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Anionic nonstoichiometry in fluorides (replacement of F1– with O2–) deteriorates the quality of optical materials. Two reviews have been devoted to the initial stage of anionic nonstoichiometry: fluorides MF2 (M = Ca, Sr, Ba) are considered in this paper, and fluorides RF3 (R are 16 rare-earth elements) will be considered in the next one. These 19 fluorides play an important role in the fluoride materials science, comprising more than 70% of 27 MFm that are used to design two-component fluoride crystalline materials. The initial stage of anionic nonstoichiometry in MF2 is the only one in which oxofluorides MF2 – 2xOx with low oxygen content are formed. Partial replacement of F1– with O2– in the fluorite structure is accompanied by thermal stabilization of the structure type when moving up the temperature scale with a maximum in the melting curves of the oxofluoride phase, which decomposes upon cooling. No other intermediate phases containing fluorine and oxygen were found in the MF2–MO systems studied. [ABSTRACT FROM AUTHOR]

Optimization of the compositions of Cd1–xRxF2+x nonstoichiometric phases (CaF2 type, R is a rare-earth element) for the ionic conductivity and thermal stability is based on the temperature measurements of the electrical conductivity of single crystals depending on the R3+ ionic radii and RF3 content and on the investigation of phase diagrams of the CdF2–RF3 systems. It is shown that the conductivity of 20 compositions out of 30 studied Cd1–xRxF2+x crystals exceeds the conditional limit σ500K = 10–5 S/cm, below which the use of solid electrolytes in electrochemical devices is believed to be undesirable because of low conductivity. The maximum conductivities σ500K = (3.0–3.2) × 10–4 S/cm and σ293K = (1.5–2.3) × 10–8 S/cm are observed for solid electrolytes with R = Ho, Er, Tm, or Yb and x = 0.22–0.27. [ABSTRACT FROM AUTHOR]

The temperature dependence (in the range of 479–827 K) of the fluorine-ionic conductivity of nonstoichiometric Na0.37Tb0.63F2.26 crystals with fluorite-type structure (sp. gr. , lattice parameter a = 5.5785(1) Å) has been investigated. The crystals were grown from melt by the Bridgman method in a fluorinating atmosphere of CF4. The formation of cuboctahedral clusters of defects {(Na,Tb)8[Tb6F37]F32} is most likely in the structure of Na0.37Tb0.63F2.26 crystals. The ionic conductivity is found to be σdc = 2.6 × 10–6 and 3 × 10–3 S/cm at 500 and 827 K, respectively. A crystallophysical model of ion transport in Na0.37Tb0.63F2.26 crystals is proposed. The concentration and carrier mobility are calculated to be, respectively, nmob = 1.45 × 1021 cm–3 and μmob = 1.1 × 10–8 cm2/(s V) at 500 K. [ABSTRACT FROM AUTHOR]

Temperature measurements (in the range of 375–830 K) of the ionic conductivity of terbium trifluoride single crystals (structure type β-YF3) along three unit-cell crystallographic axes (a, b, and c) have been performed. It is found that TbF3 crystals (orthorhombic system, sp. gr. Pnma) have a weak anisotropy of electrical conductivity: σ||b/σ||a = σ||b/σ||c ≈ 2. The conductivity along the b axis is σ||b = 8 × 10–6 S/cm at 500 K. The anisotropy of fluorine-ion conductivity in rare-earth fluorides with β-YF3, LaF3 (tysonite), and β-BaTm2F8 structures is discussed in the contest of the specific features of their atomic structure. [ABSTRACT FROM AUTHOR]

Sm1 –ySryF3 –y (0 < y ≤ 0.31) crystals have been grown from melt by directional solidification in a fluorinating atmosphere. The crystals have been studied by X-ray diffraction and optical spectroscopy, and their fluorine-ion conductivity σdc, density ρ, and refractive indices nD have been measured. It is established that Sm3+ ions are not reduced to Sm2+ during crystal growth. The reversible polymorphic α ↔ β-SmF3 transition does not make it possible to obtain bulk (>1–3 mm3) samples of the tysonite phase (of LaF3 type) at y < 0.02. The dependences ρ(y) and nD(y) for the crystals are descending. The dependence σdc(y) exhibits nonmonotonic behavior; the maximum σdc value (1.6 × 10–4 S/cm) at 293 K is observed for the Sm0.98Sr0.02F2.98 crystal. At y = 0.31, an eutectic composite 69SmF3 × 31SrF2 is formed, whose conductivity is σdc = 6 × 10–8 S/cm, a value smaller than σdc for the crystal with y = 0.02 by a factor of ~3 × 103. The carrier concentration nmob and its mobility μmob have been calculated for Sm1 –ySryF3 –y (0.02 ≤ y ≤ 0.25) within the hopping conductivity model. For the crystal with the highest conductivity (Sm0.98Sr0.02F2.98), nmob = 4.0 × 1020 cm–3 and µmob = 2.5 × 10–6 cm2/(V s) at T = 293 K. [ABSTRACT FROM AUTHOR]

Crystals of the Cd0.77Sr0.23F2 solid solution (sp. gr. Fm m, lattice parameter a = 5.4882(1) Å), corresponding to the minimum in the liquidus and solidus curves in the CdF2–SrF2 phase diagram, have been grown from a melt by vertical directional solidification and their thermal conductivity has been measured in the temperature range 50–300 K. The thermal conductivity κ(T) of the Cd0.77Sr0.23F2 crystals is a weak function of temperature: κ50 = 1.9 W/(m K) and κ300 = 1.5 W/(m K) at 50 and 300 K, respectively. We have carried out a comparative analysis of the thermophysical properties of congruently melting M1 −xSrxF2 (M = Cd, Ca, Ba) crystals with the fluorite structure, containing SrF2 as a common component. The κ300 of the Cd0.77Sr0.23F2 crystals is lower than that of Ba0.66Sr0.34F2 and Ca0.59Sr0.41F2 by a factor of 2.1 and 2.7, respectively. [ABSTRACT FROM AUTHOR]

Nd1 –yEuyF3 –y crystals (y is the molar fraction of EuF2; 0 ≤ y ≤ 0.15) with a tysonite (LaF3) structure have been grown from melt by the Bridgman technique; their optical transmission spectra and dependences of the lattice parameters, density, refractive index, and fluorine-ion conductivity on the Eu2+ content have been investigated. Nd1 –yEuyF3 –y crystals are single-phase; they are crystallized into the trigonal system (sp. gr. , Z = 6) at y ≤ 0.12, whereas at y = 0.15 one can observe stabilization of a high-temperature tysonite α-phase (sp. gr. P63/mmc, Z = 2). The crystals under study are transparent in the IR range up to 12 µm. The EuF2 doping into the NdF3 matrix leads to a monotonic decrease in the density and refractive index. The dependence of the conductivity on the composition σdc(y) has a nonmonotonic character. The Nd0.97Eu0.03F2.97 crystal has the maximum σdc value at 293 K (2.0 × 10–4 S/cm). Its charge-carrier concentration is nmob = 5.8 × 1020 cm–3 and the carrier mobility is μmob = 2.2 × 10–6 cm2/(V s) (at 293 K). The σdc value for Nd0.97Eu0.03F2.97 is smaller (by a factor of 2.5) than that for Ce0.97Sr0.03F2.97, which has the best electrolytic characteristics among R1 –yMyF3 –y solid electrolytes (M = Ca, Sr, or Ba). [ABSTRACT FROM AUTHOR]