B. P. Sobolev

Phase diagrams of thE CaF2- (Y, Ln) F3 svstems I. Experimental

Abstract The phase diagrams of the CaF 2 - (Y, Ln) F 3 systems, where Ln is any one of the lanthanides except Pm and Eu, were studied by thermal and X-ray phase analysis methods. The investigations were made from 800 (600)°C up to the melting temperature. The oxygen concentration of the specimens was checked after thermal treatment. For the systems with Ln = Ce, Pr, Tb, Ho, Tm and Lu the fusibility data have been obtained for the first time. Phases with the following structure types are present in the systems investigated: (a) solid solutions Ca 1− x (Y, Ln) x F 2+ x with the fluorite structure type, which undergo ordering in the systems with Ln = HoLu, Y; (b) non-stoichiometric phases (Y, Ln) 1− y Ca y F 3− y with the tysonite (LaF 3 ) structure type, which undergo ordering in the systems with Ln = TbLu, Y; (c) solid solutions with the α-UO 3 structure type, based on α modifications (Y, Ln) F 3 (Ln = ErLu); and (d) modifications (Y, Ln) F 3 (Ln = GdLu) of the β-YF 3 type. Successive transitions, from solid solutions of the components of the system to the berthollide phases and the stoichiometric compound, were found for phases with the tysonite structure type. The maxima on fusion curves of non-stoichiometric phases were noticed in Sm, Gd, Tb, Ho and Er for Ca 1− x - (Y, Ln) x F 2− x and for Ln = PrLu in (Y, Ln) 1− y Ca y F 3− y . The coordinates of non-variant points for equilibria of the liquid-solid type and also the equations describing the dependence of the lattice parameters of nonstoichiometric phases on the composition are given for the systems investigated.

On the problem of polymorphism and fusion of lanthanide trifluorides. II. Interaction of LnF3 with MF2 (M = Ca, Sr, Ba), change in structural type in the LnF3 series, and thermal characteristics☆

Abstract The results of a study on the interaction of Ln F 3 with M F 2 ( M = Ca, Sr, Ba) for 34 binary systems of the Ln F 3  M F 2 type in the concentration range 60–100 mole % Ln F 3 are presented. It is shown that in this range the interaction of the components in the Ln F 3  M F 2 systems is similar to that in the Ln F 3  Ln 2 O 3 systems. The problem of stabilizing different structural types of Ln F 3 (tysonite and α-YF 3 ) during the isomorphous substitution of Ln 3+ by M 2+ and 2F − by O 2− with formation of solid-solid solutions Ln 1− x M x F 3− x and Ln F 3−2 x O x , respectively, is discussed also. In congruent fusion of these phases, the coordinates of the maximum on the fusibility curve (according to composition) are regularly displaced to the side of pure Ln F 3 with a decrease in the atomic number of the lanthanide. The vacancy-stabilized phases are typical examples of variable composition compounds (berthollides). On the basis of data on the interaction of components in the Ln F 3  Ln 2 O 3 and Ln F 3  M F 2 systems, problems of polymorphism and changes of structural type in the Ln F 3 series are discussed. Fusion and polymorphic transformation temperatures are given for Ln F 3 with control of oxygen content in the specimens after thermal analysis.

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 MFm-RFn 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.

On the problem of polymorphism and fusion of lanthanide trifluorides. I. The influence of oxygen on phase transition temperatures

Abstract The details of using thermal analysis (TA) in the investigation of lanthanide trifluorides are examined. By examining the literature on the problem of phase transitions in LnF3 (Ln = lanthanum and the lanthanides), it is established that the basic reason for disagreement among the data of various authors is, in most cases, lack of control of pyrohydrolysis of LnF3. For the first time, parts of the phase diagrams of the LnF3Ln2O3 systems (Ln = Gd, Tb, Ho, Er, and Y) have been studied with TA methods. Phases LnF3−2xOx where 0 ≤ x ≤ 0.2, which were not known earlier, are discovered. According to the type of interaction of LnF3 with the corresponding Ln2O3, the trifluoride series is broken down into several groups corresponding to the structural type in which the LnF3 crystallizes. The parts of the phase diagrams of the LnF3Ln2O3 systems that were studied permit us to explain the reasons for contradictions in existing data on the temperatures of phase transformations in LnF3.

Phase diagrams of the SrF2(Y, Ln)F3 systems part I.—X-ray characteristics of phases

Abstract The phase equilibria in the subsolidus part of 14 binary systems of SrF2(Y, Ln)F3 type (Ln—all lanthanides except Pm and Eu) were studied at temperatures over 850°C in equilibrated and quenched specimens by the X-ray analysis method. The oxygen concentration in the specimens before and after the thermal treatment was checked. The crystallographic characteristics of phases formed in the systems i.e. nonstoichiometric phases Sr1−xLnxF2+x with fluorite type structure, phases with fluorite derived type structure, nonstochiometric phases Ln1−ySryF3−y with tysonite (LaF3) type structure are given in this paper.

CdF2:In: A novel material for optically written storage of information

We demonstrate that semiconducting CdF2 crystals doped with indium is an efficient medium for optical storage of information in static and dynamic regimes. A metastable phototransformation of 1018 cm−3 In centers from a localized deep state to a hydrogenlike shallow state leads to a change of the refractive index Δn of about 10−4 for the probe beam at the wavelength of 500 nm. The diffraction efficiency is temperature dependent due to spontaneous decay of the grating caused by thermal recovery of the In impurity from the metastable hydrogenic state to the localized ground state.

Specific features of ion transport in nonstoichiometric Sr1−xRxF2+x phases (R=La, Y) with the fluorite-type structure

Abstract Temperature and concentration dependence of ionic conductivity (δ) was studied for the crystals of nonstoichiometric Sr 1− x R x F 2+ x phases (R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu, Y;0 x ⩽0.40) having the fluorite structure ( Fm 3 space group). The temperature dependence of σ of the crystals in the region 300–800 K obeys the Arrhenius equation. In order to explain the ion transport features observed (absolute σ and ΔH σ values, their concentration dependence) in the anion excess solid solutions, a model of “defect regions” is proposed. Within the framework of the model, the conductivity of a solid solution is determined by the ratio of the conductivities of the “defect region” and the matrix. It is shown that in relation to electrical characteristics, the whole set of nonstoichiometric phases can be divided into three groups, R=La, Ce, Pr, Nd: R=Gd, Tb, Dy, Ho; R=Er, Lu, Y. This division correlates well with a number of other crystallophysical and crystallochemical characteristics. The average concentration of charge carriers (F − ) and their average mobility in the Sr 0.8 R 0.2 F 2.2 crystals were calculated: n =2.2×10 27 m −3 , μ 500 K =2.8×10 −11 m 2 / s/V for solid solutions with R=La, Ce, Pr, Nd and n =6.8×10 26 m −3 , μ 500 K =3×10 −14 m 2 / sV for solid solutions with R=Er, Lu, Y.

Phase equilibria in BaF2(Y, Ln)F3 systems

Abstract Data are presented on phase equilibria at 877 ± 10°C in the systems BaF2(Y,Ln)F3, where Ln = SmLu. All the systems show cubic solid solutions based on BaF2 and variable composition phases of structure derived from the CaF2 type (rhombohedral distortion). Syngony and unit cell dimensions have been determined on monocrystals; crystallographic parameters of ten trigonal phases have been adduced. The existence of solid solutions of BaF2 in high-temperature α-LnF3(LaF3 type) at the given isothermal section indicates stabilization of the LaF3 structure type by heterovalent isomorphous replacement. In the systems BaF2(Y,Ln)F3 with Ln = DyYb, monoclinic compounds BaR2F8 are formed. X-ray parameters, derived from single crystal and polycrystalline specimens, of six monoclinic BaR2F8 compounds are presented. In the BaF2LuF3 system we have isolated and studied for the first time a compound of the approximate composition BaLu2F8 which crystallizes in rhombic syngony and has a marked range of homogeneity.

New phases with fluorite-derived structure in CaF2(Y, Ln)F3 systems

Abstract Crystallographic characteristics of six new phases of idealized composition Ca8R5F31 which are formed in the CaF2RF3 (R = Y, HoLu) systems are reported. All the phases have a similar structure derived from CaF2 with slight distortion and pseudocubic unit cell parameters aord = 13adis (where adis is the parameter of the fluorite subcell). The degree of ordering increases in the lanthanide series with decrease of ionic radius and, in every system given, with an increase in the RF3 content of the solid solution. Significant influence of temperature and time of annealing on the degree of ordering was not detected.

Structural aspects of fast ionic conductivity of rare earth fluorides

Abstract The structure and ionic conductivity of orthorhombic β-YF 3 structured rare earth fluorides and of tysonite-structured ones are investigated. Temperature dependencies of the conductivity of various orthorhombic crystals are close to one another. The conductivity at 500 K, σ 500 , and the conduction activation enthalpy, Δ H , are equal to 1.1(4)×10 −5 S/cm and 0.70(3) eV, respectively. The conductivity of tysonite-structured crystals is significantly higher ( σ 500 =(1.4(8))×10 −3 S/cm, Δ H =0.36(1) eV), and can be further enhanced by doping with alkaline-earth fluorides. The highest conductivity is found for LaF 3 :5 m/o SrF 2 ( σ 500 =2.4×10 −2 S/cm, Δ H =0.35(1) eV). In both structural types, the conductivity is isotropic. A significant difference between the conductivities of both structural types of rare earth fluorides is discussed.