Dissociation of PtSi,NiSi and PdGe in presence of Pt,Ni and Pd films,respectively

⁴He⁺ ions backscattering spectrometry and x-ray diffractometry were used to study interactions between PtSi and Pt, NiSi and Ni, PdGe and Pd. Due to the dissociation of the compound the formation of a phase richer in metal was observed to grow at the original compound/metal interface in the temperature range considered, 280–325°C for Pt₂Si, 325°C for Ni₂Si and 180–260°C for Pd₂Ge. The growth kinetics of these new phases (Pt₂Si and Pd₂Ge) follow a parabolic relation between thickness and annealing time. At a given temperature the growth rate of Pt₂Si and Pd₂Ge in compound-metal structure is a factor higher than in the usual semiconductor-metal structure. Partially supported by Consiglio nazionale delle Ricerche (Italy) and by Commission of the European Communities.     

Effects of annealing on structural and morphological properties of e-beam evaporated AgGaSe2 thin films

Polycrystalline AgGaSe₂ thin films were deposited by using single crystalline powder of AgGaSe₂ grown by vertical Bridgman-Stockbarger technique. Post-annealing effect on the structural and morphological properties of the deposited films were studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA) measurements. XRD analysis showed that as-grown films were in amorphous structure, whereas annealing between 300 and 600 °C resulted in polycrystalline structure. At low annealing temperature, they were composed of Ag, Ga₂Se₃, GaSe, and AgGaSe₂ phases but with increasing annealing temperature AgGaSe₂ was becoming the dominant phase. In the as-grown form, the film surface had large agglomerations of Ag as determined by EDXA analysis and they disappeared because of the triggered segregation of constituent elements with increasing annealing temperature. Detail analyses of chemical composition and bonding nature of the films were carried out by XPS survey. The phases of AgO, Ag, Ag₂Se, AgGaSe₂, Ga, Ga₂O₃, Ga₂Se₃, Se and SeO₂ were identified at the surface (or near the surface) of AgGaSe₂ thin films depending on the annealing temperature, and considerable changes in the phases were observed.     

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

⁷¹Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga₂O₃ samples prepared by sol-gel and ball-milling techniques. ²⁹Si and ²⁷Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. ⁷¹Ga NMR parameters are reported for the α- and β-Ga₂O₃ phases, and more tentatively for the δ-Ga₂O₃ phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga₂O₃ using Gaussian distributions in χ_Q, the extent of disorder in the Ga₂O₃ crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the ⁷¹Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga₂O₃, as well as the smallest nanocrystal sizes. Authors' address: Mark E. Smith, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Determination of binary diffusion coefficients of gases using photothermal deflection technique

A photothermal deflection (PD) technique was applied to measure the binary diffusion coefficients of various gases (CO₂–N₂, CO₂–O₂, N₂–He, O₂–He, and CO₂–He). With an in-house-made Loschmidt diffusion cell, a transverse PD system was employed to measure the time-resolved PD signal associated with the variation of the thermal diffusivity and the temperature coefficient of the refractive index of the gas mixture during the diffusion. The concentration evolution of the gas mixture was deduced from the PD amplitude and phase signals based on our diffraction PD model and was processed using two mass-diffusion models explored in this work for both short- and long-time diffusions to find the diffusion coefficient. An optical fiber oxygen sensor was also used to measure the concentration changes of the mixtures with oxygen. Experimental results demonstrated that the binary diffusion coefficients precisely measured with the PD technique were in agreement with the literature values. Moreover, the PD technique can measure the diffusion coefficients of various gas mixtures with both short- and long-time diffusions. In contrast, the oxygen sensor is only suitable for the long-time diffusion measurements of the gas mixtures with oxygen. PACS 78.20.Nv; 51.20.+d     

High-temperature superelasticity in CoNiGa,CoNiAl, NiFeGa,and TiNi monocrystals

The influence of the crystal orientation on the thermoelastic martensitic transformations developing under load was investigated for Co₄₉Ni₂₁Ga₃₀, Co₄₀Ni₃₃Al₂₇, Co₃₅Ni₃₅Al₃₀, Ni₅₄Fe₁₉Ga₂₇, and Ti_49.4Ni_50.6 (аt. %) monocrystals. It has been shown that the superelastic temperature range depends on the crystal orientation and reaches a maximum for [001]-oriented crystals. In monophase crystals of Co₄₉Ni₂₁Ga₃₀, Co₄₀Ni₃₃Al₂₇, Co₃₅Ni₃₅Al₃₀, and Ni₅₄Fe₁₉Ga₂₇ (at. %), segregation of dispersion particles takes place at test temperatures T > 623 K. A criterion for high-temperature superelasticity has been proposed which implies the attainment of high strength of the high-temperature phase due to a proper choice of the crystal orientation, deviation from stoichiometry, and segregation of dispersion particles. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10. pp. 19–37. October, 2008.     

CsCl effect on the optical properties of the 80GeS2–20Ga2S3 base glass

Optical properties of chalcogenide glasses belonging to the series (80GeS₂–20Ga₂S₃)_100−x (CsCl) _x with x=0;5;10;15;20 were investigated. The linear refractive indices (n ₀) were determined by prism measurements at four wavelengths: 633 nm, 825 nm, 1311 nm, and 1511 nm. Z-scan experiments were performed at 800 nm to measure the non-linear indices (n ₂) and the absorption coefficients (β). CsCl additions in the base glass (80GeS₂–20Ga₂S₃) are characterized by a white shift of the transmission in the visible range and a strong decrease of both n ₀ and n ₂. As the same time, β is also decreasing and this results in a figure of merit FOM=2βλ/n ₂ that remains relatively low at 800 nm, meaning that this series of highly non-linear glasses should be very suitable for optical switching applications at telecommunication wavelengths.     

IS α-Al2O3/ La0.8Sr0.2Ga0.8Mg0.2O3–δ really a new ionic conductor composite?really a new ionic conductor composite?

The recent interest in the (Sr, Mg) doped LaGaO₃ family compounds as electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) has suggested the study of the system α-Al₂O₃/La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ as a possible new ionic conductor composite. The studies concerning the electrical and microstructural characterization of the system α-Al₂O₃/La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ, in a wide compositions range, are reported and discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Proton-exchange influence on the polar phonons of LiTaO3 optical waveguides

Refractive index change and Raman scattering of Z-cut proton-exchanged LiTaO₃ optical waveguides with different composition have been studied. Probing of the waveguide depth was carried out by using micro-Raman spectroscopy in order to distinguish layers with different phase states. Varying the proton-exchange and post-exchange annealing regimes, two phases –α and β– in addition to the pure LiTaO₃, were clearly observed. The behaviour of the A ₁ vibrations of the TaO₆ groups was found to be very sensitive to the phase changes. Using data from waveguide Raman spectra, normal and oblique E(TO, LO) phonons were sorted and their properties were followed in the α and β phases. The OH-stretchings are considered to correspond to dipoles turned out of the oxygen planes. Received: 9 July 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Statistical theory of forming structural vacancies in palladium hydride

Based on molecular-kinetic representations, the theory of forming structural vacancies and vacant-ordered superstructure of Cu₃Au type in palladium hydride is developed. Free energies of PdH and Pd₃VH₄ phases are calculated. The constitution diagram is constructed that determines ranges of temperature and concentration in which phases with A1 and L1₂ structures are formed and regions of two A1 + L1₂ phases are realized. Results of theoretical calculations are in agreement with the available experimental data.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Two superconducting phases in the d=3 Hubbard model

The phase diagram of the d=3 Hubbard model is calculated as a function of temperature and electron density 〈n_i〉, in the full range of densities between 0 and 2 electrons per site, using renormalization-group theory. An antiferromagnetic phase occurs at lower temperatures, at and near the half-filling density of 〈n_i〉= 1. The antiferromagnetic phase is unstable to hole or electron doping of at most 15%, yielding to two distinct“τ" phases: for large coupling U/t, one such phase occurs between 30–35% hole or electron doping, and for small to intermediate coupling U/t another such phase occurs between 10–18% doping. Both τ phases are distinguished by non-zero hole or electron hopping expectation values at all length scales. Under further doping, the τ phases yield to hole- or electron-rich disordered phases. We have calculated the specific heat over the entire phase diagram. The low-temperature specific heat of the weak-coupling τ phase shows an exponential decay, indicating a gap in the excitation spectrum, and a cusp singularity at the phase boundary. The strong-coupling τ phase, on the other hand, has a critical exponent α≈-1, and an additional peak in the specific heat above the transition temperature possibly indicating pair formation. In the limit of large Coulomb repulsion, the phase diagram of the tJ model is recovered.     

Diffusion of 111Cd probes in Ga7Pt3 studied via nuclear quadrupole relaxation

Diffusion of ¹¹¹In/Cd impurity atoms was studied in Ga₇Pt₃ at the atomic level using the method of perturbed angular correlation of gamma rays (PAC). As in previous measurements on Ga₇Pd₃, probes were observed to occupy two inequivalent Ga-sublattices. Quadrupole interaction perturbation functions for each site exhibited damping at elevated temperatures that is attributed to nuclear relaxation caused by stochastic jumps of the probe atoms accompanied by reorientation of axes of the electric field gradient tensor. Fitted relaxation frequencies, proportional to mean jump frequencies of the probe, were thermally activated. Arrhenius plots revealed jump-frequency activation enthalpies of 0.94 (8) and 0.67 (4) eV for the two sites and frequency prefactors close to 1 THz. Results were similar to those found previously for Ga₇Pd₃, although jump frequencies are about 100 times lower in Ga₇Pt₃.     

Water vapor overtones pressure line broadening and shifting measurements

By using a spectrometer having as source a commercial etherostructure Al _x Ga_{1 − x} As diode laser operating in “free-running mode”, line shape parameters of some water vapor ro-vibrational overtones at 820–830 nm have been measured at room temperature. These weak absorption lines have been detected by using the wavelength modulation spectroscopy technique with second-harmonic detection. The broadening and shifting coefficients have been obtained by fitting the collected second-harmonic absorption features while varying the pressure of different foreign gases.     

Silica-based composite and mixed-oxide nanoparticles from atmospheric pressure flame synthesis

Binary TiO₂/SiO₂ and SnO₂/SiO₂ nanoparticles have been synthesized by feeding evaporated precursor mixtures into an atmospheric pressure diffusion flame. Particles with controlled Si:Ti and Si:Sn ratios were produced at various flow rates of oxygen and the resulting powders were characterized by BET (Brunauer–Emmett–Teller) surface area analysis, XRD, TEM and Raman spectroscopy. In the Si–O–Ti system, mixed oxide composite particles exhibiting anatase segregation formed when the Si:Ti ratio exceeded 9.8:1, while at lower concentrations only mixed oxide single phase particles were found. Arrangement of the species and phases within the particles correspond to an intermediate equilibrium state at elevated temperature. This can be explained by rapid quenching of the particles in the flame and is in accordance with liquid phase solubility data of Ti in SiO₂. In contrast, only composite particles formed in the Sn–O–Si system, with SnO₂ nanoparticles predominantly found adhering to the surface of SiO₂ substrate nanoparticles. Differences in the arrangement of phases and constituents within the particles were observed at constant precursor mixture concentration and the size of the resultant segregated phase was influenced by varying the flow rate of the oxidant. The above effect is due to the variation of the residence time and quenching rate experienced by the binary oxide nanoparticles when varying the oxygen flow rate and shows the flexibility of diffusion flame aerosol reactors.     

Microscopic scattering centers in AgGaS2

Summary Microscopic scattering centers in as-grown AgGaS₂ were investigated by optical and scanning electron microscopy. The defects consisted of rectangular platelets lying in the (100) and (010) planes. Their longest dimension (up to 100 μm) was parallel to the [001]-direction. Their shorter dimension was parallel to the [100] and [010] directions, respectively. The scattering centers were usually fractions of a micrometer thick and did not appear to have a different composition than the matrix phase according to results by energy-dispersive analysis. The defects can be eliminated by annealing in Ag₂S or quenching from elevated temperature, which suggests that the defects may themselves consist of a two-phase region, part of which should be Ga₂S₃-rich, where rodlike or platelike second-phase particles are embedded in the matrix. The quantity and the size of this second phase make a definitive compositional analysis difficult. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Thermodynamics and surface properties of liquid Al–Ga and Al–Ge alloys

The surface properties of Al–Ga and Al–Ge liquid alloys have been theoretically investigated at a temperature of 1100 K and 1220 K respectively. For the Al–Ga system, the quasi chemical model for regular alloy and a model for phase segregating alloy systems were applied, while for the Al–Ge system the quasi chemical model for regular and compound forming binary alloys were applied. In the case of Al–Ga, the models for the regular alloys and that for the phase segregating alloys produced the same value of order energy and same values of thermodynamic and surface properties, while for the Al–Ge system, the model for the regular alloy reproduced better the thermodynamic properties of the alloy. The model for the compound forming systems showed a qualitative trend with the measured values of the thermodynamic properties of the Al–Ge alloy and suggests the presence of a weak complex of the form Al₂Ge₃. The surface concentrations for the alloys show that Ga manifests some level of surface segregation in Al–Ga liquid alloy while the surface concentration of Ge in Al–Ge liquid alloy showed a near Roultian behavior below 0.8 atomic fraction of Ge.     

X-ray photoelectron spectroscopy study of the electronic structure of La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3+δ</Subscript> solid solutions

The electronic structure of La_1–x Ga _x MnO_3+δ. solid solutions is studied by X-ray photoelectron spectroscopy (XPS). The valence state of the manganese is estimated by various methods: by analyzing the difference in the binding energies of the Mn2p_3/2 and O1s electronic levels, analyzing exchange splitting in the spectrum of Mn3s, and from the dependence of the binding energy of the XPS spectrum of Mn2p_3/2 on the calcium concentration. The state of oxidation of the manganese in the compositions containing calcium lies between Mn³⁺ and Mn⁴⁺. The efficacies of these methods are compared. A correlation is found between the type of crystalline structure of La_1–x Ga _x MnO_3+δ. (0 ≤ x < 1) and the binding energy of the Mn2p_3/2 peak. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 419–427, May–June, 2009.     

Microscopic indium distribution and electron localization in zinc blende InGaN alloys and InGaN/GaN strained quantum wells

In order to give an atomic level understanding of the light emission mechanism and seek In distribution patterns closely related to the elusive electron localization centers, we optimize the crystal structure of zinc blende In _x Ga_1−x N (0≤x≤1) alloys with different In distributions and investigate their electronic structures using first-principles calculations. Our results show that In _x Ga_1−x N forms a random alloy, in which several-atom In–N clusters and In–N chains can exist stably with a high concentration due to their small formation energy. These In–N clusters and chains form more easily in zinc blende structure than in wurtzite structure. The band gap of zinc blende In _x Ga_1−x N alloys insensitively depends on the In distribution. Moreover, we find that both small In–N clusters and straight In–N chains with three or more In atoms, acting as radiative recombination centers, highly localize the electrons of the valence band maximum state and dominate the light emission of Ga-rich In _x Ga_1−x N alloys. The strains of In _x Ga_1−x N layers can enhance the electron localization in In _x Ga_1−x N/GaN strained quantum wells. Our results are in good agreement with experiments and other calculations.     

Geometries and electronic structures of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>, Ga<Subscript>3</Subscript>Se<Subscript>2</Subscript> and their anions. Theoretical insights

We hereby report a theoretical study on the equilibrium geometries, electronic structures and harmonic vibrational frequencies of Ga₂Se₃, Ga₃Se₂ and their anions. The ground and low-lying excited states of Ga₂Se₃⁻, Ga₂Se₃, Ga₃Se₂⁻ and Ga₃Se₂ are studied at the B3LYP and/or MP2 and CCSD(T) levels in conjunction with 6-311+G(d) and 6-311+G(2df) one particle basis sets. Ga₂Se₂⁻ adopts the C_2v kite geometry while Ga₂Se₃ has a ‘V’ geometry. Ga₃Se₂⁻ has a three-dimensional ‘D_3h ’ geometry and Ga₃Se₂ prefers the three-dimensional ‘C_2v ’ structure. Electron detachment energies from the ground electronic states of the anions to several neutral states are reported and discussed. At CCSD(T)//MP2 level, the adiabatic electron affinity (AEA) of Ga₂Se₃ is calculated to be 3.23 eV when using the 6-311+G(2df) basis set and that of Ga₃Se₂ is 2.77 eV with the 6-311+G(d) basis set. The findings of this research are analyzed and compared with gallium oxide and sulfide analogues.     

Luminescent crystals of MIIGa2S4:Er3+ (MII = Eu, Yb, Ca)

We have studied the photoluminescent properties of M^IIGa₂S₄:Er³⁺ polycrystals (M^II = Eu, Yb, Ca) for excitation by radiation with λ = 976 nm as a function of temperature. The samples were obtained by solid-state reaction. We have studied the comparative characteristics of the anti-Stokes and IR luminescence of these luminophores. We have determined the mechanisms for anti-Stokes emission of M^IIGa₂S₄:Er³⁺ crystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 332–335, May–June, 2007.