Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

A study of Ba x Mg1 − x F2 optical films

Ba _x Me_{1 ? x} F₂ binary fluoride films (“Me” denotes calcium or magnesium fluoride) are studied. A method of processing the reflection and transmission spectra is proposed to determine the optical constants. The dispersion dependences of the refractive indices and absorption coefficients of films in the range of 1.3–12 μm are found. Dispersion in films in the regions of additional absorption bands, which are absent in single crystals, is observed for the first time. It is shown that the films of binary fluorides have a higher packing density, a lower absorption, and better operating characteristics than do films of pure fluorides. The films are promising for application as optical interference coatings in the mid-IR spectral region.     

Specific features of the temperature characteristics of electrical conductivity and photoconductivity of polymer composites containing Cu(II)/Cr(III) heterometallic complexes

The temperature characteristics of the electric current and the photocurrent in films of composites based on electrically neutral poly(vinyl butyral) with additions of Cu(II)/Cr(III) heterometallic cation-anion complexes are investigated. The electrical conductivity and photoconductivity of the polymer composite films in the visible optical range increase with a decrease in the distance between the metal centers in the complexes and upon introduction of acceptor additions of the C₆₀ fullerene into the composition of the polymer binder and increase exponentially with increasing temperature. The activation energy of electrical conduction and photoconduction exceeds 1 eV and depends weakly on the strength of the external electric field. The temperature characteristics of the electrical conductivity and photoconductivity of the materials under investigation are explained by the specific features of trapping of charge carriers at the interface between particles of the heterometallic complex and the polymer binder.     

Effect of uniaxial pressure on the Raman spectra of fluoro perovskites containing manganese with sodium or potassium

ABSTRACT

Perovskite structured mixed metal fluorides containing manganese/sodium or potassium have been synthesized in pure form by a greener precipitation route and characterized by high-resolution powder X-ray diffraction and Raman spectroscopy techniques. While all the reflections in the powder X-ray diffraction pattern of potassium manganese fluoride could be indexed in cubic symmetry with a = 4.1889 Å, sodium manganese fluoride showed reflections at positions typical of orthorhombic symmetry (Pnma space group) with a = 5.751, b = 8.008, and c = 5.548 Å. Potassium manganese fluoride in powder form showed bands at 209, 291, 386, 558, 621, and 733 cm^?1 in the Raman spectrum at room temperature. All these bands disappeared and second-order band at 1151 and 1298 cm^?1 emerged when the powders were compacted under pressure ranging between 1 and 4 tons (uniaxial). A similar change was noticed for sodium manganese fluoride in which bands at 1099, 1149, 1203, and 1286 cm^?1 were observed for the compacted samples. The response of the vibrational modes of these compounds to uniaxial pressure revealed the existence of large structural disorder in them. Additionally, the need for the extreme care to collect and interpret Raman data of polycrystalline samples of these systems has been illustrated through this study.     

The effect of oxide additions on the electrical conductivity of the low-temperature form of Ta2O5

The effect of foreign oxide additions on the electrical conductivity of L-Ta₂O₅ is discussed. It is concluded that lower valent oxides have no doping effect (i.e. increased and extended ionic conductivity by substitutional dissolution of lower valent cations) on L-Ta₂O₅. The conductivity data have been interpreted in terms of different dispersion models reflecting conduction in a mixture of L-Ta₂O₅ and tantalate phases.     

Physical properties of multicomponent fluoride glasses for photonic and superionic applications

Heavy metal fluoride glasses are promising materials for ultra-low loss mid-infrared optical fibers. The fibers are applied in remote spectroscopy, laser surgery, and thermal imaging. Upon doping with rare earth ions, heavy metal fluoride fibers are suitable for a development of high power laser materials, up-conversion lasers, and optical amplifiers for telecommunications systems. As heavy metal fluorides are prospective fast fluoride ion conductors, fluoride glasses based on ZrF₄, BaF₂, LaF₃, AlF₃ and NaF (ZBLAN), PbF₂, InF₃, BaF₂, AlF₃, LaF₃ (PIBAL) or ZnF₂, BaF₂, InF₃, SrF₂, AlF₃, NaF (ZBISAN) are interesting for a development of glassy or fibrous ionic conductors. In this paper, the ionic conductivity and dielectric response of the abovementioned multicomponent fluoride glasses is studied. The influence of the glass composition on the glass transition temperature (T_g) and on the crystallization temperature (T_cr) is also reported. The optimum composition and drawing temperature for fluoride glass fibers is specified. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Electrical conductivity of polycrystalline BiVO4 samples having the scheelite structure

Polycrystalline samples of the scheelite oxide BiVO₄ have been prepared with and without 1 m/o additions of CaO. Using impedance spectroscopy the electrical conductivity of these samples was measured in the temperature range 140–550°C and data for the low temperature regime (140–300°C) interpreted in terms of ionic conductivity due to oxygen ion vacancies. This interpretation suggested that the enthalpy of motion of anion vacancies in these materials is relatively low (∼ 0.33 eV).     

Dielectric parameters of mesoporous sieves filled with NaNO<Subscript>2</Subscript>

This paper reports on a dielectric study of MCM-41 molecular sieves with cellular channels of different sizes filled with the NaNO₂ ferroelectric. The temperature dependences of the permittivity and electrical conductivity of sodium nitrite in cellular channels are calculated from experimental data on the permittivity and electrical conductivity of the composite. The calculations are performed using the relationships obtained for the hexagonal matrix with parallel cylindrical inclusions within pores. The observed increase in the conductivity of sodium nitrite in confined geometry at high temperatures is attributed to partial melting. It is shown that the increase in the permittivity of the composite is caused by Maxwell-Wagner relaxation processes.     

Substitution of aluminium by gallium in β-alumina

In the two-block structure of β-alumina the effects of the replacement of aluminium by gallium have been investigated. As alkali ions sodium and potassium have been chosen. The alkali to trivalent metal ratio varied between 1:6 and 1:7. In this composition range for the sodium derivative substitution could be accomplished up to an aluminium to gallium ratio of about 1:9 without the presence of a second phase. For potassium the two-block β-alumina phase does exist for the aluminate and for the gallate systems. A complete range of solid solutions appears to exist for these two compounds.From sintered, polycrystalline pellets (with a density between 85 and 90 per cent of the theoretical density), X-ray powder diffraction diagrams have been recorded. The unit cell parameters increase linearly with the gallium content in accordance with the larger radius of the trivalent gallium ion. The expansion of the lattice parameters did not change the electrical conductivity of the sodium β-alumina samples. However, for the analogous potassium specimens a decrease of the electrical conductivity has been observed with increasing gallium content.     

Anisotropy of the ionic conductivity and dielectric response in orthorhombic and monoclinic inorganic fluorides

The anisotropy of the ionic conductivity and permittivity of (1) BaR₂F₈ (R=rare earth element) single crystals with monoclinic BaTm₂F₈ structure and of (2) (β-YF₃ structured) rare earth trifluorides is studied. Single crystals, eutectic composites and ceramics are investigated in a broad temperature range. In the monoclinic BaR₂F₈ crystals, a pronounced anisotropy of the ionic conductivity and a slight anisotropy of the permittivity are found. The fastest ionic transport with the lowest activation energy (0.563 eV) and the highest value of the permittivity are observed along thea axis. The temperature dependencies of the fluoride ion conductivities of various orthorhombic rare earth trifluorides differ only slightly from one another. For the bulky single crystals, the conductivity at 500 °C and the conduction activation enthalpy are equal to 1.1(4)×10⁻⁵ S/cm and 0.75(3) eV, respectively. The ionic conductivity is almost isotropic, but the anisotropy of the permittivity is significant. For the fluorides of both structural types, plausible conduction mechanisms are proposed, networks of most probable conduction paths are presented and the origin of the observed anisotropy of the ionic conductivity is elucidated. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Electrical and optical properties of pure and KClO3-doped P(MMA-CO-4VPNO) polymer electrolyte films

Ion-conducting polymer electrolyte films based on a copolymer poly(methyl-methacrylate-co-4-vinyl pyridine N-oxide) [P(MMA-CO-4VPNO)] complexed with potassium chlorate (KClO₃) were prepared by solution cast technique. The complexation of KClO₃ salt with the polymer was confirmed by X-ray diffraction and infrared studies. The electrical conductivity and optical absorption of pure and KClO₃-doped P(MMA-CO-4VPNO) polymer electrolyte films have been studied. The electrical conductivity increased with increasing dopant concentration, which is attributed to the formation of charge transfer complexes. The variation of electrical conductivity with temperature shows two regions with two activation energies. Optical properties like direct band gap, indirect band gap, and optical absorption edge were investigated for pure and doped polymer films in the wavelength range 300–550 nm. It was found that the energy gaps and band edge values shifted to lower energies on doping. The behavior is in an agreement with the activation energies obtained from the conductivity data.     

Raman and Infrared Spectroscopic Characterization of the Silicate Mineral Lamprophyllite

The mineral lamprophyllite is fundamentally a silicate based upon tetrahedral siloxane units with extensive substitution in the formula. Lamprophyllite is a complex group of sorosilicates with general chemical formula given as A₂B₄C₂Si₂O₇(X)₄, where the site A can be occupied by strontium, barium, sodium, and potassium; the B site is occupied by sodium, titanium, iron, manganese, magnesium, and calcium. The site C is mainly occupied by titanium or ferric iron and X includes the anions fluoride, hydroxyl, and oxide. Chemical composition shows a homogeneous phase, composed of Si, Na, Ti, and Fe. This complexity of formula is reflected in the complexity of both the Raman and infrared spectra. The Raman spectrum is characterized by intense bands at 918 and 940 cm^?1. Other intense Raman bands are found at 576, 671, and 707 cm^?1. These bands are assigned to the stretching and bending modes of the tetrahedral siloxane units.     

Ionic conductivity in the single crystals of non-stoichiometric fluorite phases M1−xRxF2+x (M=Ca,Sr, Ba; R=Y,La-Lu)

The ionic conductivity has been measured of single crystals of rare earth fluoride solid solutions in Ca, Sr and Ba fluorides described by the formula M_1?xR_xF_2+x (M=Ca, Sr, Ba; R=Y, La-Lu). The measurements have been done using dc and ac within the temperature range 300–850 K. With the increase of RF₃ concentration up to x=0.05–0.15 (in different systems) conductivity steeply increases. With the further concentration growth the conductivity increases only slightly, reaching saturation value for the solid solution Ba_1?xR_xF_2+x. The dependence of solid solution conductivity on the chemical composition has been studied for isoconcentrates M_0.9R_0.1F_2.1. It has been established that in CaF_2- based series the crystals with R=Gd, Tb possess maximum conductivity and minimum activation energy. For SrF₂ and BaF₂-based series the highest conductivity was observed for crystals containing LaF₃. It has been established that for all the crystals, independent of the chemical composition and defect concentration, the conductivity logarithm at definite temperature linearly depends on the activation energy. Within the investigated class of substances the optimum composition for solid electrolytes is Sr_0.69La_0.31F_2.31.     

Growth and dc conductivity studies of tripotassium sodium dichromate single crystal

DC electrical conductivity studies were carried out along the three crystallographic axes for tripotassium sodium dichromate (K₃Na(CrO₄)₂ or KNCr). Earlier studies of phase transition in this crystal show successive phase transitions at 239 K and 853 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303–430 K along the crystallographic axes. An anomaly in conductivity was obtained around 326 K along both the axes. This may be attributed as due to a newly observed phase transition in the crystal. DSC taken for the sample also shows exothermic peak supporting the occurrence of newly observed phase transition.     

X-ray C K-emission band spectra of graphite fluoride

X-ray C K-emission band spectra of graphite fluoride were measured for the first time by the use of the electron beam excitation. The problem of first importance in obtaining X-ray spectra was the decomposition of graphite fluoride induced by electron beams. The decomposition of the samples during the measurements was controlled by moving the samples at a constant rate. The spectra thus measured showed strong high energy satellite bands. Peak positions of these satellites were 5 eV and 8 eV higher relative to that of the main band. Each satellite was considered to be attributable to the CF and CF₂ bonding, respectively. A similar spectrum was obtained from graphite fluoride, (C₂F)_n. Owing to the zig-zag form of the C-C bond, the polarization of the C K-emission bands of well-ordered flaky graphite fluorides was not observed distinctly.     

Etude des proprietes structurales et electriques de la solution solide Pb1−xBixOxF2−x

An investigation of the PbF₂BiOF system at 600°C has allowed to isolate a Pb_1-xBi_xO_xF_2-x solid solution for x ? 0.67 the structure is of fluorite type and for 0.67 < × ? 0.80 it presents a fluorite derived rhombohedral distortion. The ionic conductivity has been examined. For × ? 0.50 Pb_1-xBi_xO_xF_2-x has electrical performances of the same order than those of the best so far known fluorides. Bragg neutron diffraction investigations have been carried out in order to account for the electrical behavior of the cubic solid solution (0 < x ? 0.67). The distribution of the anions between normal and interstitial sites is given as a function of substitution rate. The structure has been described on the basis of local BiOF-type clusters. The formation of isolated antiprisms BiX₈ (X = O,F) allows to understand the evolution of the electrical properties.     

Influence of composition nonstoichiometry on the electrical conductivity of LiNaGe4O9 crystals

The electrical conductivity σ of Li_{2 ? x} Na _x Ge₄O₉ (x = 1, 0.5, 0.2) crystals in an alternating-current electric field has been investigated at a frequency of 1 kHz in the temperature range of 300–800 K. A considerable anisotropy of the electrical conductivity has been revealed for crystals with a sodium concentration x = 1 at T > 500 K. It has been shown that the electrical conductivity σ along certain crystallographic directions increases by more than three orders of magnitude with a change in the sodium concentration from x = 1 to x = 0.2. The results have been discussed taking into account the specific features of the structure of the crystals under investigation. Presumably, the major charge carriers are interstitial Li ions migrating along channels of the framework structure of the Li_{2 ? x} Na _x Ge₄O₉ crystals.     

The Role of Infrared Spectroscopy and X-Ray Diffraction Analysis in the Investigation of the Influence of Fluorides on the Process of Calcination of Aluminium Hydroxide

The aim of this work was to check the possibility that by use of infrared spectroscopy the influence of fluorides on the calcinations process of aluminum hydroxide can be followed. The DTA method shows it clearly. It was, namely, found that the added fluoride influences the process of polymorphous transformation of the formation of α-Al₂O₃ from Al(OH)₃. According to DTA, IR, and X-ray analyses, it was found that the added fluoride lowers the temperature of the mentioned transformation for some 300 degrees.

For this purpose recorded were IR spectra and X-ray difractograms of pure AI(OH)3 as well as of such samples with added fluoride, in this case of AlF₃, in the range of 0. 1 to 10 per cent. The samples were examined in the temperature range of 298 to 1673 K in different time intervals.     

Mössbauer spectroscopy of perovskite-related oxide fluorides of composition Ba0.5Sr0.5FeO2F at elevated temperatures

The discovery of superconductivity in perovskite-related oxide fluorides of composition Sr₂CuO₂F_2?+?x has stimulated interest in the synthesis and characterisation of other inorganic oxide fluorides with related structures. Recently a new low temperature preparation of inorganic oxide fluorides has been reported, which entails heating the precursor oxide with the polymer poly (vinylidene fluoride) to fluorinate oxygen deficient strontium ferrite, SrFeO_{3??? δ} . The oxygen-deficient compounds Sr_0.5Ba_0.5FeO_{3??? δ} has been fluorinated to give a compound of composition Sr_0.5Ba_0.5FeO₂F. The magnetic transition temperature for this compound is determined by Mössbauer spectroscopy to be 670(±10) K.