Structure of rare-earth phosphate glasses by X-ray and neutron diffraction

Previous diffraction studies of the structures of rare-earth phosphate glasses (R₂O₃)_x(P₂O₅)_1−x are extended to glasses with smaller R³⁺ ions with R = Sm, Gd, Dy, Er, Yb, Y for x = ∼0.25 and with R = Nd, Sm, Gd for x = ∼0.15. Parameters for the P–O, R–O and O–O first-neighbor peaks were obtained by Gaussian fitting. P–P and R–P distances were estimated from the positions of peak maxima. Effects of residual silica or alumina contents present as a result of glass processing were taken into account for selected samples. The P–O coordination number, N_PO, and the P–O, O–O, P–P distances are consistent with the presence of phosphate tetrahedra and are insensitive to the R species and the R₂O₃ content. Rare-earth coordination numbers, N_RO, decrease from ∼8 to ∼6.5 when x is increased from ∼0.15 to ∼0.25. N_OO and N_PP decrease with increasing R₂O₃ content indicating the network disintegration. The numbers N_RO of the metaphosphate glasses (x = ∼0.25) decreases from ∼7 to ∼6 when R is changed from La to Yb. This change is also indicated by the behavior of the R–O distances and by constant number densities of atoms. The decrease in N_RO with increasing R₂O₃ content is due to the reduction in the number of terminal O (O_T) available for coordination of the R³⁺ ions (six at metaphosphate composition). Especially for smaller R³⁺ ions sharing O_T between two R sites is not favored. The decrease by ∼0.04 nm of the prominent R–R first-neighbor distance with a change of R from La to Yb at the metaphosphate composition is indicated by a shift to higher magnitude of scattering vector of the shoulder occurring in front of the first main diffraction peak.     

Positronium study of porous structure of sol–gel prepared SiO2: influence of pH

Positron annihilation lifetime and Doppler broadening of annihilation line techniques have been used to obtain information about the small-pore structure of SiO₂ prepared by the alkoxide method in different experimental conditions. Samples prepared in strong acidic environment (pH = 2) contain only small pores with mean radius R∼5 Å, while those prepared at pH = 6 and pH = 9 contain pores of two sizes, R∼5 and R∼17–26 Å. The influence of pH, water/alkoxide molar ratio and temperature of heat-treatment of the samples on their pore structure has been studied.     

Statistical analysis of threshold load for radial crack nucleation by Vickers indentation in commercial soda-lime silica glass

The threshold load for nucleation of primary radial cracks in commercial soda-lime silica glass was investigated at room temperature by using Vickers indentation. The Weibull statistics with three-parameters was employed to analyze the number of cracks nucleated around each indentation as function of the applied load. The used probability estimator was (n − 0.5)/N, where n is the total number of radial cracks nucleated at a specific load and N is the total number of possible cracks. The minimum applied load for radial crack nucleation that best fitted the experimental data was equal to 180 mN. Comparison between experimental data distribution and the binomial distribution was also performed. The results were interpreted according to the fracture mechanics models for sub-threshold indentation flaws of Lathabai et al. and for crack initiation in elastic/plastic indentation of Lawn and Marshall.     

The local atomic environment of oxygen in silicate glasses from molecular dynamics

Recent molecular dynamics (MD) results for (Na₂O)_x(SiO₂)_1−x and (CaO)_x(SiO₂)_1−x glasses show that co-ordination of bridging oxygen (O_b) by modifiers (M) is a normal structural feature. This can be explained as a consequence of the limitation on oxygen co-ordination in network oxides, a common rule of thumb being that total co-ordination of oxygen by (Si + M) is ⩽4. This gives an upper limit on co-ordination of non-bridging oxygen (O_nb) by modifiers of N_OnbM ⩽ m with m = 3, corresponding to N_MOnb ⩽ mv, where v is modifier valence. If modifier co-ordination exceeds this limit, i.e. N_MO > mv, then there is bonding of O_b to modifiers, i.e. N_ObM > 0. This is the case in alkali and alkaline earth silicate crystals (apart from Be and Mg), and is predicted to be a feature of glasses in these systems. An illustration of the influence of oxygen co-ordination is given from MD models of (CaO)_0.33 (SiO₂)_0.67 glass at pressures of 5 and 10 GPa. The main effect of densification is to increase the co-ordination of Ca by O_b. This can be understood because at 0 GPa the co-ordination of O_nb by Ca is already high, with N_OnbCa ∼ 2.7, but the co-ordination of O_b by Ca is less high, with N_ObCa ∼ 1, and so can more easily increase.     

Electromechanical and electro-optic properties of xBiScO3–yBiGaO3–(1−x−y)PbTiO3 single crystals

Single crystals in the xBiScO₃–yBiGaO₃–(1−x−y)PbTiO₃ (BS–BG–PT) system were grown by the high temperature solution method using Pb₃O₄ and Bi₂O₃ as the flux. The dielectric permittivity (ε_r) at room temperature for unpoled tetragonal crystals was determined to be 500–600 with dielectric loss tangents less than 0.3%. The Curie temperature was found to be around ∼420–450°C, with a dielectric maximum, exhibiting relaxor behavior. The longitudinal piezoelectric coefficient (d₃₃) was found to be ∼300 pC/N for 〈0 0 1〉 oriented tetragonal crystals with electromechanical coupling factor (k₃₃) of 75%, with a shear mode, d₁₅∼290 pC/N and k₁₅∼45%, lateral mode, d₃₁∼−55 pC/N and k₃₁∼−37%. The remnant polarization (P_r) was 46 μC/cm² with a coercive field (E_c) of 43 kV/cm at 1 Hz and DC field of 60 kV/cm. The linear electro-optic (E-O) coefficients of poled crystals determined using an automated scanning Mach–Zehnder interferometer method at room temperature and wavelength of 632.8 nm were r₃₃=36 and r₁₃=4 pm/V, respectively.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

Surface morphology of spin-coated As–S–Se chalcogenide thin films

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R_q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm⁻¹) and C–H (at 2965, 2935 and 2880 cm⁻¹) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T_g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.     

Determining the primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system

The primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system are determined on the basis of induction time measurements. The induction time of cloxacillin sodium is experimentally determined by the laser scattering method at different supersaturations and temperatures. The measured induction times are then treated using the models of mononuclear and polynuclear mechanisms. It is discovered that the primary nucleation mechanism of cloxacillin sodium is identified as polynuclear mechanism, which relates the induction time and the supersaturation for various growth mechanisms. On the basis of these analyses, the growth mechanism of cloxacillin sodium is two-dimensional nucleation-mediated growth at 285 K, and changes into spiral growth with increasing temperature (at 291 and 297 K).     

Na–Pb liquid alloy structure at low lead concentrations: Neutron-diffraction studies

Na–Pb liquid alloys with the lead concentration of 1.5 and 7.9 at.%, and pure sodium as a reference system are studied by neutron-diffraction at 700 K. A characteristic prepeak of static structure factor, S(Q), at Q ∼ 1.2 Å⁻¹, known from neutron-diffraction studies of the Na–Pb alloys with the lead concentration of 20 at.% and higher, is not observed for the alloys under consideration. The experimental S(Q) values are compared with the results of molecular-dynamics simulations.     

Aging of oriented polymer glasses

Dimensional (D) and enthalpy relaxation (ΔH) of oriented polymer glasses (PS and PC) have been studied as function of temperature, between T_g and T_g−20 °C, and aging time t, ranging to several weeks. The dimensional relaxation (shrinkage) and enthalpy relaxation curves verify the logarithm law D(t) ∼ H(t) ∼ log t, between an incubation τ_i and a final relaxation time τ_f. The time τ_f to reach the equilibrium (D and ΔH) follows the Vogel–Tamann–Fulcher (VFT) law. Enthalpy relaxation and shrinkage exhibit important differences. Enthalpy relaxation of oriented and isotropic polymers follows the same logarithm law, independent of the draw ratio λ and the mode of deformation, the relaxation time τ_f coincides with the relaxation time of the α segmental motions. Shrinkage depends on λ and the mode of deformation, the relaxation time τ_f is attributed to the normal mode, the relaxation time of the whole chain. Finally the shrinkages of PS and PC show some differences. PC at short aging times presents another type of dimensional relaxation which would be due to the β motions. This would be in close connection with the ductile (PC) and fragile (PS) behavior of these two polymers far below T_g.     

Enhancement of photoluminescence in SiCxNy nanoparticle films by addition of a Ni buffer layer

This work reports the effect of the presence of a Ni buffer layer on the photoluminescence (PL) of SiC_xN_y nanoparticle films prepared by RF plasma magnetron sputtering process in a reactive N₂ + Ar + H₂ gas mixture. An introduction of a Ni buffer of 80 nm or thicker remarkably improves the PL of the films. Annealing in a temperature range of 400–1100 °C is found to significantly affect the PL intensity. Optimal PL is achievable at 600 °C. X-ray photoelectron and Fourier-transform infrared spectroscopy suggest that the strong PL is directly related to the composition of the SiC_xN_y nanoparticle and the concentration of Si–O, and Si–N bonds. The results are relevant to the development of wide bandgap optoelectronic devices.     

Molecular dynamics,diffraction and EXAFS of rare earth phosphate glasses compared with predictions based on bond valence

Rare earth (RE) phosphate glasses, (R₂O₃)_x(P₂O₅)_1−x, commonly form with x < 0.3. As previously discussed by Hoppe, in this composition range Q² groups provide 6 O_nb (non-bridging oxygens) per RE, but higher RE coordinations can occur if RE are bonded to O_nb on Q³ groups, or if RE–O_nb–RE configurations occur (where Qⁿ refers to a PO₄ tetrahedra with connectivity of n). The values of N_RO and R_RO from the majority of the previous diffraction and EXAFS studies of RE phosphate glasses have been surveyed. Overall, the experimental results for 0.18 ⩽ x ⩽ 0.28 indicate RE coordination is from 6.5 to 7.0 for large RE, and slightly lower for small RE. For x ⩾ 0.23 this implies the occurrence of RE–O_nb–RE configurations, as observed in recent diffraction studies. The experimental results for x ⩽ 0.15 indicate RE coordination is from 7.5 to 8, which can be attributed to RE bonding to O_nb on Q³ groups. RE coordination of 8 for x ∼ 0.15 requires 2 Q³ groups (per RE) to be connected to Q² groups. Comparison with estimated connectivities between Q² and Q³ groups indicate that chemically ordered Q²–Q³ linkages are required. A recent MD model of a Tb metaphosphate glass has N_RO = 6, but includes Tb bonded to O_nb on Q³ groups and Tb–O_nb–Tb configurations, because it has a broad Qⁿ distribution.     

Space charge limited conduction in a-(Ge20Se80)1−xSnx thin films

The present paper reports the dc conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of (Ge₂₀Se₈₀)_1−xSn_x glassy alloys where 0 < x < 1. Current–voltage (I–V) characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behavior is observed. However, at high electric fields (E ∼ 10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. These results show that the effect of incorporation of Sn in the Ge–Se system is quite different at its low and high concentration. This peculiar role of third element Sn as an impurity in the pure binary Ge₂₀Se₈₀ glassy alloy is also discussed.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Kinetics of the exchanges at the solid–liquid interface during the dissolution process of gallium orthophosphate

Gallium orthophosphate is a piezoelectric material with an α-quartz structure. In order to manufacture bulk acoustic wave devices (BAW), controlled chemical dissolution is often used to reduce the thickness of the piezoelectric membranes. This paper presents the kinetics of the chemical exchanges, which occur at the solid–liquid interface during the chemical dissolution of GaPO₄ in phosphoric acid. Based on chemical composition of phosphoric acid solvent, the pure dissolution rate is determined. A strong anisotropy of chemical reactivity is formed. The dissolution rate is the lowest for the crystallographic z-plane (0 0 0 1) but this orientation is the most sensitive with respect to the proton concentration and the temperature. In accordance with the crystal growth rates, the nucleation at the interface for the (1 0 2 0) plane, named X-plane, is also the most rapidly dissolved. Assuming the activation energies corresponding to dissolution and to nucleation are like standard activation energies, the different values of the standard enthalpy variation are calculated. The most important variation is obtained for the z-plane (Δ_rH=−14.3 kJ/mol) and the lowest for the X-plane (Δ_rH=−5.4 kJ/mol).     

Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO4 glass–ceramic

A Li_1.5[Al_0.5Ge_1.5(PO₄)₃] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (T_g), crystallization onset (T_x) and melting (T_m) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature T_gr = T_g/T_m = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between T_g and T_x. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.     

Phase composition,microstructure and conductivity of (85-α)VO2–15(Vanadium–Phosphate–Glass)–αCu glass–ceramics

Electric conductivity, microstructure and phase composition of (85-α)VO₂–15VPG–αCu glass–ceramics (VPG = Vanadium–Phosphate–Glass) with copper content in the interval 0 wt% ⩽ α ⩽ 15 wt% were investigated. VPG is the glass (molar %) 80V₂O₅–20P₂O₅. Only two phases: VO₂ and VPG were identified when α ⩽ 5 wt%. VO₂ crystallites, VPG and pores are observed in these ceramic microstructures. Glass forms layers 1–2 μm thick in the space between VO₂ crystallites. The copper is dissolved in VPG during glass–ceramic synthesis. It increases the electric conductivity of the glass and provides improvement of electrical bonds between VO₂ crystallites. Therefore glass–ceramics conductivity exhibits an abrupt change of approximately 100× in the vicinity of the phase transition temperature, T_t, in VO₂. A new crystalline phase appears in (85-α)VO₂–15VPG–αCu ceramics when α ∼ 6 wt%. This phase is observed as small crystallites with the sizes of 1–5 μm. The increase in such phase content, with an increase in copper content is accompanied by a decrease in the content of VO₂. Percolation along the new phase is a primary contributor to electric conductivity when α ⩾ 8 wt%. In this case the conductivity exhibits no abrupt change in the vicinity of the temperature T_t. The new phase is probably the bronze Cu_xV₂O₄. It crystallizes from a liquid phase during glass–ceramics synthesis.     

Role of ballistic transport in photoluminescence excitation of Si nanocrystals

Photoluminescence of Si NCs with the size (10–300 nm) bigger than the exciton Bohr radius in the bulk Si crystals (4.8 nm) has been considered. Photoluminescence in such NC systems is analyzed from the point of view of new concept based on the effect of hot carrier ballistic transport in excitation of suboxide defect-related photoluminescence at the Si/SiO_x interface. The dependence of the 1.70 eV PL band integrated intensity on Si NC sizes was numerically calculated on the base of the hot carrier ballistic PL model. The well correlation between calculated and experimental results has been obtained for Si NCs with the size from the 30–150 nm range.     

Low temperature neutron diffraction and magnetization of Fe25Cu75 solid solutions

Metastable Fe₂₅Cu₇₅ solid solution obtained by high-energy ball milling has been studied by means of neutron thermo-diffraction and magnetization measurements. This material, which crystallizes in a face centered cubic (FCC) crystal structure, is ferromagnetic, with a value for the Curie temperature, around 190 K. The linear thermal expansion coefficient, α_T, has been estimated from the temperature dependence of the lattice parameter in the temperature range 5–300 K. An increase in the value of α_T following the normal trend observed in other metallic compounds is observed below 150 K, however, above this temperature α_T remains almost constant indicating a slight magneto-volume effect in this material.     

Experimental determination of the solid–liquid equilibrium,metastable zone,and nucleation parameters of the flunixin meglumine–ethanol system

Measurements of the metastable zone and solubility for flunixin meglumine–ethanol system were obtained. The solubility was measured within the temperature range from 288.15 to 328.15 K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: x_eq=2.35×10^?12e^0.07121T. The value of enthalpy of dissolution, enthalpy of fusion and enthalpy of mixing were determined to be 49.04, 64.03 and ?14.99 kJ mol^?1 respectively. The metastable zone width of flunixin meglumine was measured by an electric conductivity method. A comparison of the nucleation temperatures from electric conductivity measurement and from focused beam reflectance measurement (FBRM) shows that both detection techniques give almost the same results for flunixin meglumine. The nucleation parameters of flunixin meglumine in ethanol were determined from the metastable zone data. Over the equilibrium temperature range from 312.28 to 325.55 K, the nucleation rate constant was varied from 0.00001 to 0.00120 #/m² min, whereas the nucleation order was varied from 2.23022 to 3.39299. The obtained high values of nucleation order indicated a high rate of nucleation.