordering can be influenced by excess water content, stacking faults and stress (ferroelastic behaviour is evidenced). The conductivity mechanism in HUP is discussed.     

EPR studies of phase transitions in perchlorates [M 2+(ClO4)2 · 6H2O] at high pressures

The EPR spectra of the Mn²⁺ ion in crystals of the perchlorate hexahydrates Zn(ClO₄)₂ · 6H₂O, Mg(ClO₄)₂ · 6H₂O, and Cd(ClO₄)₂ · 6H₂O were studied in the temperature range 77–320 K under hydrostatic pressure. It is shown that the octahedron of six molecules H₂O surrounding this paramagnetic ion is contracted along the c axis and that pressure decreases this distortion. The second-order phase transition that occurs near 200 K in the perchlorates and in other crystal hydrates is shown to be associated with changes in the bonds in the nearest ligand environment. As the pressure is increased, the phase-transition temperatures shift and the perchlorate crystals tend to a single-phase state. The low-temperature phase is assumed to disappear as the pressure increases, and this phase exists in a closed T-P region in the phase diagram. As the pressure increases, the character of the high-temperature transition in the Cd(ClO₄)₂ · 6H₂O changes: the jumplike transition at T ₁ with a 1-K hysteresis changes into a smooth transition and then disappears as the pressure increases further.     

Electrical and Optical Properties of Cu<Subscript>2</Subscript>Zn(Fe,Mn)SnS<Subscript>4</Subscript> Films Prepared by Spray Pyrolysis

We have analyzed the electrical and optical properties of Cu₂ZnSnS₄, Cu₂FeSnS₄, and Cu₂MnSnS₄ films with the p-type electrical conductivity, which were prepared by spray pyrolysis at temperature TS = 290°C using 0.1 M aqueous solutions of salts CuCl₂ · 2H₂O, ZnCl₂ · 2H₂O, MnCl₂ · 2H₂O, FeCl₃ · 6H₂O, SnCl₄ · 5H₂O, and (NH₂)CS. The energy parameters have been determined from analyzing the electrophysical properties of the films using the model of energy barriers at grain boundaries in polycrystalline materials, and the thickness of intercrystallite boundaries has been estimated. The extent of the influence of the hole concentration p₀ in the bulk of crystallites and height E _b of the energy barriers between grains on the electrical conductivity has been determined. The optical bandgap width for thin Cu₂Zn(Fe,Mn)SnS₄ films has been calculated based on analyzing the spectral dependences of the absorption coefficient.