Nanostructured crystals of Sr1?xRxF2+x fluorite phases and their ordering: 6. Microindentation analysis of crystals

Hardness, crack resistance, brittleness, and effective fracture energy have been studied for crystals of 24 fluorite phases Sr_{1 − x} R _x F_{2 + x} (R are 14 rare earth elements (REEs); 0 < x ≤ 0.5) and SrF2 grown by the Bridgman method from a melt. These characteristics change nonlinearly with an increase in the REE content for Sr_{1 − x} R _x F_{2 + x} (0 < x ≤ 0.5) with R = La, Nd, Sm, Gd, and Lu; it is maximum in the range x < 0.1 for all REEs. The changes in a number of REEs have been traced for an isoconcentration series of Sr_0.90 R _0.10F_2.10 crystals (R = La, Nd, Sm, Gd, Ho, Er-Lu, or Y) and crystals (similar in composition) with R = Tb and Dy. The hardness of Sr_{1 − x} R _x F_{2 + x} crystals is higher by a factor of ∼2–3 than that of SrF2. The effect of decrease in microstresses in SrF₂ crystals is confirmed by the isomorphic introduction of R ³⁺ ions into this crystalline matrix.     

Nanostructured crystals of fluorite phases Sr1 ? xRxF2 + x (R = Y, La-Lu) and their ordering: Part III. A study of the refractive indices

The refractive indices n of Sr_{1 − x} R _x F_{2 + x} crystals (R = Y, La-Lu; 0 ≤ x ≤ 0.5) have been measured at wavelengths of 0.436, 0.546, and 0.589 μm. It is established that n increases when there is an increase in the RF₃ content x according to a weakly quadratic law for each R. For the isoconcentration series of Sr_0.9 R _0.1F_2.1 crystals, the change in n in the series of rare earth elements has a pronounced nonlinear character, which reflects the nonmonotonous change in the properties of compounds in the R series. It is shown that the method of molecular refraction additivity can be used to calculate n for Sr_{1 − x} R _x F_{2 + x} crystals. By varying the RF₃ content in them, one can obtain optical media with a gradually varied refractive index n in the range 1.44–1.55, thus filling the gap in the n values between high ones for RF₃ crystals and low ones for crystals of alkaline earth fluorides MF₂. Original Russian Text ? T.M. Glushkova, D.N. Karimov, E.A. Krivandina, Z.I. Zhmurova, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 642–647.     

Nanostructured crystals of fluorite phases Sr1 ? xRxF2 + x (R are rare-earth elements) and their ordering: IV. Study of the optical transmission spectra in the 2–17-μm wavelength range

Transmission spectra of two-component crystals of Sr_1−x R _x F_2+x (R = Y, La-Lu; 0 ≤ x ≤ 0.5) in the 1–17-μm wavelength range were studied. The spectral characteristics of these crystals and of single-component crystals of MF₂ (M = Ca, Sr, or Ba) and RF₃ (R = La-Nd) were compared. The transmission cutoff of Sr_1−x R _x F_2+x crystals is shifted to shorter wavelengths with increasing x. The same tendency is observed with the increasing atomic number R of rare-earth elements for two isoconcentration series of Sr_1−x R _x F_2+x (x ∼ 0.10 and 0.28). This tendency is pronounced at large x. The transmission cutoff of Sr_1−x R _x F_2+x crystals can be varied in the range of from 10.7 to 12.2 μm by changing their qualitative (R) and quantitative (x) composition. Hence, these crystals can be assigned to multicomponent fluoride optical materials with controlled optical characteristics. The Sr_1−x R _x F_2+x crystals, where R = Ce-Sm, were shown to be promising materials for the design of selective optical filters in the 2–10-μm spectral range.     

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.     

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 Cd_0.9 R _0.1F_2.1 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 Sr_0.9 R _0.1F_2.1 crystals. It is shown that the refractive indices in Cd_{1 − x} R _x F_{2 + x} crystals, depending on the RF₃ content, can be estimated using the method of molecular refraction additivity. Original Russian Text ? A.F. Konstantinova, T.M. Glushkova, I.I. Buchinskaya, E.A. Krivandina, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 648–651.     

Defect structure and ionic conductivity of Ca1 ? xScxF2 + x (0.02 ≤ x ≤ 0.15) single crystals

Single crystals of the Ca_{1 − x} Sc _x F_{2 + x} (x = 0.106, 0.132, 0.156) solid solutions (CaF₂ structure type, space group Fm m) are investigated using X-ray diffraction. It is revealed that the crystals under investigation contain vacancies in the 8c positions and interstitial fluorine ions in the 48i positions. The coordination number of Sc³⁺ ions in the structure of the Ca_{1 −x} Sc _x F_{2 + x} solid solutions is equal to eight. The specific features of the concentration dependences of the ionic conductivity and the activation energy of ion transfer for the Ca_{1 − x} Sc _x F_{2 + x} (0.02 ≤ x ≤ 0.15) solid solutions are explained in the framework of the percolation model of conducting “defect regions.” The percolation threshold equal to 3–5 mol % ScF₃ corresponds to the model of [Ca_{14 − n} Sc _n F₆₈] octacubic clusters containing fluorine ions in the 48i positions. The ionic conductivity of the Ca_{1 − x} Sc _x F_{2 + x} solid solutions is analyzed in comparison with the change in this characteristic for the series of Ca_0.8 R _0.2F_2.2 crystals with rare-earth elements. Original Russian Text ? E.A. Sulyanova, V.N. Molchanov, N.I. Sorokin, D.N. Karimov, S.N. Sulyanov, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 612–622.     

Nanostructured crystals of fluorite phases Sr1−x R x F2+x and their ordering: VIII. Imperfect crystal structure of Sr0.71Ce0.29F2.29

A congruently melting single crystal of nonstoichiometric phase Sr_0.71Ce_0.29F_2.29 crystallizing into the CaF₂ structural type (sp. gr Fm $\bar 3$ m) has been studied by X-ray diffraction analysis. Vacancies in the main fluorine position 8c and interstitial anions in two 32f positions have been found. The ratio of the structural defects in the Sr_0.71Ce_0.29F_2.29 solid solution corresponds to the tetrahedral configuration of the defect cluster {Sr_{4 ? n} Ce _n F₂₆}.     

Growth dynamics of Ge1?xSix single crystals obtained by directional constitutional supercooling of the melt

The problem of growth dynamics for crystals of binary solid solutions, obtained by the constitutional supercooling of the melt with a silicon feeding rod, has been solved within the Pfann approximation. The dependences of the change in the axial growth rate of Ge_1−x Si _x crystals (0 ≤ x ≤ 0.3) are calculated. It is shown that the Ge_1−x Si _x crystallization rate significantly changes during growth. The results make it possible to determine the optimal conditions and technological parameters for growing Ge_1−x Si _x single crystals (0 ≤ x ≤ 0.3) with a specified concentration gradient along the crystallization axis.     

Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x and their ordering: 9. The defect crystal and real structure of quenched fluorite phases Sr1 − x Ce x F2 + x (x = 0–0.5)

A Sr_0.7Ce_0.3F_2.3 crystal (CaF₂ type, sp. gr. $Fm\bar 3m$ ), obtained by quenching from melt, has been studied for the first time by X-ray diffraction. Fluorine vacancies and interstitial anions are found in the 8c and 32f sites, respectively. The defect ratio in the Sr_0.7Ce_0.3F_2.3 structure corresponds to the tetrahedral cluster configuration of defects {Sr_{4 ? n} Ce _n F₂₆}. The defect structure of quenched (at a rate of ～25 K/min) crystal differs from that of a crystal grown from melt (cooling at a rate of ～3 K/min) by the displacement of some cations (presumably Ce³⁺) along the threefold axis to the 32f site and the anisotropy of thermal vibrations of ions in the cluster core (F _int(32f)3). The concentration dependence of the lattice parameters of quenched Sr_{1 ? x} Ce _x F_{2 + x} phases (x = 0–0.5) is described by a third-order polynomial: a = 5.80009 + 1.166518 × 10^?3 x ? 1.124969 × 10^?5 x ² + 8.258155 × 10^?8 x ³. The compositional dependence of microdistortions is also nonlinear; maximum microdistortions are observed in the SrF₂ crystal. They decrease with an increase in the cerium concentration x to ～ 0.35. The minimum in the range x = 0.30–0.35 correlates with a composition corresponding to the peak (at x ～ 0.29) in the melting curves of the fluorite phase estimated from the phase diagram of the SrF₂-CeF₃ system (the method of thermal analysis).     

Nanostructured crystals of the fluorite phases Sr1 − x R x F2 + x (R—rare-earth elements) and their ordering: II. Crystal structure of the ordered Sr4Lu3F17 phase

Crystallography Reports - The crystal structure of the ordered phase Sr4Lu3F17 prepared by directed crystallization of the melt has been investigated. The crystals have a trigonally distorted...     

Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x (R are rare earth elements) and their ordering: 5. A study of the ionic conductivity of as-grown Sr1 − x R x F2 + x crystals

Crystallography Reports - The ionic conductivity σ of Sr1 ? x R x F2 + x crystals (R = Y, La-Lu) has been measured in the temperature range of 324–933 K. The isomorphic...     

Mechanochemical synthesis of nonstoichiometric nanocrystals La1 ? yCayF3 ? y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

The nonstoichiometric phases La_{1 − y} Ca _y F_{3 − y} (y = 0.15, 0.20) with a tysonite (LaF₃) structure have been prepared for the first time by the mechanochemical synthesis from CaF₂ and LaF₃ crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF₂-LaF₃ system. The “mechanohydrolysis” of the La_{1 − y} Ca _y F_{3 − y} phase has been observed for the first time. Under these conditions, the La_{1 − y} Ca _y F_{3 − y} phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La_{1 − y} Ca _y F_{3 − y} nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 10⁸ Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10⁻⁴ S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials. Original Russian Text ? B.P. Sobolev, I.A. Sviridov, V.I. Fadeeva, S.N. Sul’yanov, N.I. Sorokin, Z.I. Zhmurova, I.I. Khodos, A.S. Avilov, M.A. Zaporozhets, 2008, published in Kristallografiya, 2008, Vol. 53, No. 5, pp. 919–929.     

Crystal and magnetic structures of the Bi1 ? xSrxFeO3 ? δ and Bi1 ? xAxFe1 ? xMnxO3 (A = Sr, Ca) solid solutions

Bi_{1 − x} Sr _x FeO_{3 − x/2} (I), Bi_{1 − x} Sr _x Fe_{1 − x} MnxO₃ (II), and Bi_{1 − x} Ca _x Fe_{1 − x} Mn _x O₃ (III) solid solutions have been obtained. Their magnetization has been measured by X-ray and neutron diffraction and M?ssbauer spectroscopy. According to the M?ssbauer spectroscopy data, iron ions are in the trivalent state in system I. Near the concentration x ≈ 0.2, rhombohedral distortions (sp. gr. R3c) are transformed into tetragonal (P4/mmm). The symmetry of system II changes at x > 0.2 (R3c → R3c), whereas orthorhombic distortions (R3c → Pbnm) arise in system III at x > 0.2. The magnetic structure is antiferromagnetic (of G type). The samples of systems II and III exhibit weak ferromagnetism at x > 0.2 due to the Dzyaloshinski-Moriya interaction.     

Investigation of Trimetallic Ligand-Pillared Oxyfluorides: Ag2Cu(pzc)2MOxF6?x (M?=?Mo, Nb, and W)

Abstract  

Three new ligand-pillared hybrid solids, Ag₂Cu(pzc)₂MO _x F_6−x (I, M = Mo, x = 2; II, M = W, x = 2; III, M = Nb, x = 1) (pzc = pyrazine-2-carboxylate) were synthesized via hydrothermal reactions at 150 °C, and their structures were determined by single-crystal X-ray diffraction (P2₁/n (No. 14), Z = 2; a = 7.2302(1), 7.2124(2), 7.2715(2) ?; b = 7.9460(1), 7.9270(2), 7.98436(3) ?; c = 13.9173(2), 13.8959(4), 13.8226(5) ?, for I, II, and III, respectively). All three are isostructural and contain unusual trimetallic (Ag₂CuMO _x F_6−x )²⁺ layers that consist of [Ag₂O₂F₂] _n and [CuMO _x F_6−x ] _n chains that alternate within the layers. Each structure also contains [MO _x F_6−x ]²⁻ octahedra with fully disordered O/F positions and with an inversion center on the M ⁿ⁺ sites, i.e., Mo⁶⁺, W⁶⁺ and Nb⁵⁺. Magnetic susceptibility measurements can be fitted to the Curie–Weiss law with a Curie constant consistent with a single non-interacting Cu(II) (S = ?) site per formula unit. Thermogravimetric analyses indicate that these hybrid compounds are stable up to ~280 °C, with each exhibiting a single weight-loss step beginning at ~300 °C that corresponds to the loss of all pyrazine-2-carboxylate ligands and additional O/F atoms via oxidation of the ligand during its removal. UV–Vis diffuse reflectance measurements show that each exhibits an optical bandgap size of ~2.8 eV, and which electronic-structure calculations show arise from excitations between the Cu(II)-based valence orbitals and the M^5+/6+-based conduction band orbitals.     

Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x (R are rare-earth elements) and their ordering. I. Crystal growth of Sr1 − x R x F2 + x (R = Y,La, Ce,Pr, Nd,Sm, Gd,Tb, Dy,Ho, Er,Tm, Yb,and Lu)

Crystallography Reports - Crystals of nonstoichiometric phases Sr1 − x R x F2 + x (R are 14 rare-earth elements) and the ordered phase Sr4Lu3F17 with a trigonally distorted fluorite lattice...     

Synthesis and Crystal Structure of a New Mixed Alkali Oxalate A1?x(NH4)x(H2C2O4)(HC2O4)(H2O)2 with A?=?K, Rb

Abstract  

In the present study we report the synthesis, crystal structure, spectroscopic and thermal analysis of mixed alkali A_1−x (NH₄) _x (H₂C₂O₄)(HC₂O₄)(H₂O)₂ with A = K, Rb. Single crystal refinements showed that the two compounds Rb_0.86(NH₄)_0.14(H₂C₂O₄)(HC₂O₄)(H₂O)₂ and K_0.53(NH₄)_0.47(H₂C₂O₄)(HC₂O₄)(H₂O)₂ adopt P − 1 space group. The nine fold coordination cationic sites are randomly occupied by Rb⁺/NH₄ ⁺ and K⁺/NH₄ ⁺ respectively for rubidium and potassium compounds. The structure consists of a three-dimensional network formed by the succession along (c) axis of corrugated sheets formed by alkali polyhedra layers in the packing of four-membered rings [A₄(HC₂O₄)₂(H₂C₂O₄)₂] linked and bridged by oxalate groups that behave as bi and tetradentate ligand under three different coordination modes. The stability of the crystal lattice is ensured by interesting hydrogen bonding contacts: N–H···O and O–H···O. The thermal behavior under air reveals two anomalies at 368 and 402 K attributed respectively to a structural phase transition probably due to the reorientation of ammonium tetrahedral and to the release of crystalline water. IR spectroscopy further confirms that this material loses crystallization water gradually in the temperature range 400–460 K.     

Refinement of the crystal structure of [Rb x (NH4)1? x ]3H(SO4)2( x = 0.11) by single-crystal X-ray and neutron diffraction: I. Phase II at 300 K

The crystal structure of a mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal was refined using single-crystal X-ray and neutron diffraction at 300 K. The refinement confirmed that the phase II of the crystal belongs to the monoclinic crystal system (space group C2/c). It was shown that, according to the single-crystal neutron diffraction data, ammonium ions can be represented as regular tetrahedra. The analysis of the constructed difference Fourier maps of the electron and nuclear densities of ammonium ions revealed the presence of additional peaks, which can indicate the dynamic disorder of ammonium ions in the phase II. The results obtained con-firmed that the phase transition to the orientational-glass state can occur in the mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal with a decrease in the temperature. Original Russian Text ? L.S. Smirnov, K. Wozniak, P. Dominiak, A. Loose, I. Natkaniec, M.V. Frontasyeva, E.V. Pomyakushina, A.I. Baranov, V.V. Dolbinina, 2008, published in Kristallografiya, 2008, Vol. 53, No. 3, pp. 450–459.     

Neutron diffraction investigation of the evolution of the crystal structure of oxygen-conducting solid solutions (Yb1 ? x Ca x )2Ti2O7 ( x = 0, 0.05, 0.10)

The crystal structure of pyrochlore-like solid solutions (Yb_{1 − x} Ca _x )₂Ti₂O₇ (x = 0, 0.05, 0.10) synthesized by high-temperature annealing of mechanically activated initial oxides at temperatures of 1300–1500°C is studied using neutron diffraction. It is found that the Ca²⁺ cations are located in the ytterbium sublattice, which apparently favors the splitting of one of the oxygen sublattices [O(2) (48f)] of the pyrochlore structure: the decrease in the occupancy of this sublattice is accompanied by the formation of a new sublattice O(3) (8b), whereas the other oxygen sublattice O(1) (8a) remains unchanged. This rearrangement of the anions in the oxygen subsystem due to the incorporation of an alkaline-earth cation explains the high ionic conductance (∼0.2 S/cm at 1000°C) for (Yb_0.9Ca_0.1)₂Ti₂O₇ ∼ 0.2, which is the maximum value observed to date for pyrochlores of the A ₂ B ₂O₇ type, where A = Ln and B is a Subgroup IVA element of the periodic system. The bulk and grain-boundary components of the conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ synthesized at temperatures of 1300, 1400, and 1500°C are studied using impedance spectroscopy. It is found that the (Yb_0.95Ca_0.05)₂Ti₂O₇ sample synthesized at 1500°C has the highest total conductivity due to the increased grain-boundary component. The bulk component of the ionic conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ is hardly affected by the synthesis temperature and depends mainly on the degree of heterovalent substitution. Original Russian Text ? A.V. Shlyakhtina, A.E. Sokolov, V.A. Ul’yanov, V.A. Trunov, M.V. Boguslavskiĭ, A.V. Levchenko, L.G. Shcherbakova, 2009, published in Kristallografiya, 2009, Vol. 54, No. 1, pp. 31–36.     

Displacements in the cationic motif of nonstoichiometric fluorite phases Ba1 − x R x F2 + x as a result of the formation of {Ba8[R 6F68–69]} clusters: I. The Ba0.78Tm0.22F2.22 crystals

The displacements of Ba²⁺ cations in the cationic motif of Ba_0.78Tm_0.22F_2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba_{1 ? x} R _x F_{2 + x} , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba₈[R ₆F_68–69]} superclusters of structural defects with nanometer linear sizes. The Ba ²⁺ cations are displaced from the fluorite crystallographic positions 4a (space group Fm $ \bar 3 The displacements of Ba²⁺ cations in the cationic motif of Ba_0.78Tm_0.22F_2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba_{1 − x} R _x F_{2 + x} , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba₈[R ₆F_68–69]} superclusters of structural defects with nanometer linear sizes. The Ba ²⁺ cations are displaced from the fluorite crystallographic positions 4a (space group Fm m) to the positions 32f. The static nature of the cation displacements is confirmed by the fact that these displacements are retained at a temperature of 110 K. The correctness of the interpretation of the correlation between the cation displacements and the formation of superclusters of structural defects is supported by the coincidence of the intercationic distances determined in the disordered phase Ba_0.78Tm_0.22F_2.22 with those found in the previously studied ordered phases Ba₄ R ₃F₁₇ (R = Y, Yb). The model with splitting of cationic positions is appropriate for testing in structural investigations of crystals of the fluorite phases M _{1 − x} R _x F_{2 + x} at room temperature. Original Russian Text ? A.M. Golubev, L.P. Otroshchenko, V.N. Molchanov, B.P. Sobolev, 2008, published in Kristallografiya, 2008, Vol. 53, No. 6, pp. 1023–1030.