Local atomic structures in Zr-Ni metallic glasses

Local atomic structures of Zr100−xNix (x=33.3, 36, 50 at%) binary metallic glasses were investigated by means of extended X-ray absorption fine structure (EXAFS) probe. Structural parameters show that the Zr-Ni bond length, RZr-Ni, keeps a constant value of 2.62 Å, regardless of alloy compositions. This result implies that there is a strongly chemical interaction between Zr and Ni atoms, leading to significant chemical short-range orders (CSROs) in the present metallic glasses. Further analysis indicates that the SRO structures in these metallic glasses are extremely similar with those in their crystalline counterparts. It is interesting to note that the CSROs in the eutectic Zr₆₄Ni₃₆ metallic glass consist of Zr₂Ni-like and ZrNi-like CSROs.     

Molecular dynamics simulation of ternary glasses Li2O-P2O5-LiCl

Molecular dynamics simulation of binary glass 52Li₂O-48P₂O₅ and ternary glasses 45Li₂O-42P₂O₅-13LiCl and 39Li₂O-36P₂O₅-25LiCl was undertaken to study the effects of the addition of LiCl to the binary phosphate glass. The results show that addition of LiCl in the glass creates more non-bridging oxygens and reduces P-O-P chain lengths and branches in these chains, leading to a weakening of the glass matrix and consequent lowering of T_g. Interchain linkages mediated by Li in the binary structure diminish, and consequently better channels are created for Li⁺ movement, enhancing the ionic conductivity σ. Structure parameters also indicate the absence of LiCl clusters in the glass matrix.     

EXAFS and XRD studies of an amorphous Co57Ti43 alloy produced by mechanical alloying

We have investigated the local atomic order of an amorphous Co₅₇Ti₄₃ alloy produced by mechanical alloying by means of x-ray diffraction and EXAFS analyses on Co and Ti K-edges. Average coordination numbers and average interatomic distances between first neighbors where found from EXAFS and compared with those determined using an additive hard sphere (AHS) model associated with an deconvolution, and also with data from bcc- Co₂Ti compound. EXAFS results obtained indicated a shortening in the Co-Ti and Ti-Ti average interatomic distances when compared to those found by using the AHS-RDF method and an increase in the Co-Co and Ti-Ti average interatomic distances and a shortening in the Co-Ti one when compared to the interatomic distances found in the bcc- Co₂Ti compound. In spite of these differences, average coordination numbers obtained from EXAFS and AHS-RDF are similar to each other and also to those found in bcc- Co₂Ti.     

SiO2-PbF2-CdF2 glasses and glass ceramics

Lead-Cadmium fluorosilicate stable glasses were prepared and the vitreous domain region determined in the composition diagram. Characteristic temperatures were obtained from thermal analysis and the structural studies performed illustrate clearly the role played by lead atoms in the glasses crystallization behavior and the glass-forming ability of cadmium atoms. The occurrence of either a cubic lead fluoride or a lead-cadmium fluoride solid solution in crystallizing samples was found to be dependent on Er³⁺ doping. The optically active ions were found to concentrate in the crystalline phase and in fact play the role of nucleating agent as suggested from X-ray diffraction and EXAFS measurements.     

Crystallization kinetics in new Sb14As29Se52Te5 amorphous glass

Differential scanning calorimetry (DSC) under non-isothermal conditions is used to study the crystallization kinetics of Sb₁₄As₂₉Se₅₂Te₅ chalcogenide glass. In addition, two approaches are used to analyze the dependence of glass transition temperature (T_g) on the heating rate (α). One is empirical linear relationship between (T_g) and ln(α). The second approach is the use of straight line vs. 1/T_g for the evaluation of the activation energy for glass transition. The phases at which the alloy crystallizes after the thermal treatment have been identified by using X-ray diffraction. The diffractogram of the transformed material shows the presence of some crystallites of As, Te, AsSb, As₂Se₃, Sb₂Se₃ and AsSe_.5Te_.5 in the residual amorphous matrix.     

Reverse Monte Carlo simulations and Raman scattering of an amorphous GeSe4 alloy produced by mechanical alloying

The short and intermediate range order of an amorphous GeSe₄ alloy produced by Mechanical Alloying were studied by Reverse Monte Carlo simulations of its X-ray total structure factor and Raman scattering. The simulations were used to compute the , and partial distribution functions and the , and partial structure factors. We calculated the coordination numbers and interatomic distances for the first and second neighbors. The data obtained indicate that the structure of the alloy has important differences when compared to alloys prepared by other techniques. There are a high number of Se-Se pairs in the first shell, and some of the tetrahedral units formed seemed to be connected by Se-Se bridges.     

Optical absorption and thermoluminescence properties of ZnF2-MO-TeO2 (MO=As2O3, Bi2O3 and P2O5) glasses doped with chromium ions

Optical absorption, thermoluminescence, infrared spectra and differential thermal analysis of three different tellurite glass systems viz., ZnF₂-As₂O₃-TeO₂, ZnF₂-Bi₂O₃-TeO₂ and ZnF₂-P₂O₅-TeO₂ containing 0.4% of Cr₂O₃, have been investigated. Results have been analysed in the light of different oxidation states of chromium ion and the most suitable host for lasing Cr³⁺ ions has been identified and reported.     

Short-to-medium-range order in Mg65Cu25Y10 metallic glass

The atomic configurations of liquid and glassy Mg₆₅Cu₂₅Y₁₀ alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.     

Local icosahedral order and thermodynamics of simulated amorphous Fe

Local icosahedral order and thermodynamics of amorphous Fe have been analyzed in detail for models containing 3000 atoms, which were obtained by the molecular dynamics (MD) method. Models were obtained by cooling from the melt. Local order in models has been analyzed by using the technique proposed by Honeycutt and Andersen; we found an existence of icosahedral order in the system. Moreover, structural properties of models were also studied via radial distribution function (RDF), static structure factor, mean atomic distances, coordination number and bond-angle distributions. Glass transition temperature, heat capacity and potential energy of the system were found in addition to the evolution of structure and mean-squared displacement (MSD) of atoms upon cooling from the melt toward the glassy state. We found the glass transition temperature of simulated liquid Fe via temperature dependence of potential energy and it is close to that observed previously in the literature, i.e. T_g≈1070 K. Calculations showed that structural properties of amorphous Fe models with the Pak-Doyama interatomic potential agreed well with the experimental data.     

Nanostructured Fe50Ni50 alloy formed by chemical reduction

A high purity Fe₅₀Ni₅₀ nanometric alloy was synthesized by ultra rapid autocatalytic chemical reduction of the corresponding transition metal ions in an aqueous solution. The ratio of metal concentration in solution is preserved in the precipitated powder alloy and no metal segregation has been detected. The alloy was characterized as a nanostructured chemically disordered taenite phase by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). Transmission electron microscopy (TEM) showed that the as prepared alloy contained spherical particles with 96 nm mean diameter size. The particles are composed of crystallites (of ∼15 nm size) and a predominant disordered interfacial region. A thermal treatment of 673 K/2 h produced a structural relaxation with a significant narrowing in the XRD and Mössbauer lines with a exothermic flow in the DSC signal and an increase in the crystallite size to 30 nm.     

Characterization and electrical properties of V2O5-CuO-P2O5 glasses

Characterization and electrical properties of vanadium-copper-phosphate glasses of compositions xV₂O₅-(40−x)CuO-60P₂O₅ have been reported. X-ray diffraction (XRD) confirms the amorphous nature of these glasses. It was observed that, the density (d) decreases gradually while the molar volume (V_m) increases with the increase of the vanadium oxide content in such glasses. This may be due to the effect of the polarizing power strength, PPS, which is a measure of ratio of the cation valance to its diameter. The dc conductivity increases while the activation energy decreases with the increase of the V₂O₅ content. The dc conductivity in the present glasses is electronic and depends strongly upon the average distance, R, between the vanadium ions. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron.     

Theoretical studies of the structure of C2H2 adsorbed on Si(0 0 1)

The carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of the acetylene (C₂H₂) at 1 ML coverage adsorbed on the Si(0 0 1)-(2 × 1) surface at room temperature have been investigated by multiple-scattering cluster (MSC). The MSC result shows that the correct adsorption model of C₂H₂/Si(0 0 1)-(2 × 1) is unique, i.e. the dimerized structure with two domains, (2 × 1) and (1 × 2).     

Structure and electrical properties of SrO-borovanadate (V2O5)0.5(SrO)0.5−y(B2O3)y glassses

SrO-borovanadate glasses with nominal composition (V₂O₅)_0.5(SrO)_0.5−y(B₂O₃)_y, 0.0≤y≤0.4 were prepared by a normal quench technique and investigated by direct current (DC) electrical conductivity, inductively coupled plasma (ICP) spectroscopy, infrared (IR) spectroscopy and X-ray powder diffraction (XRD) studies in an attempt to understand the nature of mechanism governing the DC electrical conductivity and the effect of addition of B₂O₃ on the structure and electrical properties of these glasses. XRD patterns confirm the amorphous nature of the present glasses and actual compositions of the glasses were determined by ICP spectroscopy. The temperature dependence of DC electrical conductivity of these glasses has been studied in terms of different hopping models. The IR results agree with previous investigations on similar glasses and it has been concluded that similar to SrO-vanadate glasses, metavandate chain-like structures of SrV₂O₆ and individual VO₄ units also occur in SrO-borovanadate glasses. The SrV₂O₆ and VO_n polyhedra predominate in the low B₂O₃-containing SrO-borovanadate glasses as B substitutes into the V sites of the various VO_n polyhedra and only when the concentration of B₂O₃ exceeds the SrO content do BO_n structures appear. This qualitative picture of three distinct structural groupings for Sr-vanadate and Sr-borovanadate glasses is consistent with the proposed glass structure on previous IR and extended X-ray absorption fine structure (EXAFS) studies on these types of glasses. The conductivity results were analyzed with reference to theoretical models existing in the literature and the analysis shows that the conductivity data are consistent with Mott's nearest neighbor hopping model. Analysis of the conductivity data shows that they are consistent with Mott's nearest neighbor hopping model. However, both Mott VRH and Greaves models are suitable to explain the data. Schnakenberg's generalized polaron hopping model is also consistent with temperature dependence of activation energy. However, various model parameters such as density of states, hopping energy, etc. obtained from the best fits were not found to be in accordance with the prediction of the Mott model.     

Adsorption model determination of N2O/Ag(1 1 0) by theoretical studies of near-edge X-ray absorption fine structure

The nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of the N₂O adsorbed on Ag(1 1 0) have been studied by the multiple-scattering cluster (MSC) and self-consistent field (SCF) DV-Xα methods. Two adsorption models, in which the N₂O molecule attached to the Ag substrate through the central nitrogen (N_C) atom and the terminal nitrogen (N_T) atom, respectively, have been checked up thoroughly. The MSC calculation and the R-factor analysis show that the N₂O molecule is attached to the Ag substrate through the terminal nitrogen atom with the adsorption height h = 3.4 ± 0.1 Å. In the overlayer the N₂O molecules arrange themselves into a tilted chain due to the interaction between the cations and the anions in the molecules. The physical cause of the resonances in the NEXAFS spectra mentioned above has been discussed by the DV-Xα method, which confirms the MSC calculations.     

Early-stage ordering in in-situ annealed Fe51Pt49 films

We evidenced an early-stage ordering (ESO) in Fe₅₁Pt₄₉ film before the appearance of superlattice diffraction (long-range-order, LRO) using 40-nm-thick films prepared by magnetron sputtering onto quartz substrate. The appearance of L1₀ phase for samples deposited at substrate temperatures (T_s) 400 °C and higher was verified by X-ray diffraction. Surface roughness of Fe₅₁Pt₄₉ films, obtained via X-ray specular reflectivity with computational fitting, increases from 3.8 to 11 Å as T_s is increased from 25 to 275 °C. As further increase of T_s to 375 °C, the roughness drops to 3.2 Å and then increases again to 38 Å with T_s up to 700 °C. Measurement on residual strain demonstrates that it is initially compressive at T_s<400 °C. Thereafter the strain transfers to a tensile one and increases in magnitude as increasing T_s up to 700 °C corresponding to LRO transformation. Local atomic rearrangement is observed for samples deposited at T_s>250 °C by using extended X-ray absorption fine structure. Coercivity of films increases from 10 to 460 Oe as T_s increase from 25 to 375 °C (ESO) and then from 460 to 10,700 Oe with T_s 375-700 °C (normal LRO). The worked out quantitative estimation of ESO engages with that of LRO before T_s 400 °C.     

Various Trap States at SiGe-SiO2 Interface Formed by a Pulsed Laser

We report fabrication of low-dimensional structures in air by a pulsed laser on SiGe alloy samples in which different oxide structures are formed by laser irradiation and annealing treatment. The micro-structures on SiGe are more complex than those on Si. A series of photolumineseence （PL） emission is observed due to various trap states at the SiGe-SiO2 interface formed under different preparing conditions. The peak centre of PL emission exhibits red=shift from Si to SiGe because of narrower gap. A model for explaining the PL emission is proposed in which the trap states of the interface between some oxide and SiGe play an important role.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Positional disorder of Ba in the thermoelectric germanium clathrate Ba6Ge25

The local structure of Ba₆Ge₂₅ has been studied by X-ray diffraction and the atomic pair distribution function technique at 40 K and room temperature. Unambiguous evidence has been found showing that two out of three types of Ba atoms in Ba₆Ge₂₅ move off their positions and become locked in split sites at low temperatures.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Study on the influence of TiO2 on the insulating strength of ZnO-ZnF2-B2O3 glasses by means of dielectric properties

ZnO-ZnF₂-B₂O₃ glasses containing small concentrations of TiO₂ ranging from 0 to 0.6 mol% were prepared. Dielectric properties (constant ε′, loss tan δ, ac conductivity σ_ac over a moderately wide range of frequency and temperature at room temperature in air medium) of these glasses have been studied. The results of these studies were analyzed with the aid of data on optical absorption, ESR and IR spectra of these glasses. The analysis suggests that when the concentration of TiO₂>0.2 mol%, the titanium ions, in addition to Ti⁴⁺ state, co-exist in Ti³⁺ state, act as modifiers and reduce the breakdown strength.