Mechanochemical synthesis,structure, and properties of solid solutions of alkaline earth metal fluorides: Ma1−xMbxF2 (M: Ca,Sr, Ba)

The capability of mechanochemical synthesis for the formation of solid solutions of alkaline earth metal fluorides M^a_1−xM^b_xF₂ (M: Ca, Sr, Ba) was tested by fluorination of metal acetates and metal hydroxides with ammonium fluoride directly at milling. Evidence was found for a mutual substitution of cations on their lattice positions in Ca_1−xSr_xF₂ and Ba_1−xSr_xF₂ samples. For the Ba/Ca-system this synthesis route is only partially successful. X-ray diffraction and ¹⁹F MAS NMR spectroscopy were used to characterize all samples concerning their crystal structure and local fluorine coordination. Calculations of ¹⁹F chemical shifts with the superposition model along with probability calculations for the intensity of the individual ¹⁹F lines, performed in dependence on the molar composition of the samples, perfectly agree with the experimental findings. The fluoride ion conductivity of as-prepared samples, determined by temperature dependent DC conductivity measurements, is significantly higher than those of crystalline binary fluorides. Moreover, a higher F⁻ ion conductivity is observed for samples with higher mixing grade in the Ca/Sr-and the Ba/Sr-systems.     

Ordre local dans quelques verres fluores du systeme PbF2MIItF2MIIItF3 par EXAFS

The local environment of transition metal (M_t) and lead has been studied by EXAFS for some fluoride glasses in the system PbF₂M^II_tF₂M^III_tF₃ (M^II_t = Mn²⁺, Zn²⁺; M^III_t = Fe³⁺, Ga³⁺). Theoretical phase shifts and backscattering amplitude are used after testing with crystallized fluorides of various structures. Transition metals are sixfold coordinated and M_tF distances are very close to those known in crystallized compounds. Lead has eight to nine fluorine neighbors forming a very distorted polyhedra. Radial distributions, partially corrected for phase shifts, show a very weak second peak but the second neighbors nature and the distances cannot be determined without ambiguity.     

Fast ionic conduction of fluorides with the fluorite-type structure

The authors have studied the Ca_1?xY_xF_2+x (0 < x ? 0.38), Pb_1?xBi_xF_2+x (0 < x ? 0.50), Pb_1?xTh_xF_2+2x (0 < x ? 0.25), MBiF₄ (M = K, Rb, Tl), and PbSnF₄ phases, which are outstanding conductors of the F^? ions. They give some quantitative information about the influence of various structural criteria on the electric properties of these materials.     

Fast Ion Conducting Nanocrystalline Alkaline Earth Fluorides Simply Prepared by Mixing or Manual Shaking

Simple mixing or shaking of alkaline earth hydroxides with ammonium fluoride results in nanocrystalline phase pure metal fluorides MF₂ (M: Ca, Sr, Ba). The formation of the alkaline earth fluorides was investigated by varying the reaction conditions. Evidence was found that just the contact between the starting materials is sufficient for the reaction to take place. X‐ray diffraction, elemental analysis, ¹⁹F MAS NMR spectroscopy, and measurements of DC conductivities were used to characterize the fluorides regarding properties like crystal structure, crystallite sizes, local fluorine coordination, and fluorine ion conductivity. The ¹⁹F MAS NMR spectra of the phase pure fluorides prepared showed several signals, which were assigned to defects, impurities, or geometric distortions. The fluorides prepared by mixing or shaking revealed fluorine ion conductivities several orders of magnitude higher than observed for the respective microcrystalline alkaline earth fluorides. Therefore, the synthesis routine presented in this study may open a path to a very quick and simple synthesis of nanocrystalline fast fluorine ion conductors.     

Structural investigations of lead-strontium fluoroapatites

Solid solutions in the system Pb_(10−x)Sr_x(PO₄)₆F₂, 0?x?10, were obtained as apatitic phases from aqueous medium. They were investigated by X-ray diffraction, chemical analysis and infrared (IR) spectroscopy. The results of the structural refinements indicated that the substitution of lead by strontium induces a regular decrease of the lattice constant “a” and a preferential strontium distribution in site M(1). A progressive shift of the F⁻ ion position along the apatitic channel was detected and confirmed by IR evidence. The different character of the M-F and M-O interactions was invoked to justify the structural differences observed as a function of composition.     

Hydrurofluorures ioniques MF2−xHx (M = Sr,Ba): Synthèse et étude structurale par diffraction des neutrons

The study of the substitution of H^? for F^? ions in SrF₂ and BaF₂ has shown the existence of the cubic hydride fluorides MF_2?xH_x (M = Sr, Ba). The variation of the lattice parameter with hydrogen replacement is stronger than it is in the case of CaF_2?xH_x. The upper limit of substitution diminishes for the series calcium, strontium, barium. Neutron diffraction confirms the simple structural model, already proposed for calcium hydride fluoride: a fluorine cubic structure with a statistical distribution of H^? and F^? on 8(c) sites.     

Fluoride-conducting Electrolytes Based on Doped Oxyfluorides Pb1 – xAlxF2 + x–2yOy

Employing materials with fast transport of fluoride ions in chemical power sources and gas sensors requires synthesizing new materials with a wide range of electric properties. High conductivity of -PbF₂can be provided by introducing trivalent ions, which create excess concentration of fluoride ions in -PbF₂. Experimental data on the ionic conductivity of Pb_{1 – x} M _x F_{2 + x} , where M is In, Sb, or Bi, are presented in [1–3]. For the first time brief information about high ionic conductivity of doped Pb_{1 – x} Al _x F_{2 + x– 2y} O _y was reported in [4]. Here, we study in greater detail ionic conduction of neat Pb_{1 – x} Al _x F_{2 + x– 2y} O _y and that doped with fluorides of transition metals Zn, Cu, Ni, Co, Mn, Cr, V, or Ti in the temperature range 272–473 K.     

Optimisation des facteurs influencant la conductivité anionique dans quelques fluorures de structure fluorine

The large influence in the M_1?xM′_xF_2+x solid solutions (M = Sr, Pb, M′ = Y, In, Sb, Bi) of the covalency of the MF₂ “starting lattice” on the electrical properties of fluorides of fluorite-type structure is clearly shown in a comparative investigation. The influence of the polarizability of the substituting trivalent ion is only significant as far as the starting lattice contains a weakly polarizable cation. Enhancement of the electrical performances of β-PbF₂ by substitution of Pb²⁺ by trivalent cations seems to be due mainly to increasing disordering within the anionic sublattice and hence the role of cationic polarizability is apparently a second-order effect.     

Lead phosphate apatites substituted by rare earth,sodium, and potassium ions

Rare earth-substituted lead apatites of the Pb_10?2xLn_xM_x(PO₄)₆Y₂(Ln = La, Nd, Eu, Gd, Dy, and Y: M = Na and K; Y = F and Cl) systems were prepared and studied by X-ray diffraction and infrared methods. The powder patterns of all the compounds show the apatite-like hexagonal structure. Singlecrystal precession data reveal that the space group of the Pb₆Ln₂Na₂(PO₄)₆F₂ compounds is probably $\text{P}\text{6}$ while that of Pb₆Ln₂K₂(PO₄)₆F₂ is $\text{P6}{}_3\text{m}$. Analysis of the ir spectra of substituted Ca, Ba, and Pb compounds show the effect of substituted ions on the spectra and support the assumption that substitution in the Ba and Pb systems is an ordered process. Ordering of the substituted ions in the systems studied is discussed in view of changes in lattice parameters, size conditions, and polarizing properties of the ions.     

Binary Lead Fluoride Pb3F8

The binary lead fluoride Pb₃F₈ was synthesized by the reaction of anhydrous HF with Pb₃O₄ or by the reaction of BrF₃ with PbF₂. The compound was characterized by single-crystal and powder X-ray diffraction, IR, Raman, and solid-state MAS ¹⁹F NMR spectroscopy, as well as thermogravimetric analysis, XP and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Solid-state quantum-chemical calculations are provided for the vibrational analyses and band assignments. The electronic band structure offers an inside view of the mixed valence compound.     

Superionic Conduction in Complex Fluorides of Antimony(III) MnSbxFy (M—Cations of an Alkali Metal, Ammonium, or Thallium, n = 1–3, and x = 1–4)

Methods of ¹⁹F NMR and impedance spectroscopy are used to investigate the internal mobility of fluoride (ammonium) ions and electrophysical characteristics of complex trivalent antimony fluorides MSb₄F₁₃, MSb₃F₁₀, MSb₂F₇, M₂Sb₃F₁₁, M₃Sb₄F₁₅, and MSbF₄ (M is an alkali cation, ammonium, thallium). The ion motion types in the cationic and anionic sublattices of the fluorides are determined at 150–500 K. The polymorphous transformations in the fluorides are usually phase transitions to a superionic state and their high ionic (superionic) conductivity (σ ≥ 10⁻⁴ to 10⁻² S cm⁻¹ at 400 K) is due to the diffusion motion of ions of fluoride, ammonium, and possibly sodium, potassium, and thallium. The high polarizability of thallium ions favors the development of high mobility of fluoride ions in the fluorides.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 560–572.Original Russian Text Copyright © 2005 by Kavun, Uvarov, Slobodyuk, Brovkina, Zemnukhova, Sergienko.     

Electric properties of oxyfluorides Ba2In2O5−0.5x F x with brownmillerite structure

Synthesis of fluoro-substituted substances based on brownmillerite Ba₂In₂O₅ is carried out. The width of the homogeneity region of the Ba₂In₂O_5?0.5x F _x (0 < x ≤ 0.25) solid solution was established using X-ray analysis. Measurement of temperature dependences of conductivity in atmospheres with different partial pressure of water vapor (pH₂O = 3.3 and 2 × 10³ Pa) showed an increase in conductivity at T ≤ 550°C in a humid atmosphere, which is due to appearance of proton transport. The dependence of conductivity on partial oxygen pressure (pO₂ = 0.21 × 10⁵ to 10^?15 Pa) is studied in the temperature range of 500–1000°C; ion transport numbers are calculated. The method of polarization measurements was used to determine transport numbers of fluoride. Total conductivity is divided into ion (proton, oxygen, and fluoride ion) and electron components. Analysis of concentration dependences of conductivities showed that low concentrations of fluoride allow increasing both the total and partial conductivities (oxygen-ion and proton) and, besides, allow shifting the “order-disorder” phase transition by 100°C to the low temperature range.     

NMR, DTA, and impedance spectroscopy studies of ion mobility, phase transitions, and ionic conductivity in K1 − x (NH4) x SbF4 crystals

The ion mobility, phase transitions, and ionic conductivity in the crystal phases in the KF-NH₄F-SbF₃ system were studied by NMR, DTA, and impedance spectroscopy. An analysis of the ¹⁹F and ¹H NMR spectra showed how the character of ionic motions in the fluoride and proton sublattices changed with temperature. The types and temperature ranges of ionic motions were determined. Diffusion of the fluoride and partially ammonium ions was found to be the dominant form of ionic motion in the high-temperature modifications formed as a result of phase transitions. According to the electrophysical data, the high-temperature K_{1 ? x} (NH₄) _x SbF₄ phases (0.05 ≤ x ≤ 0.75) are superionic, their conductivity reaching ～10^?2–10^?3 S/cm at 450–500 K.     

Cation Effects in some series of mixed alkali fluoride glasses

An investigation of electrical properties of two series of fluoride glasses containing simultaneously LiF and NaF has been undertaken. The alkali fluoride contents are respectively x_AF = 0.10 and x_AF = 0.30 : (ZrF₄)_0.58(BaF₂)_0.25(LaF₃)_0.07(LiF)_0.10-x(NaF)_x and (ZrF₄)_0.50(BaF₂)_0.15(LaF₃)_0.05(LiF)_0.30-x(NaF)_x. In both series, a mixed alkali effect revealed by the existence of a conductivity minimum is apparently observed. It is shown, however, that the minima have a quite different nature and none of them is actually compatible with the classical ?mixed alkali effect”? of mobile cations. In the series x_AF = 0.10, where charge transport is essentially assured by the F^?-ions, this effect results from the presence of non mobile alkali ions of different nature in the vicinity of the F^?-ions. In the series x_AF = 0.30, the electrical properties of the extreme compositions x_LiF = 0.30 and x_NaF = 0.30 are due respectively either to Li⁺-ions only or to mobile F^?-ions only. The variation of the ratio r = Na/(Li + Na) from 0 to 1 corresponds to progressive cross over from a cationic conducting glass to an anionic conducting one. The presence of a minimum of conductivity for 0.50 < r < 0.75 has been explained.     

Surface degradation of fluoride conducting crystals MF2:UF4:CeF3 (M = Ca,Sr, Ba)

The ac electrical response of cell systems composed of single crystals of the concentrated solid solutions M_1?x?yU_xCe_yF_2+2x+y (M = Ca, Sr, Ba and 2.7 < 2x + y < 26.5 m/o), and ionically blocking electrodes has been studied as a function of frequency and temperature. At elevated temperatures the crystals react with traces of oxygen or water vapor. Complex admittance analysis reveals the formation of low-conducting surface layers, contrary to diluted solid solutions which under similar conditions react to form high-conducting surface layers (2). The activation enthalpy for the layer conductivity is substantially larger than that for the bulk conductivity, and equals that for interstitial fluoride ion motion in dilute solid solutions. A mechanism of charge compensation in the layers is presented. After reaction the solid solutions based on CaF₂ show also a surface electronic conductivity. Scanning electron micrographs clearly reveal the surface degradation.     

Etude par résonance magnétique des phénomènes diffusifs dans les phases du système NaFBiF3

¹⁹F and ²³Na wide-line NMR studies are reported on NaBiF₄ and the solid solution Na_1?xBi_xF_1+2x for x = 0.62 and x = 0.68 in the ?100° to +200°C temperature range. No sodium mobility could be detected in NaBiF₄, but the mobility is significant in the solid solution. The fluorine motions are weakly activated (0.28 eV) in NaBiF₄ and result only from short-range motion (correlated motions or reorientations). In the solid solution the concentration of mobile fluoride ions increases with temperature. Compared to the values given by complex impedance measurements, the activation energies in this thermal range (0.18 eV for x = 0.62 and 0.20 eV for x = 0.68) may be correlated with phenomena preceding the long range conductivity which appears at higher temperature.     

19F NMR Study of Antimony(III) Fluoride Complexes in Solution

The state of SbF₃, Sb₃O₂F₅, MSb₃F₁₀, MSb₂F₇, M₃Sb₄F₁₅, MSbF₄, and M₂SbF₃, as well as HF and MF (M = Na, K, Rb, Cs, NH₄, and Tl), in a 0.25 M aqueous solution at room temperature was studied by ¹⁹F NMR. It is found that chemical shifts of the ¹⁹F NMR signals (relative to CFCl₃) for the compounds under investigation vary in the 73.3–157.5 ppm range. For the Sb(III) fluoride complexes, chemical shifts in the ¹⁹F NMR spectra vary in the 73.3–85.5 ppm range (relative to CFCl₃). Although the crystals of Sb(III) fluoride complexes contain different polyhedra that are joined in different ways, the NMR spectra of all compounds under study show one narrow signal. The spectra of aqueous solutions of Sb(III) fluoride complexes do not contain signals of free fluoride ions and oxofluoride Sb₃O₂F₅.     

New anion-conducting fluorite-like solid solution Bi1 − x Te x (O,F)2 + δ (0.28 < x < 0.43) in the BiF3-BiOF-TeO2 system

A new fluorite-like solid solution, II-Bi_{1 ? x} Te _x (O,F)_{2 + δ}, was produced by solid-phase synthesis at 873 K with subsequent annealing, its concentration boundaries were determined, and a scheme of an isothermal (873 K) section of the BiF₃-BiOF-TeO₂ system was proposed. The new phase was characterized by X-ray powder diffraction, electron microscopy, and impedance spectroscopy. Making heterovalent substitutions simultaneously in the cation and anion sublattices, Te⁴⁺ ? Bi³⁺ and O^2? ? F^? allowed one to vary the tellurium cation content x (at constant anion nonstoichiometry δ) or the anion nonstoichiometry δ (at constant tellurium cation content x or constant fluoride ion content), which enabled one to describe the effect of these parameters on the properties of the solid solution. The anion excess δ was found to dominate the unit cell parameter of the solid solution and its ionic conductivity. The conduction within the studied temperature range was proven to be mainly by fluoride ions. It was assumed that the ordering of superstoichiometric anions, or clustering, can manifest itself as the structural modulations of the phase II-Bi_{1 ? x} Te _x (O,F)_{2 + δ} that were detected in this work.     

Bismuth substituted calcium,strontium, and lead apatites

Bismuth substituted apatites of two general types have been prepared: M_10?2xBi_xNa_x(PO₄)₆Y₂ (M = Ca, Sr, Pb; Y = F, Cl) and lead apatites with the Y ion completely vacant Pb_10?(2x+2)BixNa_x+2(PO₄)₆. X-ray powder diffraction patterns of all the compounds show the $\text{P6}{}_3\text{m}$ hexagonal apatite type structure. The change of the lattice parameters and $\text{c}\text{a}$ values with compositions indicate the preference of the bismuth ions to occupy the 6h triangular positions. Bi³⁺ tends to incorporate in apatites with unoccupied halide positions.     

Ionic mobility and conductivity in PbSnF<Subscript>4</Subscript> doped with CaF<Subscript>2</Subscript> from the NMR and impedance spectroscopy data

The ionic mobility and conductivity in the crystalline phases of PbSnF_4–xCaF₂ systems (x = 2.5 mol.%, 5 mol.%, 7.5 mol.%, and 10 mol.%) in the temperature range of 150-500 K are studied by NMR and impedance spectroscopy. The parameters of ¹⁹F NMR spectra, types of ion motions, and ionic conductivity in the PbSnF₄ compound doped with calcium fluoride are found to be determined by the temperature and concentration of calcium fluoride. The specific conductivity of the crystalline phases in the PbSnF₄–CaF₂ systems is rather high at room temperature, and hence, one cannot exclude the possibility to use them for the creation of functional materials with a high ionic (superionic) conductivity.