Disordered matter under extreme conditions: X-ray diffraction,electron spectroscopy and electroresistance measurements

In the last decade, we have developed a set of experimental techniques for in-house X-ray diffraction measurements under high-temperature (up to about 2300 K) and high-pressure (up to 10 GPa and 1500 K, up to 100 GPa and 700 K) conditions, electron energy-loss and Auger measurements for surface electronic structure measurements at high-temperature (up to about 1800 K), and electrical resistance measurements covering both low-temperatures (2 K) and high-temperatures/high-pressures (1500 K, 7 GPa) conditions. In this paper we discuss some important technical features and possibilities of these new equipments and present novel data collected for phase transitions and structural modifications occurring in liquid and solid systems. In particular, we present new results about phase transitions and undercooling of bismuth under pressure, extreme undercooling and metastable states in gallium films, and surface phase transitions of Si at high-temperatures. The relevance of these experiments to the exploitation of the potential of equipments available at synchrotron radiation facilities is emphasized.     

Evaluation of structural and mechanical properties of aluminum oxide thin films deposited by a sol–gel process: Comparison of microwave to conventional anneal

Thin films of Al₂O₃ have been deposited on polished silica glass substrates at room temperature by sol–gel dip coating technique followed by two different exposure methods. One set was annealed at different temperatures ranging from 200 °C to 800 °C for 10 h and a second set was exposed to microwave (2.45 MHz) radiation at different powers for 10 min. The lower temperature and shorter time with microwave irradiation might be ascribed to the activating and facilitating effect of microwaves on solid phase diffusion. Unlike other preparation methods, microwave heating is generally quite faster, and energy efficient. X-ray diffraction (XRD) and scanning electron microscopy (SEM), energy dispersive X-ray analysis techniques have been employed to characterize structural, morphological and elemental compositions of the films. Adhesion strength failure measurements on films performed by scratch test in progressive loading sequence have shown critical loads up to 25 N (partial perforation) for both annealed films and films exposed to microwave irradiation. Nanohardness indentation tests of the films exposed (800 W) to microwave have shown hardness of 8.3 GPa with elastic modulus of 120 GPa compared to the conventional annealed film (800°) of 4.5 GPa with elastic modulus of 90 GPa.     

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li₂O · 2SiO₂ (LS₂) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

The structure of densified As2O3 glasses

In situ high energy X-ray diffraction and Raman experiments have been carried out to probe the structure changes of vitreous As₂O₃ under pressure. The first sharp diffraction peak reduces in intensity up to 10 GPa, indicating a breakdown of intermediate range order with pressure. All features in the Raman spectra broaden with increasing pressure up to 11.6 GPa. The mode at 378 cm⁻¹ associated with As₄O₆ molecule-like vibrations increases in intensity up to 6.2 GPa and decreases at higher pressures. In addition, X-ray and neutron structure factors have been measured for normal density and permanently densified As₂O₃ glasses recovered from 10 and 23 GPa. The results show the local AsO₃ pyramids and 3-membered rings essentially remain intact after compression. The increase in density is mainly associated with an inward shift of the third nearest neighbor peak in the X-ray radial distribution function, which indicates an increased packing of 3-membered AsO₃ rings.     

Silica xerogel films hybridized with carbon nanotubes by single step sol–gel processing

Synthesis of multi-walled carbon nanotubes (MWCNTs) doped silica xerogel films was reported in this work. A crucial step of introducing MWCNTs was achieved by functionalizing them by acid treatment to form stable and homogenous SiO₂/MWCNTs sol. Scanning electron microscopy showed spherical particles in honeycomb network structure for undoped xerogel films whereas dispersion and wrapping of MWCNTs in silica matrix was observed for MWCNTs doped films. Various bond formations during the sol–gel process and surface modification were confirmed using Fourier transform infra-red and detailed study on the chemical bonding state of the films was carried out using X-ray photoelectron spectroscopy. Nanoindentation studies showed that the mechanical properties of MWCNTs doped xerogel film increase dramatically: higher modulus (E = 2.127 ± 0.095 GPa) and hardness (H = 0.035 ± 0.017 GPa) values than those of pristine xerogel film (E = 0.234 ± 0.058 GPa, H = 0.01 ± 0.003 GPa).     

A study of Roman glass from Mala Barutana/Belgrade Fortress irradiated with pulsed CO2, Nd:YAG and ruby laser — Comparison

Application of non-contact and rapid laser technique, which is minimally invasive, non-contaminant and efficient method, for ancient glass investigation and cleaning is highly desirable for restoration purposes. Irradiation of Roman glass dated from 1st to 4th/5th century AD with TEA CO₂ (wavelength 10.6 μm; pulse duration t_p = 100 ns), Nd:YAG (wavelength 1064 nm and 532 nm; t_p = 150 ps) and ruby laser (wavelength 694 nm; t_p = 30 ns) in air ambience was studied. For all three lasers, moderate energy densities (15–30 J/cm²) induced significant changes of morphology — from superficial exfoliation and occurrence of mosaic structure after few pulses to deep damages and hydrodynamic features after higher number of accumulated shots. Irradiation with moderate energy density, accompanied with plasma appearance in front of the samples, is convenient for numerous potential applications, particularly surface elemental analysis such as laser induced breakdown spectroscopy. On the other hand, lower densities are more suitable for Roman glass cleaning. Calculations of Roman glass surface temperature have shown that pulsed CO₂ laser is favorable for surface cleaning and optimal fluence is ~ 2 J/cm². This was confirmed by additional experiments for fluences 1.5 and 3 J/cm². Morphological changes on the Roman glass surface induced by lasers were studied by optical microscopy (OM) and scanning electron microscopy (SEM). The composition of Roman glass was determined by energy dispersive X-ray analysis (EDX) and inductively coupled plasma (ICP) method. Chemical analysis confirmed that the investigated glass dates from the Roman period.     

Glass forming region in the GeSe2–GeTe–PbTe system and some physicochemical properties of glassy alloys

New chalcogenide glasses from the GeSe₂–GeTe–PbTe system were synthesized. The glass forming region was determined using visual, X-ray diffraction and scanning electron microscopy analyses. It is extended towards the GeSe₂ and lies partially on the GeSe₂–GeTe (0–58 mol% GeTe) and GeSe₂–PbTe (20.0–57.5 mol% PbTe) sides. No glasses were obtained in the GeTe–PbTe system. The investigated physicochemical properties vary between 4.04–6.21 g/cm³ (density, d); 97–121 kgf/mm² (microhardness, HV); ?0.187 ÷ 0.007 (compactness, C) and 14.3–17.8 GPa (elasticity modulus, E), respectively.     

High pressure elastic and plastic deformations of silica: In situ diamond anvil cell Raman experiments

Normal silica glass is usually referred to as low density amorphous silica as it can be converted to high density amorphous silica by a hydrostatic pressure (polyamorphic transition). In this work in situ Raman experiments are performed in a diamond anvil cell up to 18 GPa. The pressure effects on the structure of silica after successive compression decompression experiments are analyzed. The mode Grüneisen parameters corresponding to the elastic compression of high density amorphous silica are obtained and compared with those of normal silica. A reorganization of the high density amorphous silica below 3 GPa is evidenced.     

Morphology and dispersion state of Ba2TiSi2O8 nanocrystals in transparent glass-ceramics and their nanoindentation behavior

The morphology and dispersion state of Ba₂TiSi₂O₈ (BTS) nanocrystals in transparent glass-ceramics (composition: 40BaO–20TiO₂–40SiO₂) were examined from high resolution transmission electron microscope observations, and their nano-scale deformation behavior was examined using Berkovich nanoindentation technique (standard-type and continuous stiffness measurement (CSM)-type). In the early stage of crystallization, BTS crystalline layers with a thickness of ~ 120 nm were formed at the surface and ellipsoid-shaped crystallites with a diameter of 100–200 nm were dispersed in the glass matrix. In the late stage, BTS crystals with a diameter of 200–400 nm were formed densely, but a glassy phase was present between BTS crystals. The Young's modulus evaluated from CSM-type nanoindentation measurements for a deformation scale of about 100 nm shows the values of 98 GPa for the glass and 110 GPa for the glass-ceramics with nanocrystals. It was suggested that CSM-type measurements are very sensitive in the nano-scaled homogeneity in the heat-treated samples.     

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.     

Mapping of mechanical stress in silicon thin films on silicon cantilevers by Raman microspectroscopy

We have used plasma enhanced chemical vapor deposition (PECVD) to deposit silicon thin films (～0.2–1 μm) with different crystallinity fractions on Nanosensors PtIr5 coated atomic force microscopy (AFM) cantilevers (450 × 50 × 2 μm³). Microscopic measurements of Raman scattering were used to map both internal stress and extrinsic stress induced in the films by bending the cantilevers using a nanomanipulator (Kleindiek Nanotechnik MM3A). Thanks to the excellent elasticity of the cantilevers, the films could be bent to curvature radii down to 300 μm. We observed the stress induced shift of the TO–LO phonon Raman band of more than 3 cm^?1 for fully microcrystalline film corresponding to the stress ～0.8 GPa. The shift of the similar film with amorphous structure was ～2.5 cm^?1.     

The preferential orientation of the directionally solidified Si–TaSi2 eutectic in situ composite

The Si–TaSi₂ eutectic in situ composite is a favorable field emission material due to relatively low work function, good electron conductivity, and three-dimensional array of Schottky junctions grown in the composite spontaneously. The preferential orientation during directional solidification is determined by the growth anisotropy. In order to obtain the preferential direction of the steady-state crystal growth, the transmission electron microscopy (TEM) is used for analysis. It is found that the preferential orientation of the Si-TaSi₂ eutectic in situ composite prepared by Czochralski (CZ) technique is [3 3¯ 2¯] Si∥[0 0 0 1] TaSi₂, (2 2 0)Si∥(2 2¯ 0 0) TaSi₂. Whereas the preferential orientation of the Si–TaSi₂ eutectic in situ composite prepared by electron beam floating zone melting (EBFZM) technique is [0 1¯ 1¯ ]] Si∥[0 0 0 1] TaSi₂,(0 1¯ 1) Si∥(0 1¯ 1 1)TaSi₂. The preferential directions of the Si-TaSi₂ eutectic in situ composites prepared by two kinds of crystal growth techniques are distinctly different from each other, which results from different solid–liquid interface temperatures on account of the different crystal growth conditions, e.g. different solidification rate, different temperature gradient, different solid–liquid interface curvature and different kinetic undercooling.     

Growth of thick,continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10⁹ cm^?2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.     

Preparation of mixed arsenic/antimony chalcogenide glasses and some optical and thermo-physical properties

The preparation of mixed glasses of As₂S_3−xSe_x (x = 0–3) and (1 − y) · As₂S₃y · Sb₂S₃ (y = 0–1) has been carried out by an in situ pouring technique. X-ray diffraction (XRD) was used to confirm the glassy nature of the materials and monitor devitrification. Visible-IR transmission, photoluminescence, refractive index and micro-Raman were measured as a function of composition. Microhardness (MH) and thermal expansion coefficient (TEC) were also measured. Raman peaks in As₂S₃ and As₂Se₃ were observed around 338 cm⁻¹ and 230 cm⁻¹, respectively in this first composition series in which S was replaced by Se. When As was replaced by Sb, in the case of second composition series, the As₂S₃ related Raman peak became broader and shifted to lower wave number, reflecting some structural change/devitrification. MH increased (1.31–1.50 GPa) with Se and Sb content while the TEC was found to decrease (2.5–1.4 × 10⁻⁵/K). The progressive increase in the content of either Se or Sb in As₂S₃ is anticipated to modify bond lengths and bond angles. The combined effect of these structural modifications would change the local structure of the glass forming a more rigid glass network thereby increasing the hardness and decreasing TEC.     

Microstructure and field emission properties of the Si–TaSi2 eutectic in situ composites by electron beam floating zone melting technique

The directionally solidified Si–TaSi₂ eutectic in situ composites, which have highly aligned and uniformly distributed TaSi₂ fibers embedded in the Si continuous matrix, are obtained by electron beam floating zone melting (EBFZM) technique at the solidification rate range 0.3–9.0 mm/min. The preferential orientation of the Si–TaSi₂ eutectic is also studied by selected area electron diffraction (SAED), which is [0 1¯ 1¯]Si∥[0 0 0 1]TaSi₂ and (0 1¯ 1)Si∥(0 1¯ 1 1)TaSi₂. Moreover, field emission properties of the Si–TaSi₂ eutectic in situ composites are investigated by transparent anode imaging technology. Approximately straight F–N curves show that this material has excellent field emission properties.     

Effects of substrate orientation on aluminum grown on MgAl2O4 spinel using molecular beam epitaxy

Al thin films have been grown on single-crystal MgAl₂O₄ spinel substrates using solid source molecular beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation. X-ray diffraction reveals that Al layers are coherently grown on both (0 0 1)- and (1 1 1)-oriented spinel substrates. However, scanning electron microscopy and atomic force microscopy show that Al layers on (0 0 1) spinel substrates display smoother surface morphology than those grown on (1 1 1) spinel substrates. Additionally, electron backscatter diffraction and transmission electron microscopy demonstrate the presence of a high density of twin domain structures in Al thin films grown on (1 1 1) spinel substrates.     

Formation of microtwins in TmP/GaAs heterostructures

Microtwins in semi-metal thulium phosphide (TmP) epilayers grown on (0 0 1) GaAs substrates by molecular beam epitaxy have been studied by transmission electron microscopy. Selected area diffraction patterns show extra spots along 〈1 1 1〉 and 〈2 2 4〉 corresponding to three times the normal rock-salt periodicity. Only one or two twin variants are found in the crystal. The occurrence of the observed microtwins in the TmP-GaAs heterostructure can be accounted for by the growth-accident mechanism, i.e., the formation of microtwins is via growth accidents in the stacking sequence on {1 1 1} and {1 1 2} planes. The growth accidents appear to occur due to rapid growth rates and/or contamination.     

The effect of heat treatment on the paracrystallinity of an opal-CT found in a bentonite

To investigate temperature dependence of paracrystallinity for opal-CT, a bentonite containing approximately 34% by mass opal-CT have been used as material. Since opal-CT can not be separated entirely, the bentonite samples have been heated at different temperatures in the interval from 200 °C to 1300 °C for 2 h, and at 1050 °C for different time intervals changing from 2 h to 24 h. The X-ray diffraction (XRD) patterns of the original and heated samples have been obtained. The increase in the paracrystallinity has been discussed according to the thermal behavior of the relative intensity (I/I₀), relative full width at half-maximum peak height (FWHM/FWHM₀ ≡ W) and d-value of the most characteristics XRD peak for opal-CT between 0.405 nm and 0.410 nm region. The increase in I/I₀ from 1 to 3, and in d(l 0 1) spacing from 0.4050 to 0.4095 and decrease in W from 1 to 0.6 show that there is an increase in paracrystallinity for opal-CT by rising the temperatures between 800 °C and 1300 °C. The increase, of I/I₀ value from 1 to 5 by heating at 1050 °C while time increases from 2 h to 24 h shows that the paracrystallinity of opal-CT increase by time and reaches steady state condition approximately 1300 °C.     

On the entropy difference between the vitreous and the crystalline state

The paper deals with the entropy difference between frozen-in phases and their equilibrium counterparts. First, the nature of data compiled in thermochemical data collections are briefly reviewed, comprising data for non-equilibrium phases. Then, experimental evidence from earlier literature is compiled showing that the conventional entropy of a frozen-in phase at zero Kelvin assumes a non-zero residual value S(0). Based on calorimetric data from multiple sources, the same evidence is elaborated for diopside glass, yielding S_glass(0) = 24.8 ± 3 J/(mol K), a value reproducing a result publishes earlier. The zero Kelvin enthalpy of this glass is H_glass(0) = 81±8 kJ/mol. For S_glass(0), a structural interpretation in terms of silicate chain mixing is proposed, yielding a lower threshold for S_glass(0). From the point of view of statistical mechanics, non-zero residual entropies of frozen-in phases can be derived from ensemble averages, however, not from time averages.