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 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.     

Modeling the distribution of Ga and Sb impurities in Ge‒Si single crystals grown by double feeding of the melt: Growth conditions for homogeneous single crystals

Mathematical modeling of the distribution of Ga and Sb impurities in homogeneous (with respect to the content of the main components) single crystals of Ge–Si alloys, grown by double feeding of the melt, has been performed in the Pfann approximation. It is shown that the axial gradient of impurity concentration in Ge–Si crystals can be controlled in wide limits by changing the ratio of crystallization rate and the rates of feeding of the melt by silicon and germanium rods. The conditions for growing alloy single crystals, homogeneous both with respect to the content of the main components and to the impurity concentration distribution, have been determined.     

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.     

Growth of Ga x In1 − x Sb solid solution single crystals by the Czochralski method

Crystallography Reports - Ga x In1 ? x Sb solid solution single crystals (x = 0.03–0.09) have been grown. The dislocation density and electrophysical parameters are measured. A...     

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.     

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.     

Growth of Nd-doped YVO4 single crystals along 〈1 0 0〉tetra by the anisotropic heating floating zone method

Nd-doped YVO₄ single crystals were grown along 〈1 0 0〉_tetra by the anisotropic heating floating zone (AHFZ) method. The relationship between the lamp power ratio (P_C/P_A) and the aspect ratio (c/a-axis diameter, D_C/D_A) of the grown crystals was investigated. D_C/D_A increased from 1.01 to 1.52 with increasing P_C/P_A from 0.7 to 1.5. The crystals grown by the AHFZ method developed a cylindrical shape with decreasing P_C/P_A, whereas those grown by a conventional FZ method developed an elliptical cylinder shape.     

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.     

Growth and characterization of near-band-edge transitions in β-In2S3 single crystals

Single crystals of β-In₂S₃ were grown by chemical vapor transport method using ICl₃ as a transport agent. The below and above band-edge transitions in β-In₂S₃ have been characterized using optical absorption and photoluminescence (PL) measurements in the temperature range 20–300 K. Thermoreflectance (TR) and photoconductivity (PC) measurements were carried out to verify the band-edge nature of the diindium trisulfide tetragonal crystals. Experimental analyses of the transmittance, PL, PC, and TR spectra of β-In₂S₃ confirmed that the chalcogenide compound is a direct semiconductor with a band gap of about 1.935 eV at 300 K. β-In₂S₃ is familiar with its defect nature. For the β-In₂S₃ crystals, two defect emissions and two above band-edge luminescences were simultaneously detected in the PL spectra at low temperatures. The energy variations of the defect emissions showed temperature-insensitive behavior with respect to the temperature change from 20 to 300 K. Temperature dependences of transition energies of the near-band-edge (NBE) transitions below and above band gap are analyzed. The origins for the NBE transitions in the β-In₂S₃ defect crystals are discussed.     

Growth of Pb1 ? xSmxX (X is Se or Te and x = 0.01–0.04) films on porous silicon

Films with the composition Pb_{1 − x} Sm _x X (X is Se or Te and x = 0.01–0.04) have been prepared and their structural properties have been investigated. A method for growing a buffer porous silicon layer is described. It is established that, for all samples (except for an amorphous film on the surface of porous silicon), the real film structure consists of blocks 10–50 μm in size and is independent of the porous layer morphology.     

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.     

The electrical and structural properties of InyGa1 ? yAs/InxAl1 ? xAs/InP quantum wells with different InAs content

In _x Al_{1 − x} As/In _y Ga_{1 − y} As/In _x Al_{1 − x} As/InP HEMT structures has been investigated with a change in the InAs molar fraction both in the quantum well and the buffer layer. The electrical parameters of the samples are measured at different temperatures. The structural parameters of the layers and the characteristics of the interfaces between them are determined by double-crystal X-ray diffraction. An increase in the Hall mobility and electron concentration, as well as in the structural quality of the samples, is observed alongside an increase in the InAs molar fraction in the quantum well. It is established that high electron mobility is retained at small (to 5%) mismatches between the buffer layer and substrate.     

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.     

Structural and electrical properties of quantum wells with nanoscale InAs inserts in InyAl1 ? yAs/InxGa1 ? xAs heterostructures on InP substrates

A complex study of the effect ofintroduction of nanoscale InAs inserts of different thicknesses into an In_0.53Ga_0.47As quantum well on the electrical properties and structural features of In_0.50Al_0.50As/In_0.53Ga_0.47As/In_0.50Al_0.50As nanoheterostructures with bilateral δ-Si doping grown on InP substrates has been performed. The layers of nanoheterostructures with a weak lattice mismatch are found to be equally (cube-on-cube) oriented. The introduction of a nanoscale InAs insert leads to an increase in mobility. At an insert thickness of about 1.8 nm, the effect of increasing mobility is saturated due to structural deterioration. The segregation of the second (apparently, wurtzite) phase is revealed; this process, as well as the formation of other defects in the nanoheterostructure layers, is due to local strains caused by variations of the indium content in the layers.     

Modeling of convection during the growth of Ga x In1 − x Sb single crystals by the Czochralski method in the presence of ultrasound

Crystallography Reports - The effect of ultrasound on convective flow in the melt at pulling of Ga0.03In0.97Sb single crystals has been modeled by the light cut method in an ebonite crucible using...     

Growth and characterization of REBa2Cu3O7−δ superconductor single crystals obtained from xenotime mineral

Single crystals of REBa₂Cu₃O_7−δ (RE=natural mixture of Y and heavy lanthanides in the single crystals) superconductor were successfully grown using rare-earth oxides extracted from xenotime mineral, allowing an alternative and simple route for superconductor single-crystal production. The methodology to extract the rare-earth mixed oxides from the xenotime mineral has three main steps: alkaline fusion, acid lixiviation and oxalic precipitation. Large single crystals with a typical 5×5×0.03 mm³ size were obtained by using a CuO-BaO self-flux and were characterized by scanning electron microscopy (SEM), energy-dispersive electron spectroscopy, X-ray diffraction, magnetic measurements and nanoindentation. The composition of the rare-earth elements of the crystal was different from the starting mixture, possibly due to the different solubilities of the elements in the melt. The final crystal stoichiometry was RE:Ba:Cu=1:2:3. X-ray diffraction analysis showed highly oriented c-axis (c=11.716±0.002 Å). The critical temperature was determined to be around T_C≅88-89 K after the crystals have been submitted to oxygen annealing. Hardness and elastic modulus for ab- and b(a)c-planes were 8.5±0.5 and 160±20 GPa, respectively.     

Structural investigation of amorphous and liquid Ge0.175Te0.825 by neutron scattering

The angular distribution of intensities of 1.003 Å neutrons scattered by an alloy of Ge_0.175Te_0.825 both in the amorphous and in the liquid state have been measured. The bulk amorphous sample, prepared by water quenching of the melt, has been examined at room temperature and just above the crystallization temperature T_x. The liquid sample has been examined at 400, 600 and 800°C. The analysis of the structure factors obtained from the corrected and scaled intensities indicates that the structural features of the amorphous sample are similar to those which would be expected if it were possible to cool the alloy to room temperature, while maintaining it in the liquid state. The analysis of the radical distribution functions, from Fourier inversion of the structure factors, indicates that a change of the coordination number n₁ from 2.43 to 3.25 occurs in passing from the amorphous to the liquid state at 400°C. This result has been interpreted in terms of coordination models. In the liquid state, it has been found that a model with fourfold coordinated Ge and threefold coordinated Te is consistent with n₁=3.25. The internal consistency of this model for the liquid state allows us to make a reasonable choice between the possibility of a 3 or 4 fold coordination for Ge in the glass. We conclude that a model based on four-fold coordinated Ge and twofold coordinated Te seems an appropriate representation for the coordination of the amorphous material.