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.     

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.     

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.     

Atomic structure of (Sr0.50Ba0.50)Nb2O6 single crystals in the series of (SrxBa1 ? x)Nb2O6 compounds

Czochralski grown (Sr_0.50Ba_0.50)Nb₂O₆ single crystals have been studied by the method of precision X-ray diffraction analysis. The structural characteristics of (Sr_xBa_{1 − x} )Nb₂O₆ compounds with x = 0.33, 0.50, 0.61, and 0.75 were analyzed. The distributions of the Sr and Ba atoms over the crystallographic positions are considered depending on their concentration. The establishment of the mechanisms of isomorphous replacements in these solid solutions allows the variation, within certain limits, of crystal properties by changing the Ba/Sr ratio. __________ Translated from Kristallografiya, Vol. 47, No. 2, 2002, pp. 249–252. Original Russian Text Copyright ? 2002 by Chernaya, Volk, Verin, Ivleva, Simonov.     

AC conductivity of (As2Te3)95Ge5

The elctrical conductivity of amorphous chalcogenide (As₂Te₃)₉₅Ge₅ is investigated at variable frequency, from 1 kHz up to 35 GHz, and a variable temperature, from 77 to 300 K. The low-temperature conductivity is constant with temperature at a fixed frequency. At a fixed temperature, it obeys a ω^0.8 law only at low frequencies. The results are analysed with respect to the polaron problem, but also with a simple model of several dilute localized levels having a wide energy distribution in the forbidden band.     

A single-crystal X-ray diffraction study of an Al70.5Pd21Mn8.5 icosahedral quasicrystal

X-ray diffraction data were measured on a single grain of a stable Al_70.5Pd₂₁Mn_8.5 perfect quasicrystal, in order to complement the neutron diffraction data obtained previously. The F-type lattice structure of the six-dimensional periodic arrangement was confirmed along with the very strong chemical order, due basically to an uneven distribution of the Pd atoms among the lattice sites. This distribution might be the origin of the high stability of the icosahedral structure in this system.     

Structural study of (As2S3)0.6(GeS2)0.4 glass

(As₂S₃)_0.6(GeS₂)_0.4 glass in non-irradiated and γ-irradiated states has been studied by using high-energy synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and positron annihilation lifetime spectroscopy. The experimental results are explained by the local changes around As and Ge atoms upon irradiation. These changes are suggested to involve chemical bonds distortion, formation of defective bonds with wrong coordination, rotation of structural units and appearance of additional free volume in the glass network.     

Structural change of liquid Si15Te85 and Si20Te80 with temperature: Ab initio molecular dynamics simulations

Using ab initio molecular dynamics simulations, the structural and electronic properties of liquid Si₁₅Te₈₅ and Si₂₀Te₈₀ at two temperatures were studied respectively. Compared with available experimental data, the calculated structure factors are acceptable. From symmetry arguments, the calculated partial bond-angle distribution functions suggest that with increasing temperature the extensive tetrahedral network structures persist longer in liquid Si₂₀Te₈₀ than those do in liquid Si₁₅Te₈₅. Our results indicate that the local tetrahedral structure around Si atoms and the Peierls-like distorted local atomic structure around Te atoms both play important roles in the structural change of liquid Si₂₀Te₈₀ and Si₁₅Te₈₅, which also suggest that the mechanisms of the structural change upon cooling in liquid Si₂₀Te₈₀ and Si₁₅Te₈₅ are of no essential difference. The results of DOS and LDOS indicate that the variation of the dip in DOS at E_F mainly results from the change of Te p orbitals.     

Structural and electrical properties of Germanium-doped Sb70Te30 eutectic thin films

Ge-doped Sb₇₀Te₃₀ eutectic films show high potential for high transfer rate recording and non-volatile memory applications. Their potential applications are based on the difference in optical and electrical properties between the crystalline and amorphous phases. However, the structure and crystallization mechanism of such films is not yet well understood.The aim of this work is to study the amorphous-to-crystalline phase transformation mechanism in eutectic thin films of pure Sb₇₀Te₃₀ and doped with 2, 5 and 10 at% of Ge. Results of isokinetics and isothermal annealing were carried out using various techniques: four-probe electrical, optical reflectance and X-ray measurements.The results of optical and electrical measurements of Ge doped Sb₇₀Te₃₀ eutectic films showed that the addition of Ge increases both, the crystallization temperature and the effective activation energy of crystallization and that the mechanism of crystallization does not depend on Ge contents.In amorphous films, Ge acts as impurity center and does not affect the optical band gap value, but decreases the pre-exponential factor in the thermally activated conductivity. In crystalline films, Ge is probably incorporated into the Sb₇₀Te₃₀ structure and Ge vacancy are responsible for p-type metallic conduction, which increases with increasing the Ge content.     

Calorimetric investigation of the structural relaxation and crystallization of amorphous Ge16Te84 alloys

Differential scanning calorimetric measurements have been performed on amorphous Ge₁₆Te₈₄ alloys; the study of the dependence of the cooling rate on the reciprocal fictive temperature yields the activation energy of the relaxation process; this value fits well that deduced from shear viscosity measurements. The enthalpy difference between the supercooled liquid and the crystal has been investigated by drop calorimetry on samples annealed at different temperatures below T_g, during different times. These results, combined with the former obtained by DSC lead to the enthalpy values of the metastable liquid system from 390 to 510 K.     

Atomic structure of Al55(Cr1−xMnx)15Si30 metallic glasses observed by neutron diffraction

An atomic structure of Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49,1) metallic glasses was studied by neutron diffraction. An advantage of the neutron diffraction technique was fully exploited by utilizing the negative scattering length of Mn to form a neutron zero scattering ‘alloy’ for the component Cr_0.51Mn_0.49 in this quaternary Al---(Cr, Mn)---Si alloy. This allows the atomic distribution of the resulting quasibinary Al---Si metallic glass to be derived directly. Moreover, the (Al, Si)---TM (TM = Mn, Cr) and TM---TM pair correlations were also extracted by taking appropriate linear combinations of the atomic structures for the Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49, 1) metallic glasses. A sharp first peak in the (Al,Si) ---TM pair correlations thus obtained led to the conclusion that a strong attractive interaction exists between (Al, Si) and TM atoms and, hence, that the presence of the TM atoms is responsible for the formation of an amorphous phase.     

Structural analysis of CaSiO3 glass by X-ray diffraction and Raman spectroscopy

The short-range structure of CaSiO₃ glass was studied by a combination of radial distribution analysis with an intensity comparison method on the basis of X-ray diffraction data. Three main peaks appeared at 1.64, 2.43 and 3.64 Å in the radial distribution function curve. The Raman bands were observed at 367, 642, 878 and 972 cm⁻¹ and the Raman spectrum was found to be similar to that observed in pyroxenes: single chains consisting of SiO₄ tetrahedra are linked by calcium ions laterally. The calcium ion is surrounded by six oxygen atoms at an average distance of 2.34 Å and the remainder Ca---O pairs at a distance of 2.54 Å. The calculated intensity curve based on this model was in good agreement with the observed one.     

Synthesis and X-ray diffraction study of (Cs0.5Ba0.25)[UO2(CH3COO)3] and Ba0.5[UO2(CH3COO)3]

Uranyl triacetate complexes (Cs_0.5Ba_0.25)[UO₂(CH₃COO)₃] (I) and Ba_0.5[UO₂(CH₃COO)₃] (II) are synthesized for the first time and their structures are determined by X-ray diffraction. Both compounds crystallize in the cubic crystal system. The crystal data are as follows: a = 17.3289(7) ?, V = 5203.7(4) ?³, space group I2₁3 and Z = 16 (I); a = 17.0515(8)?, V = 4957.8(4) ?³, space group I $ \bar 4 $ \bar 4 3d, and Z = 16 (II). In I and II, as in all uranyl triacetates studied earlier, the coordination polyhedron of the uranium atom is a hexagonal bipyramid whose vertices are occupied by the oxygen atoms of the uranyl and three acetate groups. The uranium-containing group belongs to the AB ₃⁰¹ (A = UO₂²⁺, B ⁰¹ = CH₃COO⁻) crystal chemical group of uranyl complexes. It was found that compound II is isostructural to the (Rb_0.50Ba_0.25)[UO₂(CH₃COO)₃] studied earlier.     

Influence of Gd and Fe on crystallization of Al87Y5Ni8 amorphous alloy

Crystallization of amorphous Al-based alloys (Al-Y-Gd-Ni-Fe) was investigated by applying differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high resolution electron microscopy (HREM). It was shown that the crystallization in the examined alloys proceeds in three stages (DSC maxima). The two first stages are attributed to formation of solid solution of fcc Al(RE) nanograins in amorphous matrix. In the third stage the precipitation of ternary compound Al₁₉Ni₅RE₃ of the orthorhombic Al₁₉Ni₅Gd₃-type structure was observed. A partial substitution of Ni by Fe causes a change of stoichiometry and crystal structure of the ternary compounds: Al₈TM₄RE (TM = Fe, Ni; RE = Y, Gd) of the tetragonal ThMn₁₂ (Al₈Mn₄Ce)-type structure. A partial replacing of Y atoms by Gd in the Al₈₇Y₅Ni₈ based alloy shifts the Al(RE) nanocrystallization to lower temperatures. In contrast to this a partial replacing of Ni by Fe shifts the nanocrystallization to higher temperatures.     

Synthesis and X-ray diffraction study of complex neptunium(VI) potassium chromate K2[(NpO2)2(CrO4)3(H2O)] · 3H2O

The crystal structure of the K₂[(NpO₂)₂(CrO₄)₃(H₂O)] · 3H₂O compound was established. The structure consists of anionic layers [(NpO₂)₂(CrO₄)₃(H₂O)] _n ²ⁿ , between which K+ ions and crystallization water molecules are located. The coordination polyhedra of Np atoms are distorted pentagonal bipyramids. All chromate ions are bound in a tridentate-bridging fashion. __________ Translated from Kristallografiya, Vol. 49, No. 4, 2004, pp. 676–680. Original Russian Text Copyright ? 2004 by Grigor’ev, Fedoseev, Budantseva, Bessonov, Krupa.     

Growth and properties of (Pb0.90La0.10)TiO3 thick films prepared by RF magnetron sputtering with a PbO buffer layer

The (Pb_0.90La_0.10)TiO₃ [PLT] thick films (3.0 μm) with a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering method. The PLT thick films comprise five periodicities, the layer thicknesses of (Pb_0.90La_0.10)TiO₃ and PbO in one periodicity are fixed. The PbO buffer layer improves the phase purity and electrical properties of the PLT thick films. The microstructure and electrical properties of the PLT thick films with a PbO buffer layer were studied. The PLT thick films with a PbO buffer layer possess good electrical properties with the remnant polarization (P_r=2.40 μC cm⁻²), coercive field (E_c=18.2 kV cm⁻¹), dielectric constant (ε_r=139) and dielectric loss (tan δ=0.0206) at 1 kHz, and pyroelectric coefficient (9.20×10⁻⁹ C cm⁻² K⁻¹). The result shows the PLT thick film with a PbO buffer layer is a good candidate for pyroelectric detector.     

Indexing of diffraction patterns and indentification of fault vector in decagonal Al65Cu20Co15 phase

Characterisations of defect structures such as pencil disorder, faulting in quasiperiodic layers and dislocation in decagonal quasicrystals has assumed importance currently. The first step towards a complete understanding of these defects requires easy and unambiguous method of indexing the diffraction patterns. The present study is devoted to this task. New rules have been framed for an unambiguous indexing of the diffraction patterns following the model of Mandal and Lele. The scheme is applied to the indexing of important electron diffraction patterns from a decagonal Al₆₅ Cu₂₀ Co₁₅ alloy and the indices for the direction of streaking in some of the diffraction patterns have been found and are of the type .     

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.