Interstitial Fluoride Ions Research Articles

Local Structure of Europium‐Doped Luminescent Strontium Fluoride Nanoparticles: Comparative X‐ray Absorption Spectroscopy and Diffraction Study

AbstractRare‐earth based luminescent materials are key functional components for the rational design of light‐conversion smart devices. Stable Eu3+‐doped strontium fluoride (SrF2) nanoparticles were prepared at room temperature in ethylene glycol. Their luminescence depends on the Eu content and changes after heat treatment. The crystallinity of heat‐treated material increases in comparison with as‐synthesized samples. Particles were investigated in solution using X‐ray diffraction, small‐angle X‐ray scattering, and X‐ray spectroscopy. After heat treatment, the size of the disordered nanoparticles increases together with a change of their local structure. Interstitial fluoride ions can be localized near Eu3+ ions. Therefore, non‐radiative relaxation from other mechanisms is decreased. Knowledge about the cation distribution is key information for understanding the luminescence properties of any material.

CONDUCTIVITY OF SOLID SOLUTIONS Pb0,86 xSmxSn1,14F4+x

In the PbF2 – SmF3 – SnF2 system, he­tero­valent substitution solid solutions Pb0.86-xSmxSn1,14F4+x (0 < x ≤ 0.15) with the structure β-PbSnF4 are formed. The unit cell parameters of solid solutions are satisfactorily described by Vegard’s rules. The electrical conductivity of the obtained samples decreases in the entire temperature range compared to Pb0.86Sn1.14F4 due to the introduction of SmF3 (at x≤0.08) in the initial structure. It brings them closer to the values of the electrical conductivity of β-PbSnF4. However, at temperatures above 520 K, the electrical conductivity of solid solutions is almost twice higher than that of the initial phase Pb0.86Sn1.14F4 (σ553 = 0.054 and 0.023 S/cm, respectively). The elect­rical conductivity of solid solutions increases with the Sm3+ content, reaching maximum values at x = 0.1. The Pb0.76Sm0.10Sn1.14F4.10 phases have the highest electrical conductivity and the lowest activation energy (σ373 = 1.08 • 10-2 S/cm). The substitution of Pb2+ ions by Sm3+ ions in the fluoride-conducting phase Pb0,86Sn1,14F4 helps to increase the electrical conductivity by almost an order of magnitude compared to the initial phase and by two orders of magnitude compared to β-PbSnF4. The ionic conductivity activation energy increases in the low-temperature region generally with increasing the SmF3 content and decreases proportionally at temperatures above 430 K. The nature of the dependence of the activation energy on the concentration of the heterovalent substituent and its value indicate that the conductivity of the obtained samples is provided by highly mobile interstitial fluoride ions in the structure of solid solutions. The Hebb-Wagner polarization saturation method was used to determine the electronic conductivity of the samples. It is 2 orders of magnitude lower than the ionic one. The fluorine ion transfer numbers are 0.99 and do not depend on the substituent content.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 15. Concentration Dependence of the Defect Structure of As Grown Nonstoichiometric Phases Sr1 – xRxF2 + x (R = Sm, Gd)

The defect structure of as grown single crystals Sr1 – xSmxF2 + x (x = 0.14, 0.26) and Sr1 – xGdxF2 + x (x = 0.12, 0.19, 0.30) grown under identical conditions was studied by X-ray diffraction. All crystals belong to the CaF2 structure type, sp. gr. $$Fm\bar {3}m$$. All phases contain interstitial fluoride ions in the 48i site and vacancies in the main anionic motif. In Sr0.70Gd0.30F2.30 interstitial fluoride anions were found also in the 4b site; in Sr0.88Gd0.12F2.12, in the 32f site. Relaxation displacements of a part of F(8c) anions to the 32f site are observed in Sr0.86Sm0.14F2.14. Displacements of cations to the 24e site occur in all the phases under study; in the phases with R = Gd, there are also displacements to the 32f site. A model of the defect structure of the phases Sr1 ‒ xRxF2 + x (R = Sm, Gd) is proposed. According to this model, interstitial fluoride anions and R3+ cations are grouped into octacubic clusters [Sr14 – nRnF64 + n]. The average number of R3+ per cluster decreases from 6 to 4.5 with increasing х, the volume of the cluster core increases from 61.2(1) to 65.9(2) A3, and the volume per cluster decreases from 2419.9(5) to 783.5(2) A3.

Conductivity of aliovalent substitution solid solutions Pb1-xRxSnF4+x (R = Y, La, Ce, Nd, Sm, Gd) with β-PbSnF4 structure

The electrical conductivity of Pb1-xRxSnF4+x solid solutions (R = Y, La, Ce, Nd, Sm, Gd), whose lattice is tetragonal and isostructural with β-PbSnF4, has been studied by 19F NMR and impedance spectroscopy. The ultimate percentage of cerium group RE fluorides in such solutions is not over 20.0 mol%, and that of gadolinium trifluoride reaches 12.0 mol%. The fluoride anions in the synthesized samples of Pb1-xRxSnF4+x solid solutions occupy, as in β-PbSnF4, three structurally nonequivalent positions. The charge transfer in complex fluorides of this type at temperatures above 350 K is done by interstitial fluoride ions. At a low substituent concentration (x ≤ 0.07), the electrical conductivity is almost an order of magnitude lower than that of β-PbSnF4. The conductivity of the synthesized solid solutions increases with increasing substituent content. The samples containing 10.0–15.0 mol% RF3 have the maximum conductivity. At the same RF3 content, the conductivity of solid solutions, except those containing gadolinium trifluoride, increases with decreasing R3+ radius (from La to Sm).

Electron Transfer between Different Lanthanide Centers in BaF2 Crystals—Part II: Transfer Mechanisms

Processes of photo-induced electron transfer from a bivalent acceptor lanthanide (Eu, Sm, and Yb) onto a trivalent donor lanthanide (Nd, Sm, Dy, Tm, and Yb) and the inverse thermally activated transfer are studied in barium fluoride crystals. At room temperature, photoinduced electron transfer is accompanied by oncoming displacement of the interstitial charge-compensating fluoride ion. On photoquenching at low temperatures, a bivalent donor lanthanide remains next to the interstitial fluoride ion, which causes a redshift of its 4f–5d absorption bands. The shift increases as the lanthanide size decreases (as in the series Nd, Sm, Dy, Tm, and Yb). A detailed analysis of the mechanisms of photo and thermal electron transfers between the different lanthanide centers in BaF2 crystals is provided.

Transport properties of solid solutions of хKYF4–(1–х)PbF2 system

In the system (1–x)PbF2–x YF3–x KF, solid solutions of aliovalent substitution with fluorite structure at 0.47 < x < 0.69 are formed, in which the fluorine ions are found to be in three structurally nonequivalent positions that differ in the local environment and mobility. There are immobile, locally mobile, and highly mobile anions. The conductivity of the synthesized polycrystalline samples is ensured by highly mobile interstitial fluoride ions, whose concentration and hence electrical conductivity increase on heating. The contribution of the surface conductivity of crystallites is not detected. The electronic component is two orders of magnitude lower than the ionic one.

Topotactic Fluorine Insertion into the Channels of FeSb2O4-Related Materials.

This paper discusses the fluorination characteristics of phases related to FeSb2O4, by reporting the results of a detailed study of Mg0.50Fe0.50Sb2O4 and Co0.50Fe0.50Sb2O4. Reaction with fluorine gas at low temperatures (typically 230 °C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb-F-Sb bridges. To date, these reactions have been observed only when Fe2+ ions are present within the chains of edge-linked octahedra (FeO6 in FeSb2O4) that form the structural channels. Oxidation of Fe2+ to Fe3+ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe3+ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents.

Crystal chemistry of ordered rocksalt-type Ca2NF

Ordered rocksalt-type Ca2NF has a cubic unit cell that is doubled along [100] relative to the rocksalt-type polymorph due to N/F ordering. Crystals for the present study were prepared from a pure, dry KCuF3 precursor by reaction with Ca metal under nitrogen, and analyzed via high resolution single crystal X-ray diffraction. From this analysis, as well as qualitative compositional analysis using energy dispersive spectroscopy, it was determined that interstitial fluoride ions previously interpreted as Frenkel defects are actually due to a non-stoichiometric defect. The refined composition is Ca2N0.925F1.23, and the ordered cubic phase has space group Fd3¯m (No. 227) with a = 10.0301(5) Å and Z = 16.

Ion mobility and conductivity in solid solutions in the KBiF4–ZrF4 system

The ion mobility and conductivity in solid solutions with the fluorite structure (100–x)KBiF4 + xZrF4, where x = 2.5–15 mol %, is studied by the methods of differential scanning calorimetry (DSC), X-ray diffraction (XRD), and 19F NMR. The character and type of ion transport in the fluorite sublattice of solid solutions are studied as well as the temperature intervals in which it is observed (150–570 K). For solid solutions containing 5 and 10 mol % ZrF4, the fluorine ion diffusion coefficient is assessed. It is found that the conductivity in the solid solution systematically decreases as the content of zirconium tetrafluoride in the sample decreases, which is probably due to the formation of strongly bound complexes formed by interstitial fluoride ions with zirconium cations. The presence of the high ionic conductivity in the tested solid solutions (from ~10–3 to 10–2 S/cm) makes this material a good candidate for preparation of materials with the high ion-conducting properties.

Ab-initio calculations of Judd–Ofelt intensity parameters for transitions between crystal-field levels

Wavefunction-based ab-initio calculations of the electric-dipole moments of 4fN–4fN transitions of lanthanide ions are performed to extract the Atpλ intensity parameters. The extraction method is an extension of our earlier calculations of crystal-field (CF) parameters for lanthanide ions in crystals. The CASSCF/RASSI-SO (Complete-Active-Space Self-Consistent-Field/Restricted-Active-Space State-Interaction Spin-Orbit) calculations have been carried out on the chosen model system of CaF2: Ce3+ with an interstitial fluoride ion (Fi−) on z-axis (Ce3+ ion occupying the C4v symmetry site). In consideration of the site symmetry and the coordination situation of Ce3+ ion at C4v site in CaF2 as well as the superposition model (SM), the calculated intensity parameters for Ce3+ ion can be classified into three categories, and detailed discussions are then given.

Atomic mechanisms of superionic conductivity in fluorite

The subsequent melting/crystallization of the two sub-lattices of CaF 2 is explored by molecular dynamics simulations. Both, heating from 100 K and cooling from 2500 K encompasses two transitions, i.e. the melting/recrystallization of the fluoride sub-lattice and the of whole crystal at different temperatures. Solid state F − ion conductivity is observed in a temperature range of about 1500–2000 K which reflects a reasonable agreement with the experiment. A systematic study of fluoride migration revealed the atomistic mechanisms of the spontaneous formation of Frenkel defects, followed by void translocation and terminated by recombination of interstitial fluoride ions and vacancies.

Lanthanum Oxyfluoride: Structure, Stability, and Ionic Conductivity

Structure and phase transition of LaO1−xF1+2x, prepared by solid‐state reaction of La2O3 and LaF3, was investigated by X‐ray powder diffraction and differential scanning calorimetry for both positive and negative values of the nonstoichiometric parameter x. The electrical conductivity was investigated as a function of temperature and oxygen partial pressure using AC impedance spectroscopy. Fluoride ion was identified as the migrating species in LaOF by coulometric titration and transport number determined by Tubandt technique and EMF measurements. Activation energy for conduction in LaOF was 58.5 (±0.8) kJ/mol. Conductivity increased with increasing fluorine concentration in the oxyfluoride phase, suggesting that interstitial fluoride ions are more mobile than vacancies. Although the values of ionic conductivity of cubic LaOF are lower, the oxygen partial pressure range for predominantly ionic conduction is larger than that for the commonly used stabilized‐zirconia electrolytes. Thermodynamic analysis shows that the oxyfluoride is stable in atmospheres containing diatomic oxygen. However, the oxyfluoride phase can degrade with time at high temperatures in atmospheres containing water vapor, because of the higher stability of HF compared with H2O.

Mechanochemical synthesis of nonstoichiometric fluorite Ca1−x LaxF2+x nanocrystals from CaF2 and LaF3 single crystals

The nonstoichiometric Ca{sub 1-x} La{sub x}F{sub 2+x} phase (x {>=} 0.1) is obtained by mechanochemical synthesis from CaF{sub 2} and LaF{sub 3} single crystals. This phase is the first representative of fluorite fluorides obtained by mechanochemical synthesis in the MF{sub m}-RF{sub n} systems (m < n {<=} 4). The average grain size ranges within 10-30 nm. The temperature dependence of ionic conductivity of the mechanochemically synthesized phase pressurized at 600 MPa (at its high-temperature portion at temperatures exceeding 200-250 deg. C) coincides with the conductivity of the single crystals of the same composition (Ca{sub 0.8}La{sub 0.2}F{sub 2.2}). The activation energy of ionic conductivity (0.95 eV) corresponds to migration of interstitial fluoride ions in the crystal bulk. Mechanochemical synthesis of a multicomponent fluoride material with nanometer grains opens a new chapter in the chemistry of inorganic fluorides. A decrease of the sintering temperature of the powders with nanometer grains is very important for preparing dense fluoride ceramics of complicated compositions and other polycrystalline forms of fluoride materials.

Study of Ca 1 −xPr xF 2+ x solid solution thin films grown on silicon substrates

Ca 1 −x Pr xF 2+ x thin films, x varying from 0.01 to 0.10, were grown on Si (100) substrates by sublimation under ultra-high vacuum of solid solution powders obtained from high purity CaF 2 and PrF 3 initial powders. First, the synthesis of the bulk solid solutions was controlled using X-ray diffraction and Raman spectroscopy. A complete transformation seems to be obtained by heating for 8 hours under vacuum the mixed initial powders at a temperature of 1400 K. However, for x=0.10, a Rutherford backscattering (RBS) of α particles study shows that to obtain homogeneous thin films a pre-evaporation of the product at 1500 K during approximately 30 min is necessary. After such a treatment, the films are of the desired composition and the Pr 3+ ions are uniformly distributed within the layers. On the other hand, for the lower praseodymium contents, the alloy produced by heating the initial powders at 1400 K sublimes congruently. Moreover, in this case a RBS study in channeling conditions shows that the films are of epitaxial quality and a photoluminescence study shows that the interstitial fluoride ions, which balance the rare earth trivalent ions in the matrix, are preferentially located in nn (nearest neighbor) sites, of C 4v symmetry (tetragonal).

Probing the Defect Structure of Anion-Excess Ca1-xYxF2+x (x = 0.03−0.32) with High-Resolution 19F Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Fluorine-19 magic-angle spinning nuclear magnetic resonance spectroscopy (19F MAS NMR) was used to study the local structure and fluoride ion environments in the anion-excess cubic fluorite material Ca1-xYxF2+x (0 < x ≤ 0.32). Four distinct 19F resonances were observed at spinning speeds of >20 kHz; these have been assigned to three different types of sites: two interstitial sites, sites that have relaxed slightly from the normal sites of the fluorite structure, and the normal sites. Very similar spectra were observed for the mineral Tveitite (x ≈ 0.25−0.27), which contains an ordered calcium yttrium−fluoride phase with excess fluoride ions located in cuboctahedron clusters. Only three resonances were observed for this high-yttrium-content material, which were assigned to the normal, relaxed, and interstitial fluoride ions that make up the cuboctahedron. The similarity between the spectra of the synthesized materials and Tveitite indicate that similar clusters are present. The resonances due to the interstitial sites are assigned to fluoride ions in cuboctahedra or square antiprisms and the additional interstitial ions associated with these clusters. The changes in intensities of these resonances are consistent with the increase in the cuboctahedron:square antiprism ratio at higher doping levels. Variable-temperature NMR shows that the anions within the clusters are mobile and undergo exchange with each other. At higher temperatures and doping levels, exchange involving the relaxed and normal fluoride ions also occurs.

Analysis of AC Conductivity Data of the Anion Excess Sr1 ? xThxF2 + 2x Solid Solution with Fluorite-Type Structure

Conductivity parameters, number of charge carriers, frequency of hopping, have been determined from an analysis of ac conductivity data of the Sr1 — xThxF2 + 2x (0 ≤ x ≤ 0.225) solid solution with fluorite-type structure. The charge carrier concentration has been estimated: (i) from the frequency dependence of conductivity, in accordance with the Almond-West approach; (ii) from the increment of the dielectric constant due to charge carriers. Similar results have been obtained by both approaches. The fraction of charge carriers per formula unit, evaluated as a function of composition, is in good agreement with the estimations made on the basis of the clustering process model which assumes that the charge carriers at long range in Sr1 — xThxF2 + 2x are interstitial fluoride ions of F′′′ type. For x > 0.05, the results have been interpreted by the additional presence of nn and nnn dipoles, the concentration and reorientation rate parameters of which have been evaluated.

Clustering and anionic mobility inside the Sr 1− xBi xF 2+ x solid solution

The conductivity parameters, number of charge carriers and frequency of hopping, have been determined from an analysis of a.c.-conductivity data of the fluorite-type Sr 1− x Bi x F 2+ x (0 ≤ x ⪅ 0.50) solid solution. The composition dependence of the fraction of charge carriers per unit cell is in agreement with the estimations made on the basis of the clustering process model which assumes that the interstitial fluoride ions of F′ type are the charge carriers at long range in Sr 1− x Bi x F 2+ x .

Estimation of conductivity parameters in Sr 1 − xTh xF 2 + 2 x solid solution with fluorite-type structure

Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr 1 − x Th x F 2 + 2 x solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F i) m ions responsible for long range motions in Sr 1 − x Th x F 2 + 2 x are interstitial fluoride ions of F‴ type.

Estimation of charge carrier concentration in the fluorite-type Pb 1− x− yIn x− yBi 2 yF 2+ x+ y (fixed x, 0 ⩽ y ⩽ x) solid solution involving two substitutional cations

The bulk impedance parameters of polycrystalline samples of the Pb 1− x− y In x− y Bi 2 y F 2+ x+ y (fixed x, 0 ⩽ y ⩽ x) solid solution with fluorite-type structure were determined from analysis of ac conductivity data measured in a wide temperature range. Hopping rates of mobile F − ions were extracted from frequency dependences of conductivity at different temperatures. The concentration of charge carriers in the samples investigated has been estimated. Independently of y, its value ( log N/ cm 3 = 21.6 ± 0.02) allows to propose, on the basis ofthe clustering model, that charge carriers in these materials are likely the interstitial fluoride ions of F″ type.

Failure of the Frenkel Defect Model to Explain the Trend in Anionic Conductivity in the MF2 Fluorite Structure and Related MSnF4 Materials

AbstractThe high fluoride ion conductivity of fluorite type MF2 has been attributed tothe fact that half of the F8 cubes present in this structure are empty and therefore, are potential vacant sites for interstitial fluoride ions in the formation of Frenkel defects. However, the model of long range ion motion through Frenkel defects by use of empty F8 cubes is in contradiction with: (i) the little difference between the conductivities of CaF2 and BaF2, (ii) the conductivity of β-PbF2 being much larger than that of BaF2, and (iii) the much higher performance of MSnF4 even though there is no empty cube to form Frenkel defects in the MSnF4 structures.