Characterization of the optical properties of LPE InxGa1−xAsyP1−y thin layers grown on InP

A series of In_xGa_1?xAs_yP_1?y single-crystal thin layers have been grown on an InP substrate in a vertical liquid phase epitaxy furnace with a rotating slide boat system. The optical properties of these LPE quaternary alloys lattice-matched to InP have been investigated mainly by photoluminescence and electroreflectance measurements. Photoluminescence spectra of In_xGa_1?xAs_y P_1?y epitaxial layers are dominated by a strong luminescence line due to band-edge emission. At low temperatures, around 4.2 K, we have observed complicated luminescence bands with many fine structures. Electroreflectance spectra for the LPE In_xGa_1?xAs_yP_1?y layers are sufficiently broad to fulfil the low-field condition, and the analysis enabled us to determine precisely the band gap energy.     

Network topological thresholds in gallium doped As–Te glasses – Electrical and thermal investigations

Electrical switching and differential scanning calorimetric studies are undertaken on bulk As₂₀Te_80−xGa_x glasses, to elucidate the network topological thresholds. It is found that these glasses exhibit a single glass transition (T_g) and two crystallization reactions (T_c1 & T_c2) upon heating. It is also found that there is only a marginal change in T_g with the addition of up to about 10% of Ga; around this composition an increase is seen in T_g which culminates in a local maximum around x = 15. The decrease exhibited in T_g beyond this composition, leads to a local minimum at x = 17.5. Further, the As₂₀Te_80−xGa_x glasses are found to exhibit memory type electrical switching. The switching voltages (V_T) increase with the increase in gallium content and a local maximum is seen in V_T around x = 15. V_T is found to decrease with x thereafter, exhibiting a local minimum around x = 17.5. The composition dependence of T_c1 is found to be very similar to that of V_T of As₂₀Te_80−xGa_x glasses. Based on the present results, it is proposed that the composition x = 15 and x = 17.5 correspond to the rigidity percolation and chemical thresholds, respectively, of As₂₀Te_80−xGa_x glasses.     

Ionic conductivity of cold-pressed ceramics from grinding of R0.95M0.05F2.95 solid electrolytes (R = La,Nd; M = Ca,Sr, Ba) synthesized by reaction in melt

Cold-pressed ceramics of fluorine-conducting solid electrolytes La_{1 ? y} M _y F_{3 ? y} (M = Ca, Sr, Ba) and Nd_{1 ? y} Ca _y F_{3 ? y} with y = 0.95 have been synthesized in a melt of RF₃ (R = La, Nd) and MF₂ components in a fluorinating atmosphere and ground in a ball mill. The as-prepared ceramics require annealing, during which their porosity decreases and the conductivity is stably increased (by a factor of 250 for the R _{1 ? y} M _y F_{3 ? y} composition at 293 K). The Nd_0.95Ca_0.05F_2.95 and Nd_0.95Ca_0.05F_2.95 compositions have a maximum ionic conductivity σ(293 K) ～ 5 × 10^?6 Sm/cm. This value is larger (by a factor of about 10) than σ (293 K) for the R _{1 ? y} M _y F_{3 ? y} ceramics of tysonite phases prepared by mechanochemical synthesis with the cold pressing of reaction products.     

A new criterion for the glass-forming ability of liquids

Based on theoretical calculations using the fragility concept and the nucleation theory for a model glass-forming system, we propose a dimensionless criterion, ?, expressed by T_rg(ΔT_x/T_g)^a, with T_rg, the reduced glass-transition temperature, ΔT_x, the width of the supercooled liquid region when heating a glass, T_g, the glass transition temperature, and a, the exponent. The application of this simple criterion to various glasses, including network, metallic, and molecular glasses (except pure water), indicates an excellent correlation between the critical cooling rate R_c and ? in a Log R_c-? single master plot with a = 0.143.     

Occurrence of Kondo-like behavior in electrical-conductivity of Al70Pd20+xMn10−x (x = 0, 1 and 2) icosahedral quasicrystals

Zero-field and in-field (at 8 T) conductivity vs temperature (σ-T), magneto-resistance (Δρ/ρ), magnetization vs temperature (M-T) and magnetization vs field (M-H) of unannealed Al₇₀Pd₂₀Mn₁₀ and annealed Al₇₀Pd₂₀Mn₁₀, Al₇₀Pd₂₁Mn₉ and Al₇₀Pd₂₂Mn₈ quasicrystalline alloys have been studied in the temperature range of 1.4-300 K. Room temperature resistivity and the low-temperature magneto-resistance show a correlation with the corresponding magnetization. The σ-T for all the studied samples shows a pair of minima and maxima. The σ-T maxima show a correlation with the total magnetization. The analysis shows that σ-T is dominated by weak-localization effects. The minima are arising due to competing inelastic scattering times τ_i (e-ph scattering in the dirty metallic limit, τ_i ∝ T⁻²) and the Kondo-type spin-flip scattering time τ_sf whereas the maxima has been attributed to ‘Kondo-maxima’, occurring due to maxima in the spin-flip rate . The magneto-resistance of these samples shows a changeover from negative to positive where the negative component shows a correlation with the magnetization of the sample. The values of parameters derived from refinement give spin-flip scattering fields, which are found to be correlated with the total sample magnetization.     

293-K conductivity optimization for single crystals of solid electrolytes with tysonite structure (LaF3): I. Nonstoichiometric phases R 1−yCayF3−y (R = La-Lu,Y)

The systematic optimization of single-crystal fluoride-conducting solid electrolytes R _{1 ? y} Ca _y F_{3 ? y} with a tysonite type structure (LaF₃) with respect to the conductivity at room temperature, σ(293 K), is based on high-temperature measurements of σ(T) of stoichiometric fluorides of rare earth elements, RF₃ (R = La-Nd), in dependence of the radius \(R^{3 + } (r_{R^{3 + } } )\) ; two-component stoichiometric La_{1 ? y} R _y F₃ phases (R = Pr, Nd) in dependence on the average cation radius (r _cat ); and two-component nonstoichiometric phases R _{1 ? y} Ca _y F_{3 ? y} (R = La-Lu, Y) in dependence of the CaF₂ content. The optimization of the composition with respect to thermal stability is based on studying the phase diagrams of CaF₂-RF₃ systems and the behavior of R _{1 ? y} Ca _y F_{3 ? y} crystals upon heating when measuring temperature dependences σ(T). Singlecrystal samples of a number of investigated R _{1 ? y} Ca _y F_{3 ? y} compounds has σ(293 K) values high enough to be applied in solid-state electrochemical devices operating at room temperature (chemical sensors, fluorine-ion batteries, and accumulators) and in devices subjected to thermal cycling.     

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.     

Electrical switching in As-Te-Ag system-composition and temperature dependence

The current-voltage characteristics related to switching phenomena in silver doped arsenic telluride glasses, As₂₀Te_80−xAg_x and As₄₀Te_60−xAg_x, have been investigated over a wide composition range (4?x?14). The samples are found to show threshold switching behavior with the number of switching cycles withstood by the samples depending on the ON state current. The switching voltages are found to decrease with increase in silver content and a sharp minimum is seen at the composition x=12 for the As₂₀Te_80−xAg_x glasses and x=11 for the As₄₀Te_60−xAg_x glasses. An effort has been made to understand the observed composition dependence on the basis of increase in the conductance of the samples with silver addition and local structural effects.     

Investigation of the interaction of dimethyl sulfoxide with lipid membranes by small-angle neutron scattering

The influence of dimethyl sulfoxide (CH₃)₂SO (DMSO) on the structure of membranes of 1,2-dimiristoyl-sn-glycero-3-phosphatidylcholine (DMPC) in an excess of a water-DMSO solvent is investigated over a wide range of DMSO molar concentrations 0.0 ≤ X _DMSO ≤ 1.0 at temperatures T = 12.5 and 55°C. The dependences of the repeat distance d of multilamellar membranes and the thickness d _b of single vesicles on the molar concentration X _DMSO in the L _β’ gel and L _α liquid-crystalline phases are determined by small-angle neutron scattering. The intermembrane distance d _s is determined from the repeat distance d and the membrane thickness d _b. It is shown that an increase in the molar concentration X _DMSO leads to a considerable decrease in the intermembrane distance and that, at X _DMSO = 0.4, the neighboring membranes are virtually in steric contact with each other. The use of the deuterated phospholipid (DMSO-D6) and the contrast variation method makes it possible, for the first time, to determine the number of DMSO molecules strongly bound to the membrane.     

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 R _{1 ? y} Ca _y F_{3 ? y} with a tysonite-type (LaF₃) structure. In the single-crystal structure of Y_0.715Ca_0.285F_2.715 with a tysonite-type small unit cell (sp. gr. P6₃/mmc, a = 3.9095(2) Å, c = 6.9829(2) Å; Z = 2; R _w = 2.16%), the displacements of Y³⁺ cations and F^2? 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 Y_0.715Ca_0.285F_2.715 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.     

On the growth mechanism of an A-B crystal: II. effect of dissociative adsorption of B2 molecules

In a previous treatment, the BCF theory which applies to a monatomic crystal has been extended to the case of an A-B crystal growing from the vapour according to a scheme A(g) + B(g) ? AB(cr).The effect of dissociative adsorption of diatomic gas molecules according to a scheme A(g) + 1/2 B₂(g)?AB(cr) is incorporated in the present work. The results obtained are as follows. (i) The growth rate for a constant driving force Δg = kT ln[P_A(P_B2)^1/2/P_A0(P_B201/2] varies with the partial pressures P_A and P_B2. (ii) In the limiting cases, P_A å P_B2 or P_A å P_B2, the major component is nearly under saturation and the growth rate is me$\text{r}$ely controlled by the minor component, which works as the so-called “growth unit”. (iii) Because of the associative desorption of B adatoms, the mean migration distance $\text{x}$_sB depends on the partial pressure P_B2. (iv) The theory is in good agreement with an experiment on the vapour growth of CdTe under controlled partial pressure of the constituent element.     

Effect of impurities on the metastable zone width of solute–solvent systems

The novel approach to interpret the metastable zone width obtained by the polythermal method using the classical theory of three-dimensional nucleation proposed recently [K. Sangwal, Cryst. Growth Des. 9 (2009) 942] is extended to describe the metastable zone width of solute–solvent systems in the presence of impurities. It is considered that impurity particles present in the solution can change the nucleation rate J by affecting both the kinetic factor A and the term B related with the solute–solvent interfacial energy γ. An expression relating metastable zone width, as defined by the maximum supercooling ΔT_max of a solution saturated at temperature T₀, with cooling rate R is proposed in the form: (T₀/ΔT_max)²=F(1−Z ln R), where F and Z are constants. The above relation can also be applied to describe the experimental data on maximum supercooling ΔT_max obtained at a given constant R as a function of impurity concentration c_i by the polythermal method and on maximum supersaturation σ_max as a function of impurity concentration c_i by the isothermal method. Experimental data on ΔT_max obtained as a function of cooling rate R for solutions containing various concentrations c_i of different impurities and as a function of concentration c_i of impurities at constant R by the polythermal method and on σ_max as a function of impurity concentration c_i by the isothermal method are analyzed satisfactorily using the above approach. The experimental data are also analyzed using the expression of the self-consistent Nývlt-like approach [K. Sangwal, Cryst. Res. Technol. 44 (2009) 231]: ln(ΔT_max/T₀)=Φ+β ln R, where Φ and β are constants. It was found that the trends of the dependences of Φ and β on impurity concentration c_i are similar to those observed in the trends of the dependences of constants F and Z on c_i predicted by the approach based on the classical nucleation theory.     

Crystal structure of the low-temperature modification of α-RbB3O5

The crystal structure of α-RbB₃O₅ was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms to R _p = 2.93, R _wp = 3.80, R _B = 2.53, R _F = 2.84, and s = 1.54. The compound is isostructural to CsB₃O₅: it is orthorhombic, sp. gr. P2₁2₁2₁, a = 8.209(1), b = 10.092(1), c = 5.382(1) Å, and V = 445.9 Å 3. The framework structure is formed by the boron-oxygen [B ₂ ^III B^IVO₅] ^? rings consisting of two [BO₃]-triangles and a [BO₄]-tetrahedron. The rings are linked to form systems of helical chains running along the twofold screw axes parallel 2₁ to the a-and b-axes and infinite channels parallel to the a-and c-axes, which accommodate Rb atoms. The data were collected on an ADP-2 diffractometer [CuK _α radiation, Ni-filter, 12.00° < 2θ < 110.00°, a step in 2θ equal to 0.02°, count time 8 s per step, and 711 reflections α₁ + α₂)]. All the calculations were performed using version 3.3 of the WYRIET program. The comparison of the structures of α-and β-RbB₃O₅ and CsB₃O₅ revealed that the type of deformations in the framework structures of alkali-metal borates due to the changes of the temperature or the substitution of cations is determined by the role played by metal atoms, and especially, by large and heavy ions.     

Thermal diffusivity measurements on As-Te-Ga glasses by photo-thermal deflection technique: Composition dependence and topological thresholds

The thermal diffusivity (α) of As₂₀Te_80−xGa_x glasses (7.5 ? x ? 18.5) has been measured using photo-thermal deflection (PTD) technique. It is found that the thermal diffusivity is comparatively lower for As₂₀Te_80−xGa_x glasses, which is consistent with the memory type of electrical switching exhibited by these samples. Further, the thermal diffusivity of As₂₀Te_80−xGa_x glasses is found to increase with the incorporation of gallium initially (for x ? 9), which is consistent with the metallicity of the additive. This increase in α results in a maximum at the composition x = 9; beyond x = 9, a decrease is seen in α leading to a minimum at the composition x = 15. The observed composition dependence of thermal diffusivity of As₂₀Te_80−xGa_x glasses has been found to be similar to that of Al₂₀As_xTe_100−x glasses, based on which it is proposed that As₂₀Te_80−xGa_x glasses exhibit an extended stiffness transition with compositions x = 9 and x = 15 being its onset and completion, respectively. Also, the composition x = 17.5 at which a second maximum is seen in the thermal diffusivity has been identified to be the chemical threshold (CT) of the As₂₀Te_80−xGa_x glassy system, as at CT, the glass is configurationally closest to the crystalline state and the scattering of the diffusing thermal waves is minimal for the chemically ordered phase.     

Evaluation of glass-forming ability for bulk metallic glasses based on characteristic temperatures

A new criterion ω₂, defined as T_g/(2T_x?T_g)?T_g/T_l (wherein T_g is the glass transition temperature, T_x the onset crystallization temperature, and T_l the liquidus temperature), has been proposed to assess the glass-forming ability (GFA) of bulk metallic glasses (BMGs) based on the classical crystallization theory and the crystallization resistance. The analysis indicates that the factors T_g/(2T_x?T_g) and T_g/T_l could reflect the crystallization resistance and liquid phase stability of metallic glasses, respectively. From the available experimental data in literatures, the new criterion ω₂ has a better correlation with the GFA of metallic glasses than all other existing criteria such as T_rg(=T_g/T_l), ΔT_x(=T_x?T_g), γ(=T_x/(T_g+T_l)), ΔT_rg(=(T_x ? T_g)/(T_l ? T_g)), α(=T_x/T_l), β(=T_x/T_g + T_g/T_l), δ(=T_x/(T_l ? T_g)), φ(=T_rg(ΔT_x/T_g)^0.143), γ_m(=(2T_x ? T_g)/T_l), β(=T_x × T_g/(T_l ? T_x)²) and ξ(=ΔT_x/T_x+T_g/T_l). It has also been demonstrated that this ω₂ parameter is a simple and efficient guideline for exploring new BMG formers.     

Refinement of the crystal structure of the high-temperature phase G 0 in (NH4)2WO2F4 (powder, X-ray, and neutron scattering)

The (NH₄)₂WO₂F₄ compound undergoes a series of phase transitions: G ₀ → 201, K → G ₁ → 160, and K → G ₂, with a significant change in entropy (ΔS ₁ ～ Rln10 at the G ₀ → G ₁ transition), which indicates significant orientational disordering in the G ₀ phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G ₀ phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WO₂F₄]^2? ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH₄]⁺ ions calls for the combined method of elastic and inelastic neutron scattering. Inelastic neutron scattering is used to determine the state of hindered rotation for ammonium ions in the G ₀ phase. Powder neutron diffraction shows that the orientational disorder of NH₄ ions can adequately be described within the free-rotation approximation.     

Coloring elimination in Sr1 − x Ce x F2 + x crystals in the visible spectral range during growth from melt

Crystals of the Sr_{1 ? x} Ce _x F_{2 + x} compositions close to the congruent one (x ～ 0.3) are fabricated by the vertical directional crystallization. It is shown that the use of CF₄ to form a fluorinating atmosphere during growth leads to additional spurious absorption in the crystals in the range 350–600 nm. The use of PbF₂ and ZnF₂ for fluorination makes it possible to obtain colorless Sr_{1 ? x} Ce _x F_{2 + x} crystals of the desired optical quality from melt. The thermal conductivity of crystal with x ～ 0.28 in the temperature range 80-500 K lies within 1.50 ± 0.03 W m^?1 K^?1. High ionic conductivity makes the Sr_{1 ? x} Ce _x F_{2 + x} crystals promising for application in solid-state ionics.     

Cluster self-organization of germanate systems: Suprapolyhedral nanocluster precursors and self-assembly of the Li8GeO6, Li4GeO4, and Li6Ge2O7 crystal structures

Nanoparticles (An clusters of atoms or Kn polyhedra, where n is the number of atoms or polyhedra) corresponding to the initial stage of evolution of a chemical system have been modeled. Four series of cluster isomers K3 and K4 formed by different T tetrahedra are selected. The clusters are topologically represented by two-colored graphs. A change in their symmetry, depending on the composition and mutual arrangement of L and G tetrahedra in the chemical isomers, is established. Our model is used to search for cluster precursors in Li germanates Li₈GeO₆ (P6₃ mc, hP30), Li₄GeO₄(Cmcm, mC36), and Li₆Ge₂O₇ (P2₁/n, mP60). Three types of nanocluster precursors are identified: (A) polycyclic, with a tetrahedral arrangement of 3L + GG polyhedra (L and G are LiO₄ and GeO₄, respectively) in the Li₈GeO₆ structure; (B) monocyclic, with a sequence of L-G-L-G polyhedra in the Li₄GeO₄ structure; and (C) polycyclic, in the form of two bound monocyclic clusters with a sequence of L-L-G-G polyhedra in the Li₆Ge₂O₇ structure. Lithium atoms are found to play three different roles during structural self-assembly: they participate in the formation of nanocluster precursors, they serve as templates stabilizing the local structure of nanocluster precursors, and they act like spacers filling the voids between nanocluster precursors.     

High-temperature relaxation mechanisms in inorganic glasses

At temperatures below T_g two relaxation processes are observed in sheet glass (200–500°C) and low-alkali glass (300–600°C): the fast R₁ and the slow R₂ processes which are not connected with the viscous flow, and the structural relaxation occurring R₃ above T_g. The processes R₁ and R₂ proceed at an invariable structure and are characterized by activation energies as high as 5 kcal mol^?1 and 13–15 kcal mol^?1, respectively. The contribution of R₂ amounts to 70–80%. The process R₃, observed near and above T_g, is accompanied with structural variations and, therefore, its activation energy depends on temperature; at T_g it is equal to 60 kcal mol^?1.The processes R₁ and R₂ are due to the mobility and rearrangement of large kinetic units. On the contrary, R₃ is characterized by a low volume of kinetic units. This shows that the ions of silicon and oxygen are involved in this process. The relaxation process R₁ is assumed to be connected with the local fluctuation deformationsof the glass network as in the case of reverse glass deformation under high pressures, and the process R₂ with the mobility of microscopic areas of the glass micro-inhomogeneous structure (structural complexes, microblocks). The continuous spectra corroborate the existence of several high-temperature relaxation processes in silicate glasses.Thus, three relaxation processes are observed in alkali-silicate glasses in the temperature range 200–600°C: the processes R₁ and R₂ are mechanical relaxations, whereas the process R₃ is a structural relaxation determining the viscous flow of glass. The contribution of R₃ to stress relaxation amounts to 5%.There exists a temperature T_k (20–30° below T_g) which is the upper limit of the process R₂. At higher temperatures beginning from T_k the stress relaxation is first determined by the two processes R₁ and R₃, and then by one process R₃. At temperatures below T_k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R₃ becomes negligible and, therefore, in the glass annealing range (below T_k) the mechanical relaxation R₂ and R₁ are mainly responsible; their contribution to the whole relaxation process is as high as 95%.     

Structure of hydrides based on V-Cr alloys

The structure of deuterides based on V_0.9Cr_0.1 and V_0.5Cr_0.5 alloys has been investigated by neutron diffraction at room and low (77 K) temperatures. It is found that V_0.9Cr_0.1D_2.0 deuteride has a CaF₂ (Fm3m) crystal structure, which corresponds to vanadium dihydride. V_0.5Cr_0.5D_0.7 deuteride has a NiAs (P6/₃ mmc) structure type, similar to chromium hydride.