Structural study of magnesium polyphosphate glasses

Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P₂O₅), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO₄ chains with rising MgO content is quantitatively described by the increasing concentration of Q¹ and Q⁰ groups determined by NMR. In the pyrophosphate region the Q¹ sites disproportionate to Q⁰ and Q² groups, whereas there is no disproportionation of the Q² sites at metaphosphate composition. The shortening of the real-space distances, r_m, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgO_n polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.     

X-ray scattering studies of the short and medium range ordering in AsxTe1−x glasses

The structures of As_xTe_1−x(x = 0.2, 0.4, and 0.5) glasses are studied using the differential X-ray anomalous scattering technique. The partial distribution functions have also been obtained for the stoichiometric As₂Te₃ glass, giving information on the medium range ordering. All the glasses show chemical disorder to differing extents, the As₂Te₃ glass being the most disordered. The Te coordination number undergoes a change at x=0.4 where it is 2.5 compared with 2 for AsTe. This change indicates the existence of about 50% of threefold Te sites in the stoichiometric glass, as well as in the Te-rich glasses. Some of the physical properties of the glasses may be explained based on these results.     

Boson peak,structural inhomogeneity,light scattering and transparency of silicate glasses

The microscopic origin of the excess of Density of Vibrational States (Boson peak) is discussed, taking into account recent analyses of the properties of glasses that were frozen-in under high pressure. Experimental data reveals a decrease of the Boson peak with increasing pressure of freezing (fictive pressure) and, simultaneously, an increase of the structural homogeneity of the glass. These results suggest new ways to improve the transmission of telecommunication fibers by decreasing the optical loss due to the scattering by the structural inhomogeneities of glasses.     

Short-range and medium-range order in amorphous barium germanate

Amorphous barium germanate of composition BaGe_2.55O_6.10 was prepared by ultra-rapid quenching using a double-roller melt-spinning device. The short-range and medium-range order was investigated by high-energy synchrotron X-ray diffraction, resonant scattering at the Ba K-absorption edge and by electron diffraction. There are indications that 6-fold coordinated germanium exists in the structure of the present amorphous alkaline earth germanate, i.e. both GeO₄ tetrahedra and GeO₆ octahedra are supposed to be basic building units. The ratio of 6-fold coordinated Ge to 4-fold coordinated Ge is approximately 1:3. A structure model is proposed which favours the existence of barium chains and/or barium layers, similar to the structure of barium silicate glasses.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

SAXS study of the micro-inhomogeneity of industrial soda lime silica glass

The microstructure of a variety of industrial multicomponent soda lime glasses for container and glazing and of a lead alkali silicate glass for pressed tableware articles was analyzed using small angle X-ray scattering (SAXS). No inhomogeneity with size >0.5 nm was found in the industrial samples, while ternary soda lime glass samples with a suitable composition when heat treated developed a phase separation which SAXS detected without difficulty. The results obtained disprove earlier reports which attributed problems of rheology and strength affecting container production (‘bad workability’) to microstructure formation.     

Nanoscale phase separation of CuI-Cu2MoO4 superionic conducting glass studied by analytical transmission electron microscopy

Microstructures of CuI-Cu₂MoO₄ superionic conducting glasses have been studied by analytical transmission electron microscopy equipped with high angle annular detector dark field (HAADF) detector and energy dispersive X-ray spectroscopy (EDS). Structural inhomogeneities of 5-10 nm in size are observed from HAADF images in the glass. Deference of composition between bright and dark contrast regions is clearly confirmed by EDS experiments. The nanoscale phase separation of 5-10 nm in size has been clarified by HAADF and EDS experiments.     

The structure of phosphate glass evidenced by small angle X-ray scattering

Numerous phosphate glass systems with compositions covering the entire glass forming range have been examined by small angle X-ray scattering. The experiments were carried out in a widely extended interval of s values between 0.055 and 31 nm⁻¹. A small angle scattering effect was detected for all samples investigated with the exception of the aluminium metaphosphate glass. The small angle scattering recorded indicates the presence of two differently-sized heterogeneity regions of electron density. The small-scale heterogeneities are present in magnesium and zinc phosphate glasses only. Their size is about 1 nm diameter in the magnesium and about 2 nm in the zinc phosphate glasses. The species do not result from liquid–liquid phase separation. Examinations to extract information on the nature of these heterogeneity regions are described in detail. There is no fundamental knowledge on the large-scale heterogeneities established. The small angle scattering suggests that their occurrence is related to water contamination. Anomalous small angle X-ray scattering experiments of a series of zinc polyphosphate glasses and a strontium metaphosphate glass sample are performed to examine the distribution of the Zn or Sr atoms, respectively.     

Structural characterization of Ni-based refractory glassy metals

X-ray scattering patterns of several Ni-based refractory alloy glasses indicated short range atomic order to at least four nearest neighbor shells. Comparisons between diffraction results from a synchrotron source vs. a standard laboratory source showed the necessity for low divergence and high intensity incident radiation in order to distinguish a low concentration of crystallites as small as ≈15 nm, which were present within an amorphous matrix. The divergence of both sources was examined by comparing the diffraction patterns of the same LaB₆ standard sample and using this as a reference standard to measure relative grain sizes. The crystallites in these glasses comprised between 0.0% and 7.5% by volume, and were relatively consistent amongst samples of the same composition. These crystalline peaks had potential matches with several metallic elements, compounds and oxides. It is seen that a very small composition range exists for near-perfect bulk metallic glass formers (as defined by a ≈0% crystallinity). Scattering electron microscopy was also performed to understand aspects of individual second phase particles such as size and distribution. Departure of radial distribution results from those predicted by hard sphere models indicated intermediate range order.     

Structural aspects of volume nucleation in silicate glasses

An interrelationship between parameters of short and intermediate range order in silicate glasses and the tendency to nucleate homogeneously in the volume is tested. Changes in the average coordination number and metal-oxygen distance of network modifying cations as well as changes in the concentration of constitutive silica tetrahedra accompanied with the crystallization of 18 stoichiometric glass compositions into their crystalline analogs are determined. The intermediate range structure of the glasses is investigated by configurational entropy and flow birefringence. The changes in structural parameters are analyzed in terms of the reduced glass transition temperature T_rg, which is negatively correlated with the maximal rate of volume nucleation. The results indicate that the short-range structure in stoichiometric glasses is, in general, very similar to the corresponding crystal structure but independent of the T_rg-scale and for this reason independent of nucleation properties. In contrast to the short range of the glass structure, birefringence induced by a forced flow above the glass transition temperature and configurational entropy are positively correlated with increasing T_rg. The results indicate increased structural order in the intermediate range for melts with a high supercool limit (T_rg < 0.58). It is concluded that this order phenomena may promote nucleation events and may help to explain the tendency to volume nucleation of silicate glasses with T_rg < 0.58.     

Bestimmung von Diffusionskoeffizienten aus der Röntgen-Kleinwinkelstreuung und Lichtstreuung von Gläsern

It is shown how to conclude from small angle X-ray scattering and light scattering of phase separated glasses what kind of phase separation process is occurring in the glass. In the cases of diffusion controlled particle growth and of spinodal decomposition, methods were presented permitting to find out from diffraction experiments the diffusion coefficients underlying these decomposition processes. The methods were demonstrated with three different glasses.     

Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.     

Analysis of diffuse background on the X-ray diffraction pattern of fullerite C<Subscript>60</Subscript>

Diffuse scattering on the X-ray diffraction pattern of fullerite C₆₀ is analyzed. The characteristics of the short-range order of the amorphous component of fullerite are determined by the Finbak-Warren method. It is established that the short-range order of the amorphous component is similar to the short-range order of lonsdaleite.     

Water penetration mechanisms in nuclear glasses by X-ray and neutron reflectometry

To determine the water diffusion at the early stage of the alteration, X-ray and neutron reflectometry have been performed on altered simplified glasses and the SON68 glass (an inactive R7T7-type French nuclear glass). For the first experiment, the simplified and SON68 glasses were altered at pH 3 and pH 6 and characterized by X-ray reflectometry as a function of the alteration duration. The evolutions of the electron density profile obtained from the reflectivity curves simulations have allowed the determination of the layers compositions. At the beginning of the alteration and for pH 3, the altered surface layer is constituted of a dealkalized zone. Upon alteration progress, the water diffuses inside the layer and hydrolyzes the Si–O–B bonds. For the second experiment, glasses were altered in D₂O (pD 3) and analyzed in D₂O saturated cell. After a D₂O/H₂O substitution, the samples were characterized one more time in H₂O saturated cell. The evolution of the scattering length density shows that in the first stage of the alteration, the layer is constituted of two parts: a dealkalized glass and a dealkalized and boron depleted glass where water has diffused. According to the glass composition and after few hours of alteration, this dealkalized glass part can disappear.     

Density fluctuations in silicate glasses

The potential use of inorganic glasses as waveguides at optical wavelengths has precipitated the need to understand the relationship between intrinsic optical loss and glass composition. The contribution to optical loss due to scattering from density fluctuations, is analyzed and discussed for various homogeneous (non-phase-separated) silicate glasses. The results show that binary alkali silicates in certain composition ranges form homogeneous glasses with density fluctuations less intense than that of pure silica glass. As third components added to these binary compositions alkali oxides (different from the one already present in the base glass), alkaline earth oxide (CaO) and Al₂O₃ cause further reductions in the magnitude of the density fluctuations.     

Preparation of TiO2–Na2O glass by sol–gel method and structural characterization

The applicability of sol–gel process in glass formation of binary system, (100 − x)TiO₂–xNa₂O (x = 10, 20, 30), was investigated and the glasses were prepared successfully by the sol–gel process for the first time. The process of glass formation was checked by using X-ray diffraction measurement and DTA–TG analysis. In the baking step, a DTA peak related to the crystallization of gel was found. The short-range structure of glassified samples was studied by neutron scattering measurement. It is found from the results of neutron scattering measurement that the coordination number of O atom around Ti atom is about 4, and the O atoms around Ti atom form a planer square rather than a regular tetrahedron.     

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO₂3Al₂O₃1.5P₂O₅3CaO2CaF₂, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca₅(PO₄)₃F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The ²⁹Si, ²⁷Al and ³¹P NMR spectra for the glasses with different strontium contents were identical. The ¹⁹F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite.     

Spectroscopic studies of gamma irradiated Nd doped phosphate glasses

Ultra violet-visible (UV-Vis) and Fourier transform infrared (FT-IR) spectra of Nd doped phosphate glasses have been studied before and after gamma irradiation in order to understand the changes in the optical properties of glasses as well as to find the characteristics frequencies of the vibrational modes of chemical bonds, which decide the structural and spectral changes. UV, Vis, IR absorption and photoluminescence spectra of these glasses show changes depending on the composition of glass matrix. These changes are correlated on the basis of oxygen (O) and neodymium (Nd) concentration ratio obtained from energy dispersive X-ray spectroscopic (EDX) measurement. Gamma irradiation shows decrease in transmission below 700 nm for all the Nd³⁺ absorption lines from all the samples. Differential absorption spectra (UV-vis) of the samples before and after gamma irradiation show generation of some new bands below 700 nm along with dips (decrease) in the spectrum at the location of main Nd³⁺ absorption lines. This is attributed to the generation of different types of defects in the glass matrix along with possibility of change in the valence state of Nd³⁺ to Nd²⁺. IR absorption spectra of these glasses are found dominated mainly by the characteristics phosphate groups and water (OH) present in the glass network. The effects of gamma irradiation on IR absorption are observed in the form of bond breaking and possible re-arrangement of bonding. EDX and X-ray photoelectron spectroscopic (XPS) measurements indicate decrease in the relative concentration of oxygen in the glass samples after γ-irradiation.     

Calculation of correlation factors for sodium self diffusion in glassy sodium disilicate

The paper presents a metgod of calculation for finding correlation factors in glasses when certain assumptions are fulfilled. The influence of long-range order on the correlation effect can be estimated for crystals by two calculations. First, the correlation effect takes place only between a few mutually correlated jumps that an ion undertakes with one defect, and these correlated sub-sequences are not mutually correlated. Second, the correlation factor converges when the calculation is performed by defining a terminating sphere radius and assuming that defects leaving the sphere no longer contribute to the correlation effect.As a specific example a glass of the composition Na₂Si₂O₅ is considered. The radius of the terminating sphere and the mean number of mutually correlated jumps per sub-sequence indicate a short-range order effect. With a suitable definition the correlation factor can then be calculated by superimposing directional randomly distributed crystal short-range orders. It turns out that a vacancy mechanism cannot explain the experiments but an interstitialcy mechanism with mixed collinear and non-collinear jumps fits the measurements.