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.     

Fluorite M1 − x R xF2 + x phases (M = Ca, Sr, Ba; R = rare earth elements) as nanostructured materials

The structural defects (M ²⁺ and R ³⁺ in the noncubic environment of F^?, interstitial F^?, and anion vacancies) in nonstoichiometric M _{1 ? x} R_xF_{2 + x} crystals with the CaF₂ structure form {M ₈[R ₆F_68-69]} superclusters of nanometer linear dimensions. This fact allows one to classify the M _{1 ? x} R _xF_{2 + x} phases as nanostructured materials (NSM). The superclusters concentrate rare-earth ions (R ³⁺ = RE). In a M _{1 ? x} R _xF_{2 + x} crystal with the fluorite cation motif, two chemically different parts can be separated: the R ³⁺-depleted matrix and the R ³⁺-enriched clusters. The M _{1 ? x} R _xF_{2 + x} phases are the first NSM among fluorides; they constitute a new type of these materials in which different chemical compositions of the matrix and nanoinclusions are combined with their isostructurality and coherent conjugation of the crystal lattices. Superclusters can also form associates with linear dimensions of tens or hundreds of angstroms. A model is suggested which describes the main characteristic of such NSMs. These materials behave as single crystals in X-ray, neutron, and electron diffraction experiments. The influence of microheterogeneity on some physical properties of the M _{1 ? x} R _xF_{2 + x} phases is also considered.     

Growth of R 1 − y Sr y F3 − y crystals with rare earth elements of the cerium subgroup (R = La, Ce, Pr, or Nd; 0 ≤ y ≤ 0.16) and the dependence of their density and optical characteristics on composition

Crystals of the R _{1 ? y} Sr _y F_{3 ? y} phases (R = La, Ce, Pr, or Nd; 0 ≤ y ≤ 0.16) with tysonite (LaF₃) structure are grown by the Bridgman method. A linear dependence of the crystal density on the SrF₂ concentration has been revealed for each series of solid solutions. A scheme of heterovalent isomorphous substitution in the tysonite structure, R ³⁺ + F^1? → M ²⁺ + V _F, with compensation of the difference in the cation charges due to the formation of anion vacancies, is confirmed experimentally. The optical transmission spectra are measured in the wavelength range λ = 2.18–22.22 μm. It is shown that the 50%-transmission edge in the IR range is 10.5 μm for all crystals. The refractive index and the polarizability of crystals decrease with decreasing SrF₂ content according to a linear law. The melting curves of the R _{1 ? y} Sr _y F_{3 ? y} phases studied have a maximum; therefore, these phases can be recommended for use as crystals with a high homogeneity and different optical properties in comparison with the corresponding RF₃ crystals.     

X-ray and neutron diffraction study of the defect crystal structure of the as-grown nonstoichiometric phase Y0.715Ca0.285F2.715

This work begins a series of papers aimed at studying the defect structure of nonstoichiometric phases R _{1 ? y} Ca _y F_{3 ? y} with a tysonite-type (LaF₃) structure. In the single-crystal structure of Y_0.715Ca_0.285F_2.715 with a tysonite-type small unit cell (sp. gr. P6₃/mmc, a = 3.9095(2) Å, c = 6.9829(2) Å; Z = 2; R _w = 2.16%), the displacements of Y³⁺ cations and F^2? anions from 6₃ symmetry axes were observed for the first time. The X-ray diffraction pattern shows weak satellites insufficient for structural calculations. The LaF₃ structure type is stabilized up and down on the temperature scale due to anion vacancies and the symmetrizing effect of Ca²⁺ cations lying on 6₃ symmetry axes. At 120°C the fluoride-ion conductivity in the nonstoichiometric phase Y_0.715Ca_0.285F_2.715 is five orders of magnitude higher than that in the stoichiometric phase β-YF₃. The transition to a superionic state is caused by a deviation from stoichiometry and is not associated with reconstructive phase transformation.     

Crystal structures of double calcium and alkali metal phosphates Ca10 M(PO4)7(M = Li, Na, K)

Crystal structures of double calcium and alkali metal phosphates described by the general formula Ca₁₀ M(PO₄)₇(M = Li, Na, K) have been studied by the Rietveld method. The lattice parameters are a = 10.4203(1) and c = 37.389(1) Å (for M = Li), a = 10.4391(1) and c = 37.310(1) Å (for M = Na), and a = 10.4229(1) and c = 37.279(2) Å (for M = K); sp. gr. R3c, Z = 6. The specific features of the distribution of alkali metal cations over the structure positions are discussed.     

Crystal structure of double vanadates Ca9 R(VO4)7. II. R = Tb, Dy, Ho, and Y

Crystal structures of the compounds Ca₉ R(VO₄)₇ (R = Tb (I), Dy (II), Ho (III), and Y (IV) have been studied by the method of the full-profile analysis. All the compounds are crystallized in the trigonal system (sp. gr. R 3 c, Z = 6) with the unit-cell parameters (I) a = 10.8592(1), c = 38.035(1), V = 3884.2(2) ų; (II) a = 10.8564(1), c = 38.009(1) Å, V = 3879.6(2) ų, (III) a = 10.8565(1) and c = 37.995(1) Å, V = 3878.3(2) ų, and (IV) a = 10.8588(1), c = 37.995(1) Å, V = 3879.9(2) ų. In structures I–IV, rare earth and calcium cations occupy three positions—M(1), M(2), and M(5). Rare earth cations occupy the R ³⁺ positions almost in the same way: 2.7–2.6(2) cations in the M(1) position; 2.7–2.3(2) cations in the M(2) position, and 0.6–1.0(1) cation in the M(5) position. At the same time, the occupancy of the M(5) position regularly increases with a decrease of the R ³⁺ radius.     

Ionic conductivity of congruently melting Ca0.6Sr0.4F2 and Ca1 ? x ? ySryRxF2 + x (R = La, Ce, Pr, Nd) single crystals with fluorite structure

Single crystals of congruently melting compositions of the Ca_0.6Sr_0.4F₂ and Ca_{1 − x − y} Sr _y R _x F_{2 + x} (R = La, Ce, Pr, Nd; x = 0.16–0.21; y = 0.07–0.16) solid solutions with fluorite structure have been grown by the Bridgman-Stockbarger method. Their electrical properties have been investigated in the range from 473 to 823 K, and it is shown that they are ionic conductors. For Ca_0.6Sr_0.4F₂ crystals, the ionic conductivity σ = 2 × 10⁻⁶ S/cm at 673 K, and the ion transport activation energy E _a = 1.1 eV. For Ca_0.77Sr_0.07La_0.16F_2.16, Ca_0.70Sr_0.11Ce_0.19F_2.19, Ca_0.65Sr_0.15Pr_0.20F_2.20, and Ca_0.58Sr_0.21Nd_0.21F_2.21 crystals, the values of σ lie in the range from 9 × 10⁻⁷ to 2 × 10⁻⁶ S/cm at 500 K, and the activation energy E _a is 0.88–0.93 eV. The concentration and mobility of ionic charge carriers in Ca_{1 − x − y} Sr _y R _x F_{2 + x} crystals have been calculated. Original Russian Text ? N.I. Sorokin, D.N. Karimov, E.A. Krivandina, Z.I. Zhmurova, O.N. Komar’kova, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 297–303.     

Investigation of multicomponent fluoride optical materials in the UV spectral region: I. Single crystals of Ca1−x R xF2+x (R = Sc, Y, La, Yb, Lu) solid solutions

Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca_1?x R _xF_2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region.     

Structural mechanisms of superionic conductivity in M 1−x R x F2+x single crystals

The relationship between the superionic transport in fluorite phases M _{1 ? x} R _x F_{2 + x} (M = Ca, Sr, or Ba; R are rare earth elements) and their defect structure has been analyzed. The superionic conductivity of M _{1 ? x} R _x F_{2 + x} crystals is provided by the high concentration of charge carriers. However, the carrier concentration is several tens of times lower than the concentration of anionic defects, which is explained by the presence of defect regions (DRs), which partially block carriers. The dependence of the superionic conductivity of M _{1 ? x} R _x F_{2 + x} phases on the RF₃(x) content has a percolation nature. Crystals of these phases are divided into two groups with respect to the percolation threshold: x _{p, 1} = 2–3 mol % RF₃ and x _{p, 2} = 7–8 mol % RF₃. The corresponding DR volumes are 3000–4000 ų (x _{p, 1}) and 500–700 ų (x _{p, 2}). The x _{p, 1}, and x _{p, 2} values correlate, respectively, with the octahedral cubic {M _{14 ? p} R _p F_{68 ? 69}} and tetrahedral {M _{4 ? p} R _p F₂₆} clusters, which are DR cores. The DR model and cluster structure are indicative of the heterogeneity of nonstoichiometric M _{1 ? x} R _x F_{2 + x} crystals at the nanoscale level with respect to the chemical composition and the electrical and crystallochemical (coordinations of M and R) characteristics.     

Mechanochemical synthesis of nonstoichiometric fluorite Ca1?xLaxF2+x nanocrystals from CaF2 and LaF3 single crystals

The nonstoichiometric Ca_1−x La_xF_2+x phase (x ≥ 0.1) is obtained by mechanochemical synthesis from CaF₂ and LaF₃ single crystals. This phase is the first representative of fluorite fluorides obtained by mechanochemical synthesis in the MF_m-RF_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°C) coincides with the conductivity of the single crystals of the same composition (Ca_0.8La_0.2F_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. __________ Translated from Kristallografiya, Vol. 50, No. 3, 2005, pp. 524–531. Original Russian Text Copyright ? 2005 by Sobolev, Sviridov, Fadeeva, Sul’yanov, Sorokin, Zhmurova, Herrero, Landa-Canovas, Rojas.     

Crystallochemical model of ion-transport percolation in nonstoichiometric M 1−x RxF2+x phases with the defect CaF2-type structure

A crystallochemical model of ion-transport percolation in M _1?x R_xF_2+x (M = Ca, Sr, Ba; R = RE) solid solutions with the “defect” CaF₂-type structure has been suggested. Within this model, the percolation thresholds in M _1?x R_xF_2+x crystals with tetrahedral R ₄F_26? and octahedral R ₆F_37? clusters are considered, whose existence is highly probable in these disordered fluorite phases. It is established that the calculated percolation thresholds x _c = 2.8 mol % for nnn (next nearest neighbors) [(R ₆F₃₇) _M6F32 ?F_i] and x _c = 4.7 mol % for nn (nearest neighbor) [(R ₄F₂₆) _M6F32 ?F_i] are in satisfactory accord with the experimental percolation thresholds determined from the conductometric data.     

Ionic conductivity of the phases of five types formed in the BaF2-GdF3 system

The ionic conductivity has been measured in ceramic phases of five structure types found in the BaF₂-GdF₃ system: the fluorite type (CaF₂), its trigonal and tetragonal distortions, the tysonite type (LaF₃), and the orthorhombic ??-YF₃ modification. The phases have been obtained by solid-phase synthesis from BaF₂ and GdF₃ mixtures in hermetic nickel containers at 925, 964, and 1067°C for 108?C360 h in a fluorine atmosphere. Their conductivity ?? is compared in correlation with the composition and structure type. The highest conductivity values are found for the tysonite Gd_{1 ? y} Ba _y F_{3 ? y} phase (0.10 ?? y ?? 0.25): (1?2) × 10^?3 S/cm at 683 K. The ordered Ba_0.60Gd_0.40F_2.40 and Ba_0.57Gd_0.43F_2.43 phases with fluorite-derived structures and different degrees of order are characterized by the lowest conductivities: i(1.5?3.5 × 10^?5) S/cm.     

Nonstoichiometry in inorganic fluorides: I. Nonstoichiometry in MF m -RF n (m < n ≤ 4) systems

The manifestation of gross nonstoichiometry in MF _m -RF _n systems (m < n ?? 4) has been studied. Fluorides of 34 elements, in the systems of which phases of practical interest are formed, are chosen. To search for new phases of complex composition, a program for studying the phase diagrams of the condensed state (??200 systems) has been carried out at the Institute of Crystallography, Russian Academy of Sciences. The main products of high-temperature interactions of the fluorides of elements with different valences (m ?? n) are grossly nonstoichiometric phases of two structural types: fluorite (CaF₂) and tysonite (LaF₃). Systems of fluorides of 27 elements (M ¹⁺ = Na, K; M ²⁺ = Ca, Sr, Ba, Cd, Pb; R ³⁺ = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; R ⁴⁺ = Zr, Hf, Th, U) are selected; nonstoichiometric M _{1 ? x} R _x F _{m(1 ? x) + nx} phases, which are of greatest practical interest, are formed in these systems. The gross nonstoichiometry in inorganic fluorides is most pronounced in 80 MF₂ ? RF₃ systems (M = Ca, Sr, Ba, Cd, Pb; R are rare earth elements). The problems related to the growth of single crystals of nonstoichiometric phases and basic fields of their application as new fluoride multicomponent materials, the properties of which are controlled by the defect structure, are considered.     

Growth and defect structure of CdF2 crystal and nonstoichiometric Cd1−x R x F2+x phases (R are rare earth elements and in): 6. Growth and ionic conductivity of Cd0.904In0.096F2.096 single crystal

Cd_0.904In_0.096F_2.096 crystals with fluorite-type defect structures have been grown from melt in a fluorinating atmosphere by the Bridgman method, and their ionic conductivity is investigated. The fluorine-ion transport activation enthalpy in Cd_0.904In_0.096F_2.096 (ΔH = 0.68 eV) is much smaller than the corresponding characteristic of the crystals belonging to the isoconcentration series Cd_0.9 R _0.1F_2.1, R = La-Lu, Y (ΔH = 0.8–0.9 eV). The ionic conductivity of Cd_0.904In_0.096F_2.096 is σ = 2 × 10^?4 S/cm (at 467 K); this value exceeds the conductivity of the CdF₂ crystal matrix and the highest conductivity Cd_0.9 R _0.1F_2.1 crystals with rare earth elements by factors of 3 × 10³ and ～10, respectively. Nonstoichiometric crystals of solid electrolyte Cd_{1 ? x} In _x F_{2 + x} have the highest conductivity out of all studied electrolytes based on the CdF₂ matrix.     

Synthesis, IR spectra, and structures of double metaphosphates MNi(PO3)3 (M = Na or K)

Double metaphosphates of the composition MNi(PO₃)₃ (M = Na or K) were prepared by spontaneous crystallization from melts in the M ₂O-P₂O₅-NiO system. Their thermal properties were studied, and the IR spectra were analyzed. The complete X-ray structure analysis of the compounds synthesized was performed. The NaNi(PO₃)₃ crystals are orthorhombic with the unit-cell parameters a = 13.781(2) Å, b = 10.584(2) Å, c = 9.873(1) Å, sp. gr. Pcca. The KNi(PO₃)₃ crystals belong to the sp. gr. R3 with the unit-cell parameters a = 10.076(2) Å and c = 9.623(5) Å. The structures of the studied phosphates are compared with the structures of the related compounds.     

Determination of the crystal structure of khanneshite by the Rietveld method

The crystal structure of khanneshite (Na_2.75Ca_0.23)_2.98(Ba_1.08Sr_0.63Ca_0.46Ce_0.46La_0.18Nd_0.15Pr_0.04)_3.00 · (CO₃)₅ found in carbonatites from the Khibiny massif (the Kola Peninsula) was solved by the Rietveld method. The X-ray diffraction data were collected on a focusing STOE-STADIP diffractometer equipped with a bent Ge(111) primary-beam monochromator (λMoKα₁ radiation, 2.00° < 2θ < 54.98°, 311 reflections). All the calculations were performed within the sp. gr. P6₃ mc; a = 10.5790(1) Å, c = 6.5446(1) Å, V = 634.31(1) ų, R _P = 2.38, R _wp = 3.26, R _B = 1.42, R _F = 1.79. The atoms of the cations were refined with anisotropic thermal parameters.     

Composition of CeF3-based crystals for a composite scintillator with a polystyrene matrix

The dependences of the refractive indices of solid solutions based on cerium fluoride (_Ce1?y Sr _y F_{3 ? y} and Ce_{1 ? y} Ba _y F_{3 ? y} ) on their composition have been studied. It is shown that at a light wavelength ?? = 0.589 ??m crystals of the Ce_0.925Sr_0.075F_2.925 and Ce_0.875Ba_0.125F_2.875 compositions have an average refractive index n _av coinciding with the refractive index of polystyrene (n = 1.5975 ± 0.001) at this wavelength and can thus be used as fillers for cerium-containing composite scintillators based on polystyrene.     

Morphotropy, isomorphism, and polymorphism of Ln 2 M 2O7-based (Ln = La-Lu, Y, Sc; M = Ti, Zr, Hf, Sn) oxides

Structural studies of compounds of variable composition and measurements of their conductivity have made it possible to identify new oxygen-ion-conducting rare-earth pyrochlores, Ln ₂Ti₂O₇ (Ln = Dy-Lu) and Ln ₂Hf₂O₇ (Ln = Eu, Gd), with intrinsic high-temperature oxygen ion conductivity (up to 1.4 × 10^?2 S/cm at 800°C). Twenty six systems have been studied, and more than 50 phases based on the Ln ₂ M ₂O₇ (Ln= La-Lu; M = Ti, Zr, Hf) oxides have been synthesized and shown to be potential oxygen ion conductors. The morphotropy and polymorphism of the Ln ₂ M ₂O₇ (Ln = La-Lu; M = Ti, Zr, Hf) rare-earth pyrochlores have been analyzed in detail for the first time. Thermodynamic and kinetic (growth-related) phase transitions have been classified with application to the pyrochlore family.     

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF _m -RF _n systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF₂) and tysonite (LaF₃) structures, which are formed in the MF₂-RF₃ systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.     

Modular structure of a sodium-rich analogue of eudialyte with the doubled c-parameter and the R3 symmetry

The structure of a new sodium-rich representative of the eudialyte group with the ideal formula (Na,Sr, K)₃₅Ca₁₂Fe₃Zr₆TiSi₅₁O₁₄₄(O,OH,H₂O)₉Cl₃ was established by the X-ray diffraction analysis (R = 0.054 based on 3503 |F|). The unit-cell parameters are a = 14.239(1), c = 60.733(7) Å, V = 10663.9 ų, sp. gr. R3. The mineral structure is characterized by in-layer order of cations resulting in doubling of the c-parameter and the formation of two modules, the composition and the structure of one of which corresponds to eudialyte (with an impurity of kentbrooksite), the prototype of the second module is alluaivite. Lowering of the symmetry is caused by ordering of Ca atoms and the Ca-replacing elements in the alluaivite module and by the displacements of the alkali cations from the m plane.