Structural distribution and rigidity of the glass network determined by EPR of Cu

The linewidth-broadening of the EPR spectra of Cu²⁺ in silicate, borate and phosphate glasses was analyzed in terms of the distribution of g_| and A_| (δ_| and δA_|) and related to the distribution of the rigidity of the network structure. X- and K-band spectra were measured for the glasses doped with ⁶³Cu²⁺ (93% abundance). The linewidth of the HFS shoulders with parallel orientation to H increased linearly with increasing m or microwave frequency. δg_| and δA_| showed a marked dependence on glass composition. For example, in Na₂O---B₂O₃ glasses, on going from x (mol% of Na₂O) being small through intermediate to large, δg_| varied from small through large to negligibly small. In contrast to these glasses δg_| was extremely large for 75PbO · 25B₂O₃ glass. The large δg_| for the Na₂O---B₂O₃ glassesof intermediate x was attributed to the coexistence of various borate groups competitively coordinating to Cu²⁺. Negligibly small δg_| for 70Na₂O · 30B₂O₃ glass and extremely large δg_| for 75PbO ·25B₂O₃ glass, both with a narrower structural distribution, reflect regidity of the glass network. The Pb---O bonding is strong enough to distort the coordination of Cu²⁺-complex. The situation is the reverse in Na₂O---B₂O₃ glasses.     

Ab initio study of NMR spectra of Li2S–SiS2 glass system

We obtained the chemical shielding constants of ²⁹Si in Li₂S–SiS₂ glass system on the basis of molecular orbital calculations. The relative chemical shieldings calculated at the HF/6-31G* level is −16.1 and −23.8 ppm for E(1) and E(0), respectively. These calculations are in good agreement with the previous NMR study: σ(E(2))>σ(E(1))>σ(E(0)). It is found theoretically that incorporation of Li₂S into SiS₂ does not influence the ²⁹Si chemical shift, while incorporation of Li₂O into SiO₂ does to a significant extent. It is also found that the existence of oxygen in Li₂S–SiS₂ glasses increases the chemical shielding of ²⁹Si by about 25 ppm, which is in good agreement with the experimental result.     

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x)(0.63B₂O₃ · 0.37Li₂O) · xFe₂O₃, with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe³⁺ concentrations (Fe₂O₃ molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g_{ef = 4.3} peak accompanied by a shoulder continuing down to g_{ef = 9.7}, are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe₂O₃ contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe³⁺ ions. It is concluded that the spectrum is due to diluted Fe³⁺ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe³⁺ ions is estimated as 4.9 Å. A diluted state of Fe³⁺ ions in the glass network in this range is also confirmed by the temperature dependence of the g_ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe³⁺ ions (Fe₂O₃ molar contents from 0.01 to 0.1), the width of the g_ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g_ef ≈ 2 line, which coexists at these concentrations with the g_ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe³⁺ ions. The temperature dependence of the g_ef ≈ 2 line intensity in 0.1 mol Fe₂O₃ glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe₂O₃ glass, which is attributed to an appearance, at higher Fe₂O₃ contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g_ef ≈ 2 line in the low-temperature EPR spectra.     

Compositional dependence of the optical gap in Ge1−xSx, Ge40−xSbxS60 and (As2S3)x(Sb2S3)1−x non-crystalline systems

An attempt is made to calculate the compositional dependence of the optical gap (E_g) in Ge_1−xS_x, Ge_40−xSb_xS₆₀ and (As₂S₃)_x(Sb₂S₃)_1−x non-crystalline systems in an alloy-like approach. From the comparison of both the experimental dependence of E_g and the calculated ones using the Shimakawa relation [E_{g AB} = YE_{g A} + (1−Y)E_{g B}] it is assumed that this equation is useful for such systems or parts of the systems which behave like almost ideal solutions.     

Carbon doping into GaAs using combined ion beam and molecular beam epitaxy method

Carbon (C) doping by combined ion beam and molecular beam epitaxy (CIBMBE) was investigated. In this technique, mass-analyzed C ions (¹²C⁺) are accelerated at low energies of 30 to 1000 eV and are irradiated onto growing GaAs substrate. Doping concentration control in CIBMBE can be very stably accomplished by simply adjusting the ion beam current density, which is independent of growth conditions of host materials. Experiments on systematic variation of C⁺ ion acceleration energy (E_C+) indicated that, in the energy range of E_C+<170 eV, net hole concentration (|N_A-N_D|) increases slightly as E_C+ increases. The highest |N_A-N_D| is obtained at E_C+ = 170 eV under the constant C⁺ ion beam current density. For E_C+>170 eV, |N_A-N_D| decreases dramatically with increasing E_C+, which can be explained in terms of enhanced sputtering effect. Although no evidence of damages induced by ion irradiation is shown for low E_C+ range of ≤170 eV, trace of damages is apparently observed for E_C+>170 eV.     

Studies of the optical properties of sodium copolyphosphate glasses

A number of samples of glassy sodium polyphosphate and copolyphosphates of sodium — cobalt, sodium — copper, sodium — nickel, sodium-manganese and sodium — calcium were prepared and their optical properties were investigated. The ultraviolet and visible spectra of these glasses were recorded at the room temperature. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet — visible region. The optical absorption edges (E₀) were calculated by using the relation h ν = B (hν − E₀)² where B is constant. The linear variation of (hν)^1/2 with hν where is the absorption coefficient and hν is the incident photon energy, is taken as evidence of non-direct interband transitions. The E₀ values obtained for copolyphosphate glasses appeared to depend on the size of the counter cation. The infrared spectra of all these glasses appeared to be almost the same, indicating the presence of characteristic P --- O --- P linkages of linear polyphosphate chains and two non bridging oxygen atoms bonded to phosphorus atoms O --- P --- O (PO₂) units in the polyphosphate tetrahedra.     

Indium content determination related with structural and optical properties of InGaN layers

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of In_xGa_1−xN (0<x0.2) grown by MOCVD on top of GaN/Al₂O₃ substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in In_xGa_1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of In_xGa_1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, E_g(x)=3.42−3.86x eV (x0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for In_xGa_1−xN single layers.     

Relationship between T0, Tg and their pressure dependence for supercooled liquids

We show that simple expressions can be derived from the Vogel–Fulcher–Tammann (VFT) law relating the glass transition temperature T_g, the VFT temperature T₀, their pressure derivatives, the steepness index of the ‘Angell plot’ and the strength parameter D of the VFT equation, in good agreement with experimental data. In the same way one can describe the dependence of the dT_g/dP on the relaxation time τ_g chosen to define the temperature T_g. Thus, this procedure allows a consistent rescaling and comparison of pressure dependent parameters obtained from different experiments and simulations.     

Modelling tracer diffusion and mobility of interstitials in disordered materials

A disordered system is described by a model in which interstitials can occupy sites of different energy. Tracer diffusion occurs in the absence of a concentration gradient and by jumps over energy barriers with the same saddle point energy. Using statistical mechanics and simple random walk considerations the tracer diffusion coefficient is D^* = γ⁰D⁰ where D⁰ is the known or unknown tracer diffusion coefficient of an arbitrary reference state, and γ⁰ is the activity coefficient with respect to the same reference state. The activity coefficient γ⁰ can be calculated for a given energy distribution. The results are in agreement with rigorous treatments of diffusion for the same potential trace. It is shown that Henry's law (γ⁰ → constant for c → 0) is valid for small interstitial concentrations c and for all types of energy distributions. Thus the diffusivity also becomes independent of concentration for c → 0.

Measurements of the hydrogen diffusion coefficient in liquid-quenched and vapour-quenched Pd₈₀Si₂₀ and its dependence upon temperature and hydrogen concentration are presented. These are in excellent agreement with the model if a Guassian distribution of energies is assumed. The width of the energy distribution is different for the two materials, but in each case agrees with results from solubility measurements. For alkali ions in oxide glasses two energy levels, one for sites adjacent to the immobile anions and one for sites far away from them, describes the so-called “weak electrolyte behaviour”, which determines diffusivity and conductivity at low alkali contents. With increasing alkali content the energy distribution changes, and predictions were made, which are in agreement with experimental findings in borate-silicate and GeO₂-glasses.     

Study of group-III binary and ternary nitrides using X-ray absorption fine structure measurements

It is demonstrated that the NEXAFS spectra are a “fingerprint” of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In_0.16Ga_0.84N and Al_yGa_1−yN. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p_x, p_y) or p_z character of the final state is identified. The energy position of the absorption edge (E_abs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E_abs of the Al_yGa_1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E_abs of In_0.16Ga_0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al_xGa_1−xN for x<0.5.     

Microstructure and growth rate of chemical vapour deposited TiN films in a vertical cold wall reactor

TiN films were grown on SUS304 substrates heated by an induction furnace in a vertical cold wall reactor. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of films obtained at different deposition conditions (temperature, gas flow rate and gas composition). Film structures obtained in the present vertical reactor had the following features compared with those in the tubular reactor: (1) Abnormally grown “star-shaped” crystals were observed on the surfaces of films deposited in the following ranges of total gas flow rate (Q_T), temperature (T) and partial pressures (P): 9.0×10⁻⁶ ≤ Q_T ≤ 1.6×10⁻⁵ m³ s⁻¹, 1223 ≤ T ≤ 1273 K, 0.92 ≤ P_TiCl4 ≤ 6.18 kPa, P_H2 = P_N2. The matrix grains were responsible for (211) preferred orientation. (2) Surface morphologies did not vary so much with P_TiCl4. On the other hand, a drastic change was brought about by adding HCl to the source gas, i.e., plate-shaped crystals dominated and the large “star-shaped” crystals were no longer present. (3) The apparent activation energy for deposition reaction was 230 kJ/mol (1173 ≤ T ≤ 1273 K) and 76.5 kJ/mol (1273 ≤ T ≤ 1373 K) at P_TiCl4 = 2.43 kPa and P_H2 = P_N2 = 49.45 kPa.     

Internal friction in vanadium-phosphate glasses doped with Na2O

The internal friction of _xNa₂O·(0.5−x)V₂O₅·O.5P₂O₅(x = 0.025–0.3) glasses was studied using the low-frequency torsion pendulum technique. The temperature spectrum of internal friction reveals three maxima. Maximum 1, the so-called “electron” maximum, is the same as observed in binary vanadium-phosphate glasses. The origin of maximum 2 can be attributed to ion migration. Maximum 3 appears for glasses containing more than 10 mol.% Na₂O and is probably connected with sodium-proton interactions.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

Influence of the nucleation time-lag on the activation energy in non-isothermal crystallization

In the present work, the influence of the nucleation time-lag on the non-isothermal glass crystallization is discussed. Differential thermal analysis (DTA) results of an iron-rich glass nucleated by Cr₂O₃ were obtained at different heating rates. The activation energy of crystallization, E_c, and the Avrami parameter, m, estimated by Kissinger's and Ozawa's equations were shown to be dependent on the heating rate. The value of E_c, obtained at 2.5, 5 and 7.5 K/min heating rates was calculated as 299 kJ/mol, while the value of E_c, obtained at 10, 15 and 20 K/min was as 499 kJ/mol. The value of m for ‘low' and ‘high' heating rates were 2.57 and 1.45, respectively. The results were interpreted on the basis of the non-steady state nature of the nucleation process. It was assumed that at high heating rates no nucleation takes place and the crystals grow on a existing fixed number of nuclei; the activation energy of crystal growth, E_g, can be estimated by applying the Kissinger equations. At low heating rates nucleation occurs and the number of nuclei formed is influenced by the heating rate; E_g can be estimated by the Matusita and Sakka equation.     

Dynamics of vitreous and molten zinc chloride

The dynamics of vitreous and molten zinc chloride have been studied with inelastic neutron scattering at the Intense Pulsed Neutron Source. The results are analyzed in terms of the scattering function, S(Q,E) and the effective vibrational density of states, G(E). The vibrational spectra of both glass and liquid are dominated by broad features centered at 15 and 35 meV which are identified with F₂ modes ZnCl₄²⁻ tetrahedra. The other two normal modes are note observed because of inadequate resolution and broadening and overlap resulting from coupling between tetrahedra. The behavior of ZnCl₂ is contrasted with other tetrahedrally coordinated glasses that have been studied with the same technique.     

Elastic constant anomaly in Fe80−xMoxB20 metallic glasses

The extensional sound velocities V_E of Fe_80−xMo_xB₂₀(x = 6, 7, 8) metallic glasses have been measured between 100 and 600 K in a saturating magnetic field. Young's modulus (E = _E², = density) shows anomalous temperature dependence at and below the magnetic transition temperature of each glass studied. These anomalies are interpreted as arising from the change of long-range spin coupling energy at the Curie temperature in a molecular field approximation. Comparison is made with similar effects in other crystalline and non-crystalline magnetic materials.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

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.     

Effect of Ag on the X-ray K-absorption edge of glassy Ga30Se70

The shift of the K-absorption edge, ΔE_k, is measured in ternary alloys of Ga₉₀Se_70−xAg_x (x=0, 5, 10, 15, 20) with respect to the binary Ga₉₀Se₇₀ alloy. The K-edge of Se progressively shifts towards the lower energy side as the silver concentration increases from 0 to 10 at.%. However, at higher concentration, it is observed that the relative shift starts decreasing. The present results follow Pauling's concepts of electronegativity which indicate that the nature of the bond is ionic-covalent in these glasses as found in many crystalline solids. The composition dependence of the edge shift is also discussed in terms of the structure of Se---Ga system.     

Alkali diffusion and electrical conductivity in sodium borate glasses

The diffusion coefficient of sodium, ²²Na, and silver, ¹¹⁰Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na₂O) has been measured from 100°C to slightly below the glass transition temperature, T_g. Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D_Na and D_Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D_Ag was slightly larger than D_Na. Na₂O-Ag₂O-B₂O₃ glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.