Viera Trnovcová

Fast ionic conductivity of PbF2:MF2 (M = Mg, Ba, Cd) and PbF2:ScF3 single crystals and composites

Abstract The fluorite-structured heterovalent PbF2:ScF3 and isovalent PbF2:MF2 (M=Mg, Ba, Cd) concentrated solid solutions were prepared as single crystals in a broad concentration range. In the eutectic systems, PbF2–MgF2 and PbF2–ScF3, the eutectic and near-eutectic composites were prepared from the melt. Saturated solid solutions of each system form two phases of these composites. The fast ionic conductivity, static permittivity and microstructure of both the single crystals and the composites are examined. Fastest ionic conductivity, with a conduction enthalpy of 0.389 eV, is found in PbF2:7 m/o ScF3 solid electrolyte. In this material, the fluoride ion conductivity at 500 K is equal to 0.13 S/cm. Plausible models of defect structures and conduction mechanisms are proposed for all examined systems.

Fluoride solid electrolytes containing rare earth elements

Abstract Relations between the structure, ionic conductivity and dielectric properties of fluoride systems of different structures containing rare earth elements were presented. Superionic conductivities, by fluoride ions, of fluorite-structured (MF2-REF3, M=Ba, Pb, RE=La-Lu, Sc, Y), orthorhombic (REF3, RE=Tb-Er,Y), tysonite-structured (REF3-MF2, RE=La-Nd, M=Sr), monoclinic (BaRE2F8, RE=Ho-Yb, Y) fluoride single crystals and eutectic composites (LiF-REF3, RE=La-Gd,Y) were compared. Anisotropy of electrical properties of crystals with a lower symmetry was explained by modeling optimum ionic paths. For explanation of concentration dependences of fast ionic conductivity, models of aggregation of defects into clusters were proposed. In fluorite-structured crystals, the highest ionic conductivity was found for PbF2: 7 mol% ScF3 (at 500 K, σ 500=0.13 S/cm). In tysonite-structured crystals, the highest ionic conductivity was found for LaF3: 3 mol% SrF2 (σ 500=2.4 × 10−2 S/cm). Different types of coordination polyhedrons and their different linking in orthorhombic and tysonite structure explained large differences between conductivities in both structures. Eutectic systems, prepared as directionally solidified composites, enabled to study some orthorhombic fluoride phases (GdF3, SmF3), which cannot be prepared as single crystals. An influence of the orthorhombic-tysonite phase transition on the ionic conductivity was shown.

Microstructure and physical properties of superionic eutectic composites of the LiF–RF3 (R=rare earth element) system

Abstract For the eutectic composites of the LiF–RF 3 (R=rare earth element, Y) system, which were prepared from the melt, the relations between (1) the electrical properties (fast fluoride ion conduction and dielectric response) of the composites and those of both corresponding single phases, (2) the composition, the preparation technique, the microstructure and the electrical properties of the composites, are examined. In the composites containing a tysonite-structured phase, the conductivity is significantly lower than that of the corresponding RF 3 single crystal. A pronounced conductivity enhancement occurs in the LiF–LiGdF 4 composite only. In the LiF–SmF 3 composite, the significant upward jump of the conductivity is incidental to the orthorhombic→tysonite phase transition at 496±5°C. Simulation of the residual stress field in the fibrous LiF–TbF 3 composite is performed. The influence of this field on the crack deflection is discussed.

Fluoride solid electrolytes

Relations between the structure and electrical properties of fluoride systems with different structures are presented. Physical properties of fluorite-structured (MF2-RF3, MF2-AF, MF2-M′F2, M = Ba, Pb, R = La-Lu, Sc, Y, A = Li, Na, K, Rb, M′ = Ba, Cd, Mg), orthorhombic (RF3, R = Tb-Er, Y), tysonite-structured (RF3-MF2, R = La-Nd, M = Sr), and monoclinic (BaR2F8, R = Ho-Yb, Y) fluoride single crystals or ceramics (ErF3, MF2-RF3, M = Ca, Ba, R = La, Gd, Tb, Y), glasses (ZBLAN, PIBAL) and eutectic composites (LiF-RF3, R = La-Gd, Y, PbF2-RF3, R = Ho, Yb, Y, Sc, PbF2-AF, A = Li, Na, PbF2-MgF2) are compared. Anisotropy of electrical properties is explained. Models of aggregation of defects into clusters are proposed. In fluoritestructured crystals, the highest ionic conductivity was found for PbF2: 7 mol % ScF3 (at 500 K, σ500 = 0.13 S/cm). In tysonite-structured crystals, the highest ionic conductivity was found for LaF 3: 3 mol % SrF2 (σ500 = 2.4 × 10−2 S/cm). Different types of coordination polyhedrons and their different linking in orthorhombic and tysonite structures explain a large difference between conductivities in both structures.

Physical Properties of Fluoride Glasses for Ionics

The ionic conductivity and permittivity of glasses based on ZrF4, BaF2, LaF3, AlF3 and NaF (ZBLAN) or PbF2, InF3, BaF2, AlF3 and LaF3 (PIBAL) are studied. The influence of the glass composition on the glass transition temperature (Tg) and on the crystallization temperature (Tx) is reported. For all ZBLAN glasses the temperature dependencies of the ionic conductivity are close one to another (s500 = 8(2)·10-6 S/cm) and their conduction activation enthalpies are equal to 0.82(1)eV. From the point of view of the ionic conductivity, the best glass compositions are the PIBAL50 (50 m/o PbF2) and PIB45 ( 45 m/o PbF2).

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 (Tg) and on the crystallization temperature (Tcr) is also reported. The optimum composition and drawing temperature for fluoride glass fibers is specified.

Electrical properties of heavily doped fluorite-structured BaF2:RF3 (R=La, Pr, Nd, Gd, Tb, Y, Sc) single crystals

The superionic conductivity and dielectric response of heavily doped fluorite-structured Ba1−xRxF2+x (R=La, Pr, Nd, Gd, Tb, Y, Sc; x=0.005–0.45) crystals are reported. The highest ionic conductivity is found for R=Sc and x=0.1. Upon ScF3 doping, small Sc3+ ions rearrange their surroundings, create excessive fluoride interstitial ions and bring about a high ionic conductivity. For each dopant, the concentration dependence of the ionic conductivity is non-linear. A monotonous concentration dependence of the ionic conductivity is found only for La3+ doping. Upon doping with Nd3+, Gd3+, Tb3+, Y3+ and Sc3+ ions, a conductivity maximum is observed at x=0.1–0.2. Upon Pr3+ doping, this maximum is split. The influence of defect clustering on the concentration dependence of the conductivity is discussed.

Martensitic α↔γ Phase Transition and Ionic Conductivity in "Pure" and Doped LiIO3 Single Crystals

The polymorphism and anisotropy of electrical, dielectric, and thermophysical properties of “pure” and doped LiIO3 single crystals are studied. The hexagonal a-LiIO3 phase is a 1-D ionic conductor (along the c axis). The anisotropy of the ionic conductivity of the a-phase can be reduced by doping with transitive metals (Fe, Mo, Ni). The martensitic character of the a«g transition is proved. In the γ-phase, only a slight anisotropy of the conductivity is found. The a«g transition is not an ionic-superionic one.

Ionic conductivity of multicomponent fluorite-structured fluorides

Influence of mixing of homovalent cations on the fast anionic conductivity, static permittivity, and thermal diffusivity is investigated in multicomponent fluorite-structured concentrated solid solutions of alkali earth and rare earth fluorides. Influence of mixing of alkali earth cations on electrical properties of fluoride superionics is studied in Ba0.7 − xSrxLa0.3F2.3 single crystals and Ca0.85 − xSrxNd0.15F2.15 transparent polycrystals. Influence of mixing of rare earth cations on electrical properties of fluoride superionics is studied in Ba0.75La0.25 − xNdxF2.25 and Ba0.35Sr0.35(R1, ..., Ri)0.3F2.3 (i = 1–6; Ri stands for rare earth elements) single crystals. It is shown that the ionic conductivity decreases with increasing difference between mean ionic radii of rare earth and alkali earth cations. Up to 350°C, the fastest ionic conductivity is found in Ba0.35Sr0.35La0.3F2.3 single crystals.

Structural peculiarities and electrical properties of multicomponent oxide crystals. Mixed anionic conductors

Electrical and dielectric properties of anisotropic, “pure” and rare earth doped, K5Bi(MoO4)4 (KBMO), β-BaB2O4 (BBO), and CsLiB6O10 (CLBO) single crystals are studied. Cationic conductivity by alkali ions is found in these crystals. Anisotropy of their electrical and dielectric properties is examined. Relations between the structure and transport mechanisms are discussed.