Determination of the crystallization ratio in a series of Fe-containing vitroceramic products by means of XAFS spectroscopies

We apply X-ray fluorescence (XRF) mapping and Fe-K X-ray absorption fine structure (XAFS) spectroscopies in order to determine the elemental distribution, the local coordination of Fe and the crystallization ratio of vitroceramic compounds that consist of Fe₂O₃, PbO, SiO₂ and Na₂O. The XRF maps reveal that Fe-rich inclusions are embedded into the vitreous matrix of the studied samples. The XAFS characterization results demonstrate that all quaternary samples are vitroceramic independently of the chemical composition and the T_cast. The ΕXAFS analysis reveals that only in the sample with 45 wt% Fe₂O₃ and T_cast = 1400 °C the microcrystalline inclusions consist of magnetite units. In all other vitroceramic samples the crystalline regions comprise both hematite and magnetite crystallites. The crystallization ratio is equal among all the vitroceramic samples, independent of the composition and T_cast. On the other hand, the increase in the concentration of hematite causes an elongation of the Fe–O bondlength in the FeO₆ polyhedra and thus improves the symmetry of the FeO₆ octahedra in the crystalline regions.     

The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study

A series of 27 silicate glasses of various compositions containing 0.2-2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.     

The structural and refractive index changes in the waveguides written by femtosecond laser in Er-doped silicate glasses

Waveguides with low propagation losses were written in erbium-doped silicate glasses by tightly focused femtosecond laser pulses. Raman spectra measurements and 2-D refractive index mapping in the micro-areas near the focus were carried out. The results supported that the refractive index change induced by fs laser pulses comes from the increase of the low rank ring structures in the silicate network.     

Study of color and structural changes in silver painted medieval glasses

Silver was introduced into medieval glass by an ancient painting process using different clay minerals (ochre, illitic, montmorillonitic, and kaolinitic clays). The colorimetric properties, studied by UV-Vis spectroscopy, were dependent on the clay mineral as a result of different concentration of Ag ions diffused into the glass surface. TEM results showed the well known formation of silver nanoclusters which give the yellow coloration of the glass. The obtained results showed that clay properties such as specific surface area, pore volume and iron concentration (Fe₂O₃), are important factors that affect the yellow coloration. It is also observed that Fe₂O₃ acts as an oxidant agent for silver atoms providing the Ag₂O formation. This oxide cannot diffuse into the glass structure and avoid the ion-exchanged process. After Ag ion diffusion some structural changes occur in the glass as it has been shown by Raman spectroscopy. It is observed that the diffusion process leads to depolymerization of the glass network as it is determined by analyzing the Qⁿ components of Raman spectra. Two Raman bands at 148 and 244 cm⁻¹ assigned to Ag-O bonds can be associated to the presence of Ag₂O on the glass painted surface.     

In‐situ investigation of the temperature dependent structural phase transition in CuInSe2 by synchrotron radiation

The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients α_a and α_c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α_L were obtained, as were α_a of the cubic lattice constant, also α_u and α_η. The transition temperature is clearly identified via a strong anomaly in α_L. The temperature dependence of the anion position parameter was found to be rather weak, nearly α_u∼0, whereas α_η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐T_trans|^0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu⁺ were found to occupy the Me1 site with a corresponding 67% of In³⁺ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐T_trans|^β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of plasticity-induced structural changes in a Zr-based bulk metallic glass using positron annihilation spectroscopy

Changes in the free volume distribution in Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass with inhomogeneous plastic deformation and annealing were examined using positron annihilation spectroscopy. Results indicate that the size distribution of open volume sites is at least bimodal prior to deformation. The size and concentration of the larger sites, identified as flow defects, changes with processing. The size of the flow defects initially increases with deformation. More extensive deformation shifts the distribution, with a third group of much larger sites forming at the expense of flow defects. This suggests that a critical strain is required for the growth of nanovoids observed elsewhere by HRTEM. Although these observations suggest the presence of three open volume size groups, further analysis indicates that all groups have a similar distribution of chemical species around them as evidenced by the same line shape parameter. This may be due to the disordered structure of the glass and the positron affinity to particular atoms surrounding the open volume regions.     

An investigation of different morphological and structural changes in neodymium hydroxide nanoparticles

Nd(OH)₃ particles of nanometer size have been prepared by using different methods. The obtained neodymium hydroxide samples have been investigated by using the FT-Raman spectroscopy, X-ray diffraction, transmission electron microscopy as well as field emission electron microscopy. Observable changes in enhanced electronic Raman bands have been found among the FT-Raman spectra of different morphological Nd(OH)₃ particles which formed under different physical–chemical conditions. The result suggests that the change of size and morphology of Nd(OH)₃ particles occur with structural change and subtle variation on the coordination environment around the neodymium ions.     

Simulation studies of structural changes and relaxation processes in lysozyme under pressure

The paper describes preliminary results of a molecular dynamics simulation study on the influence of non-denaturing hydrostatic pressure on the structure and the relaxation dynamics of lysozyme. The overall compression and the structural changes are in agreement with results from recent nuclear magnetic resonance experiments. We find that moderate hydrostatic pressure reduces essentially the amplitudes of the atomic motions, but does not change the characteristics of the slow internal dynamics. The latter is well described by a fractional Ornstein–Uhlenbeck process, concerning both single particle and collective motions.     

Pressure-induced structural changes in a borosilicate glass-forming liquid: boron coordination, non-bridging oxygens, and network ordering

Two of the most important structural controls on the properties of borosilicate glasses and glass melts are the variation between three- and four-coordination of network-forming boron cations, and the extent of mixing of Si and B. The effects of composition on these key parameters are relatively well studied. However, proposed mechanisms could be better constrained by testing with another, independent parameter that can also strongly affect the network. Here we present some of the first quantitative structural data on the effects of high pressure on the network structure of a sodium borosilicate glass. Using high-resolution ¹¹B and ¹⁷O NMR on a sample melted at 5 GPa, we demonstrate that the formation of tetrahedral boron from trigonal boron is indeed closely coupled to the conversion of non-bridging to bridging oxygens. The increased fraction of tetrahedral boron at high pressure also causes increased mixing of boron and silicate structural units, as oxygens bridging between two BO₄ groups are energetically relatively unfavorable.     

Temperature gradient induced phase transitions and morphological changes in diamond thin film

The diamond films were deposited onto a wurtzite gallium nitride (GaN) thin film substrate using hot-filament chemical vapor deposition (HFCVD). During the film deposition a lateral temperature gradient was imposed across the substrate by inclining the substrate. As grown films predominantly showed the hexagonal phase, when no inclination was applied to the substrate. Tilting the substrate with respect to the heating filament by 6° imposed a lateral temperature gradient across the substrate, which induced the formation of a cubic diamond phase. Diamond grains were predominantly oriented in the (100) direction. However, a further increase in the substrate tilt angle to 12°, resulted in grains oriented in the (111) direction. The growth rate and hence the morphology of diamond grains varied along the inclined substrate. The present study focuses on the measurements of dominant phase formation and crystal orientation with varying substrate inclination using orientation-imaging microscopy (OIM). This technique enables direct examination of individual diamond grains and their crystallographic orientation.     

Molecular dynamics simulation of temperature-induced structural changes in cristobalite,coesite and amorphous silica

Structural changes in cristobalite, coesite, and amorphous silica have been investigated from 100 to 3000 K by molecular dynamics simulations. These have been made with ‘soft potentials’ that yield melting and glass transition temperatures in fair agreement with experimental data. The simulated density of cristobalite increases abruptly at the α–β transition, that of coesite increases gradually, whereas that of amorphous silica goes through a density maximum around 2000 K. In the three phases, the complex temperature-induced structural changes have been analyzed in terms of variations of the abundances of four ‘structons’ which represent different kinds of Si–O bond lengths and torsional angles between two SiO₄ units. These changes in cristobalite, coesite and amorphous silica can then be rationalized in terms of loosening of network and of dynamical rotation of SiO₄ units. The relation between structural changes and ring statistics is also discussed.     

Structural changes in MgO by activation in a planetary mill

Mechanical activation of MgO in a planetary mill results in structural changes comparable to those observed in a vibration mill. Under the given conditions they are, however, achieved in considerable less time. Furthermore, a so far unreported lattice dilatation could be observed. The activated MgO showed an increased reactivity with respect to the ambient atmosphere. As a result magnesium hydroxide and a basic magnesium carbonate are formed.     

Investigation of structural and compositional changes in silver-doped GeO2 thin films

The processes going on in silver-doped GeO₂ films during air-heating were investigated by XPS, TEM, IR and UV-visible spectroscopy methods. Silver was shown to interact with the GeO₂ matrix at 500-600 °C to give silver germanate which was decomposed on further heating to form GeO₂ and silver nanoparticles, 10-35 nm in size, absorbing in the plasmon resonance region (λ_max = 415 nm). The silver nanoparticles are located deep in the films and encapsulated by oxide particles.     

Effects of mixed alkaline earth oxides additive on crystallization and structural changes in borosilicate glasses

The effects of mixed alkaline earth oxides on crystallization and structural changes in a multi-component borosilicate glass system are studied by using X-ray diffraction (XRD) and transmission electron microscope (TEM). It is found that the crystallization is decreased with increasing alkaline earth oxide content and there are also a series changes occurred in TEM images. This paper introduces the conception that alkaline earth ions tend to occupy their preferred sites regardless of glass systems. This conception is assisted to explain the TEM images to some extent by suggesting a simple structural model about non-bridging oxygen in glass network. The conception also indicates a ‘blocking effect’ existing in such a multi-component borosilicate glass system, which may be responsible for XRD results in chief. In addition, the structural model suggested by TEM results refers a new unit of Si–O–M²⁺–O–B, which helps in understanding the minimum exhibits in XRD results. Moreover, a dielectric test is taken to study glass properties in detail.     

Influence of compositional and structural changes on hydrogen bonding in silicon and silicon–germanium alloys

Hydrogen bonding in amorphous silicon and silicon–germanium alloys is investigated using Raman and gas effusion measurements. Special emphasis is given to the influence of structural changes on H bonding that occur in the cause of hydrogen effusion measurements. The data show that the analysis of the effusion spectra in terms of a hydrogen density-of-states distribution delivers reliable results for amorphous semiconductors.     

Pressure and Temperature Induced Frequency Shifts of Raman-Active Phonons in the MX Solid Pt2Br6(NH3)4

Abstract

We report the dependencies on hydrostatic pressure up to 100 kbar, and on temperature from 10 K to 300 K of the frequencies of Raman-active stretching modes in the single crystal MX chain solid Pt₂Br₆(NH₃)₄ (PtBrn). The data show that a predicted pressure-induced metallization does not occur, and suggests instead that either a 3-D structural distortion or a transition to a broken-symmetry ground state different from the charge-density wave (CDW) takes place.     

NMR investigation of rapid structural changes and non-metal-to-metal transition in the molten eutectic Ge–Te alloy

Resonance shift, K¹²⁵, spin-lattice relaxation time, T₁ and spin–spin relaxation time, T₂ for ¹²⁵Te nuclei in molten Ge₁₅Te₈₅ and Te in deeply under-cooled state have been measured as a function of temperature. The temperature dependence of K¹²⁵ is compared with that of the extent of structural change, C, and it is shown that the rapid increase of K¹²⁵ with increasing temperature can be attributed to an increase of the density of states at the Fermi level proportional to the extent of structural change. At high temperatures, the dependences on the temperature of K¹²⁵ and T₁ conform to the Korringa relation with an enhancement factor or the relaxation mechanism expected metallic nature. At low temperatures, in contrast, the Redfield theory assuming localized spins of which the density is proportional to C can explain the overall temperature dependences of T₁ and T₂. These results provide the first clear evidence for a crossover transition from extended to localized states in the electron system accompanying the rapid structural changes.     

宽温区内ZnO纳米线的CVD可控生长方法研究

以Zn粉为材料,采用CVD法在宽温区内可控生长ZnO纳米线.利用SEM对产物进行了微观分析,考察了反应温度与升温时间对ZnO纳米线形貌的影响.用ZnO纳米线制成光电导型紫外光探测器,并测试了该器件的性能,考察了所得ZnO纳米线的光电特性.研究工作表明:用CVD法制备ZnO纳米线时的反应温度不限于某一个特定值,而是常压下在419.5℃以上的温区内均可进行,该宽温区ZnO纳米线CVD合成法的关键在于优化和匹配生长温度与加热时间两个参数.对紫外光探测器的性能测试结果表明,ZnO纳米线具有良好的紫外光电响应特性.     

Atomic structure and high anisotropy of thermal expansion in NiSi single crystals. II. Temperature-induced structural changes

X-ray diffraction analysis of the atomic structure of NiSi crystals was performed by diffraction data obtained at 295 and 418 K. The temperature-induced changes in the bond lengths and valence angles were analyzed and the high anisotropy of thermal expansion of these single crystals was interpreted in terms of crystal chemistry.