I. I. Buchinskaya

Mechanisms of writing and decay of holographic gratings in semiconducting CdF2:Ga

We consider the mechanisms responsible for the photoinduced change in the optical properties of semiconducting CdF2 crystals with metastable Ga impurities forming DX centers. Unlike the case of compound semiconductors with DX centers (GaAlAs:Si, GaAlAs:Te, CdZnTe:Cl), this change is caused not by free electrons but by a redistribution of electrons between deep and shallow localized states. The resulting modification of the refractive index of the crystal allows writing of persistent holographic gratings at temperatures up to 200 K, high for this class of holographic materials. Holographic characteristics of CdF2:Ga crystals such as refractive index change, sensitivity, and grating decay are described.

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.

Concentration dependences of the unit-cell parameters of nonstoichiometric fluorite-type Na0.5 − xR0.5 + xF2 + 2x phases (R = rare-earth elements)

Unit-cell parameter a of the cubic phases with a varying composition Na0.5 − xR0.5 + xF2 + 2x (R = Gd-Lu and Y) and with a fluorite-type structure, is described within an accuracy of ±0.003 Å, by the formula a = 4.454 + 0.874r3 + x(6.7238r3 − 7.259) where r3 is the Shannon “crystal ionic radius” R3+ at c.n. = 8.

Electrical conductivity of a CaF2-BaF2 nanocomposite

A CaF2-BaF2 nanocomposite material has been prepared via 70CaF2 · 30BaF2 (mol %) melt solidification. The material has a lamellar structure due to the eutectoid decomposition of the solidified melt. The 500°C ionic conductivity of the composite is 25 and 330 times higher than those of the parent BaF2 and CaF2, respectively. The enhanced conductivity of the composite can be understood in terms of the electrical properties of its interfaces.

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 R1 − yCayF3 − y with a tysonite-type (LaF3) structure. In the single-crystal structure of Y0.715Ca0.285F2.715 with a tysonite-type small unit cell (sp. gr. P63/mmc, a = 3.9095(2) Å, c = 6.9829(2) Å; Z = 2; Rw = 2.16%), the displacements of Y3+ cations and F2− 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 Y0.715Ca0.285F2.715 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.

A new class of holographic materials based on semiconductor CdF2 crystals with bistable centers. Part II. Growth of optically perfect crystals

The crystal-chemical processes occurring during the growth of cadmium fluoride crystals doped with bistable gallium and indium impurities and during conversion of the crystals to the semiconductor state are considered. It is found that doping with indium does not strongly affect the optical performance of the crystals, while gallium, due to its low solubility in cadmium fluoride, strongly impairs this performance. It is shown that co-doping of the crystals with highly soluble impurities (yttrium, scandium, and gadolinium fluorides) makes it possible to obtain optically perfect gallium-doped crystals.

Distribution coefficients of impurities in cadmium fluoride

Experimental data on the liquidus curve in cadmium fluoride-metal fluoride systems were used to calculate the distribution coefficients of di- and trivalent impurities in CdF2. A similar method was proposed for calculating the distribution coefficient from solidus data. It was shown that the variations of the calculated distribution coefficients with the ionic radii of M2+ and R3+ are well fitted by Gaussians. Phase relations in the CdF2-rich part of the CdF2-InF3 system were studied by differential thermal analysis and x-ray diffraction.

Thermophysical characteristics of Pb0.679Cd0.321F2 solid-solution crystals

The thermal conductivity (at temperatures of 50–300 K) and specific heat (at 58–307 K) of Pb0.679Cd0.321F2 solid solution crystals with a fluorite structure, as well as the specific heat of crystals of fluorite modification β-PbF2 (in a temperature range of 79–311 K), have been experimentally investigated for the first time. The temperature dependences of the phonon mean free path in these crystals are determined.

Electrical and thermal conductivities of congruently melting single crystals of isovalent M1 − xM′xF2 solid solutions (M, M′ = Ca, Sr, Cd, Pb) in relation to their defect fluorite structure

The electrical and thermal conductive properties of two-component single crystals of Pb0.67Cd0.33F2, Ca0.59Sr0.41F2, and Cd0.77Sr0.23F2 solid solutions with fluorite-type structure (CaF2), characterized by congruent melting (presence of minima in melting curves) and uniform distribution of components in the crystal bulk. Pb0.67Cd0.33F2 crystals, in contrast to isostructural Ca0.59Sr0.41F2 and Cd0.77Sr0.23F2 crystals, are characterized by high fluorine-ion electrical conductivity (σ = 0.02 S/m at 293 K); low ion-transport activation enthalpy (ΔH ≈ 0.4 eV); low thermal conductivity (k = 1.1 W/mK at 300 K); and glassy behavior of heat transfer, which is atypical for crystalline state. This anomalous behavior of the electrical and thermal conductivities of Pb0.67Cd0.33F2 crystals is due to the strong structural disordering of the anionic subsystem (which is retained at room temperature) as a result of isovalent replacements of Pb2+ cations with Cd2+ cations.

The growth and defect structure of CdF2 and nonstoichiometric Cd1 − xRxF2 + x phases (R are rare earth elements or in): Part VI. The optical properties of Cd0.9R0.1F2.1 crystals

The optical properties of the isoconcentration series of Cd0.9R0.1F2.1 crystals (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) grown from a melt by the Bridgman method have been investigated. The crystals have an anomalous birefringence (∼10−6) nonuniformly distributed over the sample diameter; the dichroism in them does not exceed 10−9. Scanning using a spectral modulator showed the nonuniform distribution of rare earth elements over the crystal diameter. The refractive indices n have been measured at wave-lengths of 0.436, 0.546, and 0.589 μm. The character of change in n along the rare-earth series is nonuniform and similar to the change in n in Sr0.9R0.1F2.1 crystals. It is shown that the refractive indices in Cd1 − xRxF2 + x crystals, depending on the RF3 content, can be estimated using the method of molecular refraction additivity.