293-K conductivity optimization for single crystals of solid electrolytes with tysonite structure (LaF3): I. Nonstoichiometric phases R 1−yCayF3−y (R = La-Lu,Y)

The systematic optimization of single-crystal fluoride-conducting solid electrolytes R _{1 ? y} Ca _y F_{3 ? y} with a tysonite type structure (LaF₃) with respect to the conductivity at room temperature, σ(293 K), is based on high-temperature measurements of σ(T) of stoichiometric fluorides of rare earth elements, RF₃ (R = La-Nd), in dependence of the radius \(R^{3 + } (r_{R^{3 + } } )\) ; two-component stoichiometric La_{1 ? y} R _y F₃ phases (R = Pr, Nd) in dependence on the average cation radius (r _cat ); and two-component nonstoichiometric phases R _{1 ? y} Ca _y F_{3 ? y} (R = La-Lu, Y) in dependence of the CaF₂ content. The optimization of the composition with respect to thermal stability is based on studying the phase diagrams of CaF₂-RF₃ systems and the behavior of R _{1 ? y} Ca _y F_{3 ? y} crystals upon heating when measuring temperature dependences σ(T). Singlecrystal samples of a number of investigated R _{1 ? y} Ca _y F_{3 ? y} compounds has σ(293 K) values high enough to be applied in solid-state electrochemical devices operating at room temperature (chemical sensors, fluorine-ion batteries, and accumulators) and in devices subjected to thermal cycling.     

Increase in the Fluorine-Ion Conductivity of Single Crystals of Tysonite-type CeF<Subscript>3</Subscript> Superionic Conductor by Substituting Polarized Cd<Superscript>2+</Superscript> Ions for Ce<Superscript>3+</Superscript> Ions

The fluorine-ion conductivity of single crystals with a tysonite (LaF₃) structure with heterovalent isomorphic substitutions of highly polarizable Cd²⁺ cations with a 18-electron shell for rare earth ions Ce³⁺ have been studied for the first time. Ce_0.995Cd_0.005F_2.995 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. The fluorine-ion conductivity of single crystal is measured by impedance spectroscopy in the temperature range from 153 to 1073 K, where it increases by a factor of 10⁹, approaching the value σ_dc = 5 × 10^–2 S/cm at 1073 K. At T₀ = 450 ± 20 K, the dependence σ_dc(T) is split into two portions with the ion-transport activation enthalpy ΔH_σ = 0.39 ± 0.01 eV (T < T₀) and ΔH_σ = 0.23 ± 0.02 eV (T > T0). It is found that at T = 293 K the conductivity σ_dc = 3 × 10^–5 S/cm of Ce_0.995Cd_0.005F_2.995 crystal is higher by a factor of 10 than the conductivity of the tysonite matrix CeF₃ and close to the σ_dc value for Ce_0.995Sr_0.005F_2.995 crystal. This finding indicates a significant effect of the substitutions of Cd²⁺ ions for Ce³⁺ on the σdc value and the advantage of Cd²⁺ ions over Ca²⁺ and Ba²⁺ from the viewpoint of increasing σ_dc.     

Präparation und Charakterisierung von Granateinkristalloberflächen

The preparation of single crystal substrates of gadolinium gallium garnet as a typical example of a brittle material is described. The mechanical polishing with iron oxide and diamond as well as a mechano-chemical polishing procedure basing on SiO₂/H₃PO₄ are investigated. Working damage is investigated by X-ray topography, double crystal spectrometry and selective etching. Results are discussed in the frame of a brittle fracture model of the abrasion process.     

Field-dependent negative-U model and switching in chalcogenide glasses

Using the Anderson negative-U model, with harmonically bound ionic charges, one finds that the electric dipoles of singly (D₀) and doubly (D₋) occupied dangling bonds can be streched by a strong electric field. The D₀ and D₋ energies become field-dependent, with the splitting E₀(F₋)−E_(F) vanishing at a critical field F_c when D₀ becomes more stable. Assuming that Zener tunneling to extended states occurs only from singly occupied states, but not from D₋ states, a qualitative picture for Ovshinsky switching follows: there will be a jump in the D₀ occupation and hence in the carrier concentration at F = F_c. Field magnitudes and delay times are estimated. Memory switching is also discussed.     

Hydrothermal synthesis and ionic conductivity of CdF2 and low-temperature modifications of PbF2 and PbSnF4

Single crystals of CdF₂ and low-temperature modifications of PbF₂ and PbSbF₄ are synthesized from high-temperature hydrothermal solutions in the MF₂-M′F₂-HF (KF, NH₄F)-(Pb)-H₂O systems, where M = Cd and Pb and M′ = Sn. Ionic conductivity of the synthesized phases showed pronounced anisotropy of fluoride-ion transport. Conductivity of α-PbF₂ measured along two directions equals σ_∥a = 1.4 × 10^?3 Sm/cm and σ_∥c = 2.3 × 10^?4 Sm/cm.     

Influence of nitrogen-containing reducing media on the optical and luminescence characteristics of sapphire

The optical and luminescence characteristics of sapphire crystals grown in nitrogen-containing reducing media based on CO have been investigated. It is shown that the nitrogen dissolution in the oxygen sublattice of sapphire under reducing conditions is accompanied by the formation of F ⁺ centers and aggregate F ₂ centers (defects that significantly influence the optical absorption and X-ray-, photo-, and thermoluminescence in sapphire). It is established that the formation of Al₂O₃:N occurs at fairly low nitrogen concentrations in the growth medium (10?C20 vol % when a crystal is grown at P ?? 0.2 Torr and 1 vol % when growth is carried out at P = 800?C950 Torr).     

Ionic conductivity of cold-pressed ceramics from grinding of R0.95M0.05F2.95 solid electrolytes (R = La,Nd; M = Ca,Sr, Ba) synthesized by reaction in melt

Cold-pressed ceramics of fluorine-conducting solid electrolytes La_{1 ? y} M _y F_{3 ? y} (M = Ca, Sr, Ba) and Nd_{1 ? y} Ca _y F_{3 ? y} with y = 0.95 have been synthesized in a melt of RF₃ (R = La, Nd) and MF₂ components in a fluorinating atmosphere and ground in a ball mill. The as-prepared ceramics require annealing, during which their porosity decreases and the conductivity is stably increased (by a factor of 250 for the R _{1 ? y} M _y F_{3 ? y} composition at 293 K). The Nd_0.95Ca_0.05F_2.95 and Nd_0.95Ca_0.05F_2.95 compositions have a maximum ionic conductivity σ(293 K) ～ 5 × 10^?6 Sm/cm. This value is larger (by a factor of about 10) than σ (293 K) for the R _{1 ? y} M _y F_{3 ? y} ceramics of tysonite phases prepared by mechanochemical synthesis with the cold pressing of reaction products.     

Indentation response of glass with temperature

The mechanical response of different glasses to a Vickers indentor has been investigated between room temperature and T_g+50 °C. The permanent deformation, from which hardness is estimated, as well as the brittle fracture characteristics, allowing for an evaluation of the fracture toughness, were measured and analysed. Comparison between a standard float glass and advanced glasses such as chalcogenide (with mainly covalent bonding) and metallic glasses was made to get a more general insight into high temperature indentation behaviour. As temperature increases, the glass response becomes more and more time-dependent, and in the vicinity of T_g the permanent deformation was observed to increase rapidly for all glasses. Further, while the standard float glass showed an enhanced apparent toughness at elevated temperatures due to a brittle to ductile transition, almost no change in apparent toughness was revealed in the GeAsSe glass emphasizing the time-dependent response of glass at elevated temperature.     

Ionic conductivity of binary fluorides of potassium and rare earth elements

The ionic conductivity s of KYF₄ and K₂RF₅ single crystals (R = Gd, Ho, Er) and KNdF₄ and K₂RF₅ ceramic samples (R = Dy, Er) has been studied in the temperature range of 340–500°C. A comparative analysis of the σ values for these objects has been performed. Binary fluorides of potassium and rare earth elements were synthesized by the hydrothermal method (temperature 480°C, pressure 100–150 MPa) in the R₂O₃–KF–H₂O systems. The σ values of tetraf luorides are 3 × 10^–5 S/cm (KYF₄ single crystal) and 3 × 10^–6 S/cm (KNdF₄ ceramics) at 435°C. A K₂ErF₅ single crystal with σ = 1.2 × 10^–4 S/cm at 435°C has the maximum value of ionic conductivity among pentafluorides. The anisotropy of ionic transport was found in K₂HoF₅ single crystals, σ_∥c/σ_⊥c = 2.5, where σ_∥c and σ_⊥c are, respectively, the conductivities along the crystallographic c axis and in the perpendicular direction.     

Nanocrystallization and fracture characteristics in Co-based ribbons

The fracture characteristics, Young’s modulus and Vickers hardness of nanocrystalline materials obtained from the amorphous precursor Co₆₆Fe₄Mo₂Si₁₆B₁₂ by thermal treatments at two different temperatures (slightly and well below the crystallization temperature) have been studied. The results show that these nanocrystalline materials are very brittle and the plastic flow involved in the fracture process practically disappears in the case of the material obtained at the temperature slightly below the crystallization one. In addition, an interesting nanolamellar structure has been observed in this sample.     

Structures of Azomethine Imines of the Pyrazolidone-(3)-type, (III) The Structure of 1-[Anthryl-(9)-methylene]-pyrazolidone-(3)-betaine

C₁₈H₁₄N₂O, Pbca, a = 19.541(4) Å, b = 14.800(2) Å, c = 9.572(3) Å, Z = 8, U = 2768.3 ų, D_x = 1.32 g cm⁻³, μ(MoKα) = 0.93 cm⁻⁻¹, crystal size 0.5 × 0.2 × 0.1 mm, colourless. Final R = 0.077 for 1177 independent reflections excluded those with |F₀| < 8 σ(F₀). The intensities were measured with an automatic diffractometer. The azomethine imine unit of the molecule has a geometry in approximate agreement with a polymethinic electron structure. The conjugation of the azomethine imine unit with the anthryl unit is only small. The anthryl ring and the five membered pyrazolidone ring include a dihedral angle of 65.4°. The main intermolecular forces are C–H … N hydrogen bridges.     

The structure of Tetraphenyl Imidodiphosphate,C24H21NO6P2

M_r = 481.24, monoclinic, P 2₁/c, a = 12.009(3), b = 12.305(5), c = 18.993(7) Å, β = = 122.1(1)⁰, V = 2377.2(6) ų, D_x = 1.349 Mgm⁻³, Z = 4. Final R = 0.051 for 2690 independent reflections excluding those with |F₀| < 4 σ(F₀). Intensities were measured with an automatic diffractometer. The two phosphoryl oxygen atoms are in trans position. The acidic hydrogen atom is bonded at the nittogen atom. The molecules form dimers connected by N H … O intermolecular hydrogen bonds. Results are compared with those of the structure of tris(tetraphenylimidodiphosphato)ytterbium (III).     

On the thermodynamic stability of disperse systems

Investigation has been carried out on the effect of the surface activity of capillary or surface active substances (impurities), called surfactants (i.e. the effect of the lowering (Δσ) of the specific surface energy after impurity adsorption at the solid/gas, solid/liquid, liquid/gas or liquid/liquid interface), on the thermodynamic equilibrium in a polydisperse system. Attention has been paid to the fact that the presence of a surfactant with an activity within the range 0 < Δσ < σ₀ (where σ₀ is the specific surface energy of an adsorption-free interface) reduces the thermodynamic driving force for the appearance of the Gibbs-Thomson effect and for the recondensation (recrystallization) processes in the heterogeneous polydisperse system. It is shown that with an impurity activity Δσ = σ₀, the supersaturation becomes zero and the condensed phase formations should be, irrespective of their size and shape, in equilibrium with one another because their vapour pressure becomes equal to that of an infinitely large phase. In this case the Gibbs-Thomson effect completely disappears, the system become thermodynamically stable and, regardless of its polydispersity, it can exist infinitely long. The behaviour of such a heterogeneous system, which is far from its critical point (the system temperature is T ≪ T_critical), is similar to the behaviour of a system in the region of its critical point: the surface tension becomes nearly equal to zero, and the fluctuations sharply increase. The case when the surfactant activity is so high (Δσ > σ₀) that σ_a = σ₀ − Δσ < 0 (σ_a, specific surface energy of the interface in the presence of a surfactant), is of special interest. It represents a paradox in the phase formation theory: as a result of the adsorption of an impurity with such a high activity, the condensed phase surface would be destabilized and the phase would be spontaneously dispersed.     

Review: Indentation size effect,indentation cracks and microhardness measurement of brittle crystalline solids – some basic concepts and trends

Indentation size effect, indentation cracks and microhardness measurement of some brittle crystals are reviewed against the background of the existing concepts of indentation deformation of crystalline solids. Several approaches reported in the literature devoted to relationships between applied indentation test load P and indentation diagonal length d are applied to analyze the experimentally observed normal and reverse indentation size effect (ISE) in brittle compounds. Using typical examples of normal and reverse ISE it is shown that the indentation induced cracking model does not give load‐independent hardness and the final expression describing the experimental data for various compounds is essentially another form of the Meyer law. Analysis of experiment data on crack lengths and indentation diagonals for different indentation loads suggests that the origin of ISE is associated with the processes of formation of indentation cracks following the general concepts of fracture mechanics. The load‐independent hardness H₀ may be determined reliably from plots of P /d against d of the proportional resistance model or of H_V against 1/d as predicted by strain gradient plasticity theories. It was found that the load‐independent hardness of depends on crystal orientation and state of the indented surface. Finally, some comments on determination of fracture toughness and brittle index of crystals are made. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sub-critical crack growth rate of soda-lime-silicate glass and less brittle glass as a function of fictive temperature

Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, K_IC, and slower crack growth rate.     

Stability of Electronic Colour Centres in NaF:Li Single Crystals

The influence of the lithium impurity on the formation and the stability of electronic colour centres is studied in NaF crystals. It is found that F- and M-absorption bands are somewhat displaced to the long wavelengthes when doping with lithium owing to the overlapping with F_A- and M_A-absorption bands. It is shown that the lithium impurity is stabilising one for the charged M-centres though their accumulation in the doped crystals occurs slower. The activation energy is determined for the thermal destruction of M⁺- centres. Thermal destruction of M⁺-centres assumed to be resulting from the anion vacancy removal.     

Rare earths in fluoride glasses: The LnF3AlF3ThF4BaF2 systems and related glasses

Glass compositions have been investigated in the BaF₂AlF₃ThF₄LnF₃ systems (Ln = rare earths or Y). Glasses have been obtained for the compositions 0.2 BaF₂, 0.3 AlF₃, 0.2 ThF₄, 0.3 LnF₃ and 0.3 BaF₂, 0.3 AlF₃, 0.2 ThF₄, 0.2 LnF₃ for the rare earths heavier than samarium. The glassy transition occurs near 450°C and fusion near 750°C; the density is between 5 and 6 and refractive index between 1.48 and 1.50. Large amounts of alkali fluorides (LiF, NaF, KF) may be included in the glass composition. The glassy area has been drawn in the pseudo-ternary system NaF(Al_0.5Y_0.5)F₃(Ba_0.5Th_0.5)F₃. Alkali incorporation leads to a decrease of the characteristic temperatures. Numerous new glassy compositions are given deriving from the basic ternary or quaternary systems with the addition of alkali fluoride, MgF₂ and CaF₂.     

Ordering of tysonite structure in an as-grown Er0.715Ca0.285F2.715 crystal and in a component of annealed two-phase crystal of the Er0.67Ca0.33F2.67 composition

The structures of the as-grown tysonite phase (TP) Er_0.715Ca_0.285F_2.715 as grown and the tysonite component in a crystal of the Er0.67Ca0.33F2.67 composition annealed at 760°C have been determined by X-ray diffraction methods from the main reflections in the sp. gr. P6₃/mmc, Z = 2. Ca²⁺ cations in the Er_0.715Ca_0.285F_2.715 structure occupy the 2c site on the 6₃ axis, thus confirming the conclusion about the symmetrizing effect of alkaline earth cation, which was made previously for the Y_0.715Ca_0.285F_2.715 compound. Er³⁺ cations are disordered over 6h sites around this axis. The Er_0.67Ca_0.33F_2.67 composition contains Er_0.67+δCa_0.33?δF_2.67+δ TP and inclusions of distorted fluorite phase rhβ-Ca₈Er₅F₃₁. The tysonite and two fluorite lattices have no “correct” relationship. Ca²⁺ cations and a half of TP Er³⁺ cations are fixed in the 2c sites, while the other Er³⁺ cations are disordered over the 6h sites. Superstructural ordering occurs in Er_0.715Ca_0.285F_2.715 crystals; however, the process is not completed, as follows from the character of the diffraction pattern. Weak satellite reflections in the diffraction pattern of the Er_0.67Ca_0.33F_2.67 composition are indicative of the development of TP ordering processes but remain unfit for structural calculations.     

Observations of crack patterns in NaSbF4 and NaSb2F7 single crystals

Crack pattern studies on NaSbF₄ and NaSb₂F₇ single crystals have been carried out using Leitz Wetzler microhardness tester. Attempts have been made to understand the nature of the cracks developed around the microindentation impression. The fracture toughness, index of brittleness and crack resistance have also been calculated.     

The Influence of High Temperature Predeformation on Ductility of LiF and BaF2 Single Crystals

The effect of high temperature predeformation on deformation characteristics of ductile LiF and brittle BaF₂ crystals was investigated. The same predeformation, as has been shown previously, leads to the increasing of the limited plastic deformation of MgO crystals by one order of magnitude. It was found that in both investigated crystals, as in the case of MgO crystals, at the test temperature T₂ ≅ 0.1 T_m limited plastic deformation before fracture obviously increases if the specimen was prestrained up to ε_p ≦ 1% at T₁ = 0.5 T_m. This increase is as higher as the plasticity of crystal is lower. In BaF₂ crystals it reaches a factor of 20 and more. The effect is connected with the creation of mobile dislocations during the high temperature predeformation and dislocaton sources for subsequent deformation at lower temperatures and with the homogeneity of the process of plastic deformation. These conditions prevent the appearance of dangerous places in which the fracture can begin.