Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by x-ray diffraction (I)

An analysis of the atomic radial distribution function of Al_0.20As_0.30Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys obtained from quenching of the molten mixture of the elements was performed. A structure in which all the Al atoms are tetrahedrally bonded to the other types of atoms in the material, would satisfy the requirements of the experimental curve. Tetrahedral groups might be linked to each other by As and Te atoms, or directly through a Te or As atom belonging to more than one tetrahedra.     

Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by X-ray diffraction (II)

A spherical-shaped model of Al_0.20As_0.50Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys has been performed by the random Monte Carlo method. These models describe quite well the experimental radial distribution functions and abide by the expected coordination numbers apart from the threefold coordinated Te, of which an excess has appeared. The structures are formed, basically, of distorted tetrahedra around the Al atoms whose corners are occupied by As or Te atoms. Also, a separated phase model for Al_0.10As_0.20Te_0.70 alloy has been built taking into account the results of thermodynamical study on this amorphous alloy system. The fitting of this model was better than that of the model generated under the hypothesis of a continuous phase.     

The neutron diffraction study of liquid GeTe and As2Te3

The radial distribution analyses for GeTe and As₂Te₃ are made at temperatures above the melting point in the range of momentum transfer between 0.7 and 10.0 Å^?1 by the neutron diffraction technique. Furthermore, the local order in amorphous GeTe is determined by analyzing the intensity data of the electron diffraction of its thin film. The result for the amorphous film indicates that the local distribution of atoms in amorphous GeTe is not characteristic of the structure of its crystalline state. The shape of the peaks of the intensity curve for liquid GeTe differs from that for the amorphous and crystalline states. However, the short bond length and the small coordination number determined from liquid RDF suggest that the covalent-like bonding between nearest-neighbor atoms remains unbroken when melting. The general form of the structure factor for liquid As₂Te₃ is similar to that for the amorphous material reported previously. The position of the first peak of RDF in the liquid state is observed to be shifted to a shorter distance than the average of nearest-neighbor atoms in crystalline As₂Te₃. The structure of GeTe differs considerably between the crystalline, amorphous and liquid states, whereas the local order in the liquid As₂Te₃ is similar to that in the amorphous state but not in the crystalline state.     

Crystallization process on amorphous GeTeSb samples near to eutectic point Ge15Te85

One of most important properties of some tellurium-based chalcogenide glasses is the optical and electrical switching between two states: the glass and the crystalline state. The understanding in these systems of the glass to crystal transition and its transformation kinetics is essential for their application in non-volatile memories. GeTeSb and GeTe amorphous samples of compositions close to the eutectic point Ge₁₅Te₈₅ were obtained by rapid solidification from the liquid state employing melt spinning technique. The glass forming ability of this system, for this cooling technique, is restricted to a small composition range nearby the binary eutectic. The crystallization kinetics of the samples was studied by means of differential scanning calorimetry (DSC) under both isothermal and continuous heating regimes. The quenched samples and the crystallization products have been characterized by X-ray diffraction with Cu(Kα) radiation. The crystallization temperature, activation energy, crystallization enthalpy and the dependence of these properties on concentration are reported. The crystallization study of Ge₁₅Te₈₅ glasses shows: a primary crystallization of Te superimposed with a secondary crystallization of GeTe. The addition of Sb (5 at.%) to the eutectic point Ge₁₅Te₈₅ modifies this behavior: the crystallization of Ge₁₃Sb₅Te₈₂ glasses consists on the crystallization of Te and Ge₂Sb₂Te_5. The crystallization of the ternary glasses was modeled.     

Hydrothermal synthesis of Bi2Te3 nanowires through the solid-state interdiffusion of Bi and Te atoms on the surface of Te nanowires

Polycrystalline Bi₂Te₃ nanowires were prepared by a hydrothermal method that involved inducing the nucleation of Bi atoms reduced from BiCl₃ on the surface of Te nanowires, which served as sacrificial templates. A Bi–Te alloy is formed by the interdiffusion of Bi and Te atoms at the boundary between the two metals. The Bi₂Te₃ nanowires synthesized in this study had a length of 3–5 μm, which is the same length as that of the Te nanowires, and a diameter of 300–500 nm, which is greater than that of the Te nanowires. The experimental results indicated that volume expansion of the Bi₂Te₃ nanowires was a result of the interdiffusion of Bi and Te atoms when Bi was alloyed on the surface of the Te nanowires. The morphologies of Bi₂Te₃ are strongly dependent on the reaction conditions such as the temperature and the type and concentration of the reducing agent. The morphologies, crystalline structure and physical properties of the product were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS).     

Static and dynamic structures of liquid GeTe mixtures

Neutron diffraction measurements of the liquid Ge–Te system and quasi- and inelastic neutron scattering experiments of the eutectic liquid Ge₁₅Te₈₅ were performed in the temperature range from 400 to 560 °C. The large temperature dependence of the static structure for liquid Ge₁₅Te₈₅ is reconfirmed; however it is found that the compositional dependence obtained at around 80 °C above the liquidus curve is rather small. The dynamic structure factors of liquid Ge₁₅Te₈₅ in the Q-region from 0.2 to 2.6 Å⁻¹ show a normal temperature dependence. From the vibrational density of states, it is found that the Te–Te covalent bond persists in liquid Ge₁₅Te₈₅; however a bending mode was not assigned clearly.     

Electrical transport in n-type semiconducting Ge120BixSe70−xTe10 glasses

Resistivity and thermoelectric power were measured as a function of temperature and composition for Ge₂₀Bi_xSe_70?xTe₁₀ glasses (x = 0–11). The results were compared with the case of of Ge₂₀Bi_xSe_80?x glasses to see on the electrical properties the influence of the substitution of Te for a part of Se. The glasses show n-type conduction for x ? 9, which was not affected by the substitution of Te. The resistivity was about three orders of magnitude lower for the glasses with x < 10, and remained almost the same for x ? 10, compared with the glasses not containing Te. From the composition dependence of the calculated concentration of covalent bonds in the glasses, it was proposed that the appearance of n-type conduction was closely related to the formation of a sufficient number of BiSe bonds and the disappearance of the bonds between two chalcogen atoms such as TeSe or SeSe bonds, and that the remarkably low resistivity in the present glasses with x < 10 was likely to be attributed to the formation of TeSe bonds.     

Radial distribution studies of glassy tellurium-silicon alloys

X-ray investigations of glassy tellurium-silicon alloys (Si content 10–40 at%) are described. Alloys with 13 to 27 at% Si were obtained in bulk form, non-crystalline samples with 10, 30, 33, 36 and 40 at% Si were obtained by rapid quenching from the melt by the splat-cooling technique. The interference functions and the radial distribution functions of all alloys show a great similarity. However, the positions of the maxima of the pair distribution function decrease continuously with growing Si content and lead to values which are similar to atomic distances in crystalline Te and Si₂Te₃. The areas of the first two peaks of the RDF, F₁ (rougnly 2) and F₂ (roughly 12), are compared with calculated areas derived from different structural models which are based on the short-range order in crystalline Te with a chain structure and in Si₂Te₃ with a tetrahedron configuration. An interstitial model with Si atoms in holes of Te chains and a random substitution model, where the Te atoms and the Si atoms have the same short-range order, both show a concentration dependence of the areas F₁ contrary to the experimental values. A structural model which assumes the tetrahedron configuration of Si₂Te₃ for Si atoms (coordination number N_Si = 4) and a coordination number N_Te = 2 for Te atoms can describe the short-range order of glassy TeSi alloys in the whole investigated concentration range with 10–40 at% Si.     

Electron diffraction RDF analysis of amorphous As2Se3,AAs2Se2Te,As2SeTe2 and As2Te3 films

The local order in amorphous films of As₂Se₃, As₂Se₂Te, As₂SeTe₂, and As₂Te₃ has been examined by scanning electron diffraction with direct recording of the intensity of the elastically scattered electrons. The radial distribution functions indicate that there is a systematic increase in mean nearest neighbor distance as the Te concentration is increased, butthe mean coordination number increases slightly around 2.4. Pair function calculation of models shows that the 3-aand 2-fold coordinations of arsenic and chalcogens are retained in these glasses and the interatomic distances are close to those predicted from the Pauling covalent atomic radii of the constituent atomic species. The short range order appears to be similar in amorphous and crystalline As₂Se₃, but different in the case of As₂Te₃ as found by previous workers on bulk materials.     

Crystallization kinetics of amorphous Se60S20Te20

The temperature and the time dependence of the electrical conductivity (6) of a-Se₆₀S₂₀Te₂₀ are investigated. Conductivity anomalies are observed during the crystallization growth from both the amorphous solid and supercooled liquid phases. The changes of 6 during different isothermal annealing of a-Se₆₀S₂₀Te₂₀ samples with time are used to determine the ratio transformed from amorphous to crystalline. A value of 0.97±0.16 eV has been obtained for the activation energy of the crystal growth in the temperature range of 120–170°C.     

Crystal structure (Pb4.8Na1.2)[Si8(Si1.2B0.8)O25] with a new double tetrahedral layer and its comparison with hyttsjoeite, barisilite, benitoite, and langasite

A new lead-sodium borosilicate (Pb_4.8Na_1.2)[Si₈(Si_1.2B_0.8)O₂₅] (a = 9.5752 and c = 42.565 ?; space group R $ \bar 3 $ \bar 3 c) is synthesized under hydrothermal conditions, and its crystal structure is determined without preliminary knowledge of the chemical formula. The anionic radical of a new type is a double layer in which one of the three independent Si-tetrahedra contains an isomorphous boron admixture. Its topological relationship with the radicals in the structures of benitoite and langasite, as well as in the structures of lead silicates barisilite and hyttsjoeite, is found based on the block consisting of an octahedron and six tetrahedra. This allows one to consider that the new layer is derived from the hyttsjoeite layer by the replacement of the octahedron with two tetrahedra and the increase of the silicon fraction. Although lead atoms are located between the layers in the intersheet space, they form relatively strong bonds with silicon-oxygen layers. This structural type is a collector of heavy metals.     

Optical properties of thin films of system (As2Se3)80−x(As2Te3)x(SnTe)20 prepared by PLD

Thin amorphous films from system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ were prepared by pulsed laser deposition (PLD) from their bulk glasses and their optical properties were studied by spectral ellipsometry. Spectral dependencies of refractive index, absorption and extinction coefficient and optical gap (1.41–1.66 eV for (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 20 resp. x = 0) were calculated from optical tansmittance, from ellipsometric data by Tauc method. High values of refractive index n₀ (2.49–2.60) and of non-linear χ⁽³⁾ coefficient of index of refraction (4.9–7.5 × 10⁻¹² esu for the glass (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 0 resp. x = 20) made studied thin films of system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ promising candidates for application in optics and optoelectronics.     

Crystallization and local order of bulk AsxTe1−x glasses

The crystallization and the local order of the bulk As_xTe_1?x (0.2 ? × ? 0.8) glasses have been investigated by DSC, X-ray and Mössbauer methods. During the crystallization a metastable phase has been observed between 0.35 ? × ? 0.5 and it was identified as fcc AsTe. The local surrounding of these As_xTe_1?x glasses characterized by the ¹²⁵Te Mössbauer measurements were compared with those of the stable monoclinic As₂Te₃, hexagonal Te and metastable fcc AsTe.     

Crystal structure of britvinite [Pb7(OH)3F(BO3)2(CO3)][Mg4.5(OH)3(Si5O14)]: A new layered silicate with an original type of silicon-oxygen networks

The crystal structure of a new mineral britvinite Pb_7.1Mg_4.5(Si_4.8Al_0.2O₁₄)(BO₃)(CO₃)[(BO₃)_0.7(SiO₄)_0.3](OH, F)_6.7 from the Lángban iron-manganese skarn deposit (V?rmland, Sweden) is determined at T = 173 K using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θ_max = 58.43°, R = 0.052 for 6262 reflections). The main crystal data are as follows: a = 9.3409(8) ?, b = 9.3579(7) ?, c = 18.8333(14) ?, α = 80.365(6)°, β = 75.816(6)°, γ = 59.870(5)°, V = 1378.7(2) ?³, space group P1, Z = 2, and ρ_calcd = 5.42 g/cm³. The idealized structural formula of the mineral is represented as [Pb₇(OH)₃F(BO₃)₂(CO₃)][Mg_4.5(OH)₃(Si₅O₁₄)]. It is demonstrated that the mineral britvinite is a new representative of the group of mica-like layered silicates with structures in which three-layer (2: 1) “sandwiches” composed of tetrahedra and octahedra alternate with blocks of other compositions, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, and other blocks. The tetrahedral networks (Si₅O₁₄)_∞∞ consisting of twelve-membered rings are fragments of the britvinite structure. Similar networks also form crystal structures of the mineral zeophyllite and the synthetic phase Rb₆Si₁₀O₂₃. In the crystal structures under consideration, the tetrahedral networks differ in the rotation of tetrahedra with respect to the layer plane. Original Russian Text ? O.V. Yakubovich, W. Massa, N.V. Chukanov, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 233–242.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Some features relevant to switching processes in the amorphous semiconductor Si12Ge10As30Te48

The current-voltage characteristics of the amorphous semiconductor glass Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various experimenal conditions. The experimental results show that the resistance of the device in the ‘off’ state and the threshold voltage for the onset of switching action decrease with increasing the maximum current (I_p) passing through the device in the ‘on’ state, and that the threshold input power to set in the switching action is practically independent of temperature. The stability and the consistency of the device depend on the magnitude of I_p. When I_p is increased to a certain value the glass within and near the current filament between the electrodes become softened, and when it reaches a critical value the device is changed from its ‘switching-on’ state to a ‘memory’ state. All the results are in good agreement with the model that the filament formed to cause switching consists of a mixture of crystalline domains and amorphous domains with phase separations.     

Capacitance-voltage characteristics of In0.3Ge2Sb2Te5 thin films

The influence of indium doping on the capacitance variation with temperature and applied bias voltage of Ge₂Sb₂Te₅ is investigated. The capacitance-voltage (C-V) measurements of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ thin films were performed for a sweep of voltages from −20 to +20 V at different temperatures. The results show different capacitance behavior of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ films. As the temperature increases the capacitance of the indium-doped sample decreases and becomes negative. The negative capacitance effect might be attributed to a significant increase of the film’s conductivity due to temperature and applied bias voltage. The nonlinearity in the capacitance and conductivity could be related to the nucleation mechanism as the temperature becomes close to the amorphous-crystalline transition temperature.     

Synthesis, Crystal Structure and Properties of the Binuclear Complex [Sm2(C10H9N2O4)2(C10H8N2O4)2(H2O)4]

Abstract  A novel binuclear Smarium (III) complex with N-(2-propionic acid)-salicyloyl hydrazone (C₁₀H₁₀N₂O₄, H₃L) was prepared and characterized. The crystal structure of [Sm₂(H₂L)₂(HL)₂(H₂O)₄] (1) was determined by X-ray single crystal diffractometry. 1 crystallizes in the monoclinic systerm, space group P2(1)/c, with a = 0.10229(2), c = 31.549(7), b = 0.70599(15) nm, and Z = 4. In the structure, Sm(III) is nine-coordinated by carboxyl O and acyl O atoms and imido N atoms of two ligands (H₂L and HL forms) and O atoms from two water molecules. H₂L and HL act as tridentate ligands which form two stable five-numbered chelating rings sharing one edge in the keto form for each ligand, and the carboxyl groups of two ligands were coordinated via bidentated bridging form. The coordination polyhedron around Sm (III) was described as a monocapped square antiprism. The inter- and intramolecular hydrogen bonds resulted in a three-dimensional network and provided extra stability for the structure. The complex was researched the interaction with calf thymus DNA by electronic absorption titration and emission titration. The results show that the complex is bound to calf thymus DNA mainly by intercalation .The complex also shows good fluorescence property. Index Abstract  The title compound, [Sm₂(H₂L)₂(HL)₂(H₂O)₄] was synthesized by the treatment of N-(2-propionic acid)-salicyloyl hydrazone (C₁₀H₁₀N₂O₄, H₃L) and Sm(NO₃) · 4H₂O and its crystal structure determined. Single crystal X-ray diffraction analysis reveals that Sm (III) is nine-coordinated by carboxyl O and acyl O atoms and imido N atoms of two ligands (H₂L and HL forms) and O atoms from two water molecules. The carboxyl groups of two ligands were coordinated via bidentated bridging form. The coordination polyhedron around Sm (III) was described as a monocapped square antiprism. The inter- and intramolecular hydrogen bonds resulted in a three-dimensional network and provided extra stability for the structure.      

Composition dependence of photoconductivity of Al20AsxTe80−x glasses

The time dependent photocurrent of Al₂₀As_xTe_80−x glasses has been studied at low temperatures. It is found that the photocurrent of all the Al₂₀As_xTe_80−x samples studied does not decrease appreciably during illumination, which is consistent with the behavior of other narrow band gap amorphous chalcogenides. Further, the photosensitivity is found to be maximized for the composition x=25, which can be associated with rigidity percolation.     

Low-temperature specific heat of amorphous and crystalline Te0.81Ge0.15As0.04

We have measured the specific heats of amorphous and crystalline specimens of Te_0.81Ge_0.15As_0.04 between 0.2 and 20 K, and of crystalline Te_0.93As_0.07 between 1 and 20 K. Amorphous Te_0.81Ge_0.15As_0.04 shows a low-temperature linear specific heat anomaly whose magnitude, 0.027 mJ/mol-K², is similar to that of other amorphous insulators. Crystalline Te_0.81Ge_0.15As_0.04 exists as a two-phase material comprised of GeTe and As-doped Te. The specific heat of this material is analyzed in terms of a weighted average of the properties of its two constituents.