Normal Lattice Positions Research Articles

Effect of in situ degradation on the atomic structure and optical properties of GaN-based green light-emitting diodes

The structure at the atomic scale and optical properties of GaN-based green light-emitting diodes (LEDs) before and after in situ degradation were investigated by spherical aberration corrected scanning transmission electron microscopy and temperature-dependent micro-photoluminescence. Indium (In) interstitial atoms existed in the degraded sample, due to the small-bond-energy In atoms deviating from their normal lattice position, caused by the relaxation of the InGaN well. Both the peak wavelengths of the original and degraded green LEDs had similar temperature-dependent behaviors, due to the localization states in the InGaN well. These wavelengths indicate that the degradation had little influence on the localization states. However, the emission peak of the degraded green LED redshifted by 1.6 nm at 300 K, and the integrated intensity decreased by 36.8%, compared to the peak and intensity of the original sample, respectively. Based on first-principles calculations, the calculated bandgap for the relaxation of the InGaN well was small. Therefore, the wavelength redshifted, and the luminous efficiency of the green LED decreased after degradation. These features are attributed to a decreased bandgap due to the relaxation of the InGaN well; increased defect density, resulting from In interstitial atoms; and an increase in the InGaN well thickness.

Highly dispersed silica-supported ceria–zirconia nanocomposites: Preparation and characterization

Ternary CeO2–ZrO2/SiO2 nanocomposite oxides were synthesized by a liquid-phase method and subjected to thermal treatment at 550 °C. The physicochemical characterization of the obtained nanosized systems was carried out by various techniques, namely: X-ray diffraction (XRD), nitrogen adsorption–desorption, Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The XRD measurements revealed the presence of cubic phase Zr0.4Ce0.6O2 in CZS1 (CeO2:ZrO2:SiO2 = 3:10:87) and CZS2 (CeO2:ZrO2:SiO2 = 10:10:80) samples. The crystallinity of this phase slightly increased with increasing ceria content in nanocomposites. Raman spectra indicated the presence of oxygen vacancies in the case of CZS2 and an absence displacement of oxygen ions from their normal lattice positions in both of the samples. XPS data indicated a significant quantity of Ce3+ ions present in the composites, which is greater in the CZS2 composite. In the case of CZS systems the TEM–HRTEM results showed well-dispersed nanosized Zr–Ce–oxides over the surface of amorphous SiO2. All the characterization techniques revealed that silica does not form any compounds with the dispersed Zr–Ce-oxides. The prepared materials were tested in catalytic reaction of CO oxidation in the presence of H2 and it was shown that CZS2 system had better performance than CZS1 one.

Phase diagram of the TbBr3–CsBr binary system. Thermodynamic and transport properties of the Cs3TbBr6 compound

Phase equilibria in the TbBr3–CsBr binary system were established from Differential Scanning Calorimetry (DSC) measurements. This binary system is characterized by three compounds, namely Cs3TbBr6, Cs3Tb2Br9 and CsTb2Br7, and two eutectics located at the TbBr3 mole fraction, x=0.095 (865 K) and x=0.552 (808 K), respectively. Cs3TbBr6 undergoes a solid–solid phase transition at 728 K and melts congruently at 1083 K with the related enthalpies 8.4 and 60.6 kJ mol−1, respectively. Cs3Tb2Br9, decomposes peritectically at 879 K, whereas CsTb2Br9 forms from Cs3Tb2Br9 and TbBr3 at 776 K and melts incongruently at 846 K. It undergoes also a solid–solid phase transition at 805 K, temperature very close to that (808 K) of the Cs3Tb2Br9–CsTb2Br7 eutectic. Separate investigations of the thermodynamic and transport properties were performed on the Cs3TbBr6 compound. These heat capacity and electrical conductivity experimental results suggest an order–disorder mechanism in the alkali metal cation sublattice whereas the TbBr6 octahedra, forming the anionic sublattice, retain their normal lattice positions. Compatibility of the experimental data was tested by the CALPHAD method. The entropy of mixing and Gibbs energies of formation of solid compounds were calculated. The temperature range of Cs3TbBr6 existence was discussed.

Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO 2–MO x (M = Mg 2+, Al 3+, Si 4+) mixed oxides for CO oxidation

The present work focuses on the combination of ceria with another oxide of different ionic valences from period 3 (Mg 2+, Al 3+, and Si 4+) using coprecipitation method, followed by calcination at 450 and 750 °C, respectively. The textural, structural, morphological and redox properties of nanosized ceria–magnesia, ceria–alumina and ceria–silica mixed oxides have been investigated by means of N 2 physisorption, XRD, Raman, HRTEM, DRS, FT-IR, and H 2-TPR technologies. XRD results of these mixed oxides reveal that only nanocrystalline ceria ( ca. 3–6 nm for the 450 °C calcined samples) could be observed. The grain size of ceria increases with the increasing calcination temperature from 450 to 750 °C due to sintering effect. The highest specific surface area is obtained at CeO 2–Al 2O 3 mixed oxides when calcination temperature reaches 750 °C. Raman spectra display the cubic fluorite structure of ceria and the existence of oxygen vacancies, and displacement of oxygen ions from their normal lattice positions in the ceria-based mixed oxides. DRS measurements confirm that the smaller the grain size of the ceria, the higher indirect band gap energy. H 2-TPR results suggest that the reductions of surface and bulk oxygen of ceria were predominant at low and high calcination temperature, respectively. Finally, CO oxidation were performed over these ceria-based mixed oxides, and the combination of CeO 2–Al 2O 3 exhibited highest activity irrespective of calcination temperature, which may due to excellent textural/structural properties, good homogeneity, and redox abilities.

Thermodynamic and transport properties of M3CeBr6 compounds (M = K, Rb, Cs)

Systematic trends in the thermodynamic properties of congruently melting M3CeBr6 compounds (molar enthalpies of the solid–solid phase transitions, molar heat capacity) following those found for another M3LnX6 compounds (Ln = lanthanide; X = halide, M = Li, Na, K, Rb, Cs) were evidenced. These data were complemented by electrical conductivity measurements over the wide temperature range. The results obtained clearly show that the M3CeBr6 compounds can be divided into two groups. The first one with K3CeBr6 compound having a single high temperature modification of cubic, elpasolite-type, crystal structure, and the second one with Rb3CeBr6 and Cs3CeBr6 compounds having both low- (monoclinic, Cs3BiCl6-type) and high-temperature (cubic, elpasolite-type) modifications. Transition from low- to high-temperature modification of these compounds is non-reconstructive phase transition. Within the two groups, the thermodynamic and transport properties of M3CeBr6 compounds are well correlated with their crystal structure. These results suggest different order–disorder mechanisms of the alkali metal cations whereas the CeBr6 octahedra, forming anionic sublattice, retain their normal lattice positions.

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.

Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO2, CeO2–ZrO2 and CeO2–HfO2 nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO2, Ce0.75Zr0.25O2 and Ce0.8Hf0.2O2 phases in case of CeO2, CeO2–ZrO2 and CeO2–HfO2 samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce3+←O2− and Ce4+←O2− respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr4+ and Hf4+ into the CeO2 lattice. The CeO2–HfO2 combination catalyst exhibited better activity than the CeO2–ZrO2. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.

Phase Diagram of the TbBr3−RbBr Binary System: Thermodynamic and Transport Properties of the Rb3TbBr6 Compound

Phase equilibria in the TbBr3-RbBr binary system were established from differential scanning calorimetry (DSC) measurements. This binary system is characterized by two compounds, namely Rb3TbBr6 and RbTb2Br7, and two eutectics located at the TbBr3 mole fractions, x = 0.117 (728 K) and x = 0.449 (718 K), respectively. Rb3TbBr6 undergoes a solid-solid phase transition at 728 K and melts congruently at 1047 K with the related enthalpies 7.8 and 58.7 kJ mol(-1), respectively. RbTb2Br7 melts incongruently at 803 K. It undergoes also a solid-solid phase transition at 712 K, a temperature very close to that (718 K) of the second eutectic, and much attention was paid in evidencing and separating these transition and eutectic effects. Separate investigations of the thermodynamic and transport properties were performed on the Rb3TbBr6 compound. These heat capacity and electrical conductivity experimental results suggest an order-disorder mechanism in the alkali-metal cation sublattice whereas the TbBr6 octahedra, forming the anionic sublattice, retain their normal lattice positions.

Unusual Nature of Lanthanide Chloride—Alkali Metal Chloride M3LnCl6 Compounds in the Solid State.

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Nanosized CeO2–SiO2, CeO2–TiO2, and CeO2–ZrO2 Mixed Oxides: Influence of Supporting Oxide on Thermal Stability and Oxygen Storage Properties of Ceria

The influence of SiO2, TiO2, and ZrO2 on the structural and redox properties of CeO2 were systematically investigated by various techniques namely, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HREM), BET surface area, and thermogravimetry methods. The effect of supporting oxides on the crystal modification of ceria was also mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahigh dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2–SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2-anatase phases were noted in the case of CeO2–TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of CeO2–ZrO2 sample. The cell ‘a’ parameter estimations revealed an expansion of the ceria lattice in the case of CeO2–TiO2, while a contraction is noted in the case of CeO2–ZrO2. The DRS studies suggest that the supporting oxides significantly influence the band gap energy of CeO2. Raman measurements disclose the presence of oxygen vacancies, lattice defects, and displacement of oxide ions from their normal lattice positions in the case of CeO2–TiO2 and CeO2–ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states, Si(IV), Ti(IV), and Zr(IV) at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states. The HREM results reveal well-dispersed CeO2 nanocrystals over the amorphous SiO2 matrix in the case of CeO2–SiO2, isolated CeO2 and TiO2 (A) nanocrystals and some overlapping regions in the case of CeO2–TiO2, and nanosized CeO2 and Ce–Zr oxides in the case of CeO2–ZrO2 sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO2 is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of mixed oxides is more than that of pure CeO2 and the CeO2–ZrO2 exhibits highest OSC.

Electron-stimulated desorption from alkali halide surfaces: Yield and kinetic-energy distributions of positive alkali ions

Low electron-current densities (average values below 100 pA/mm 2) and a time-of-flight technique were used to receive electron stimulated desorption spectra from LiF/Si(1 0 0) and from NaCl/Si(1 1 1) samples kept at room temperature. Such an experimental approach allows to obtain the results which are not distorted by the charging-up effects, are free of ions originated in post-ionization of atoms, and for which specific oscillatory structures can be distinguished in the kinetic-energy distributions of 6Li +, 7Li + and 23Na + ions, respectively. These oscillatory structures reveal that during an ion desorption sequence the wave packet squeezing effects are present and that, moreover, they are strong enough to be detected experimentally. Taking into account that three different potentials (ground-state, excited-state, final-state) are involved in the ion desorption scenario, it is shown that the obtained results can directly be used to determine the final-state potential from which the ion desorption occurs. Moreover, when the reduction in the ion desorption probability, related to the two-hole hopping process, is applied, not only the ion kinetic-energy distributions can be very well reproduced but also the calculated desorption yields are in agreement with experiments (including the isotope effect). The analysis performed indicates that most of the desorbed ions come from adatom sites and that only their small fraction is originated from normal lattice positions on planar parts of the sample surface.

Surface Stabilized Nanosized CexZr1-xO2 Solid Solutions over SiO2: Characterization by XRD, Raman, and HREM Techniques

Ce(x)Zr(1)(-)(x)O(2) solid solutions deposited over silica surface were investigated by X-ray diffraction (XRD), Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques in order to understand the role of silica support and the temperature stability of these composite oxides. For the purpose of comparison, an unsupported Ce(x)Zr(1)(-)(x)O(2) was also synthesized and subjected to characterization by various techniques. The Ce(x)Zr(1)(-)(x)O(2)/SiO(2) (CZ/S) (1:1:2 mole ratio based on oxides) was synthesized by depositing Ce(x)Zr(1)(-)(x)O(2) solid solution over a colloidal SiO(2) support by a deposition precipitation method and unsupported Ce(x)Zr(1)(-)(x)O(2) (CZ) (1:1 mole ratio based on oxides) was prepared by a coprecipitation procedure, and the obtained catalysts were subjected to thermal treatments from 773 to 1073 K. The XRD measurements disclose the presence of cubic phases with the composition Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) in CZ samples, while CZ/S samples possess Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), and Ce(0.5)Zr(0.5)O(2) in different proportions. The crystallinity of these phases increased with increasing calcination temperature. The cell a parameter estimations indicate contraction of ceria lattice due to the incorporation of zirconium cations into the CeO(2) unit cell. Raman measurements indicate the presence of oxygen vacancies, lattice defects, and displacement of oxygen ions from their normal lattice positions in both the series of samples. The HREM results reveal, in the case of CZ/S samples, a well-dispersed nanosized Ce-Zr-oxides over the surface of amorphous SiO(2). The structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic geometry and exhibit high thermal stability. Oxygen storage capacity measurements by a thermogravimetric method reveal a substantial enhancement in the oxygen vacancy concentration of CZ/S sample over the unsupported CZ sample.

Unusual nature of lanthanide chloride–alkali metal chloride M 3LnCl 6 compounds in the solid state

Thermodynamic and transport properties of M 3LnCl 6 compounds (M = K, Rb, Cs; Ln = La, Ce, Pr, Nd) were determined experimentally. The results obtained clearly show that the M 3LnCl 6 compounds can be divided into two groups, with compounds having only high temperature modification of cubic, elpasolite-type, crystal structure, in the first one, and compounds having both low-temperature (monoclinic, Cs 3BiCl 6-type) and high-temperature (cubic, elpasolite-type) modifications in the second one. Within the two groups, the thermodynamic and transport properties of M 3LnCl 6 compounds are well correlated with their crystal structure. These results suggest different order–disorder mechanisms of the alkali metal cation sublattice whereas the LnCl 6 octahedra, forming anionic sublattice, retain their normal lattice positions.

Local Arrangements in Fe-31% at. Pd above Ms

The diffuse x-ray scattering from an Fe -31 at. pct. Pd single crystal quenched from 1173 K was analyzed to obtain information about the local atomic environment around each atom including the local chemical order and the average atomic displacements from the normal lattice positions. To better reveal the meaning of these average quantities, computer simulations were employed to provide the structure using these values. The results indicate an interconnected distribution of ordered and clustered regions, no more than a few atoms in dimension, and only slightly more developed than for a random arrangement. The pattern of atomic displacements from the average lattice is revealed to be small embryos of a displacement array similar to the FCT martensitic phase and aligned in directions, which may be the premartensitic phase that has been predicted by several authors.

An AES and XPS study of the initial oxidation of polycrystalline magnesium with water vapour at room temperature

The initial oxide formation on polycrystalline magnesium surfaces at room temperature has been quantitatively followed from the earliest stage using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Clean surfaces were prepared by sputtering with a minimum dose of argon ions to avoid the creation of heavily disordered crystal planes. Crystallographic orientations of various grain faces were determined using the electron back scattering diffraction technique in order to allow AES measurements to be made on grains of known orientation. By using calibrated doses of deuterated water vapour, three stages of early oxide growth were recognised: (1) a chemisorption stage during doses up to approximately 0.7 langmuir (L) ( 1 L = 1.3 × 10 −4 Pa · s); (2) a rapid oxide nucleation and island growth stage which is complete by about 5 L at an average island height of four monolayers and (3) a slow, diffusion-controlled growth stage after coalescence of the islands. Modelling results suggest that the rate of initial oxygen uptake is faster on grain faces that have more open-packed, high index orientations, particularly in the second (island nucleation and growth) stage. In addition, the slow growth stage has been adequately described by a logarithmic type growth law for exposures up to 1 × 10 6 L, suggesting a limiting oxide thickness in water vapour of approximately 11 monolayers of MgO. The slow growth process was found to be controlled by the movement of metal cations through the oxide film from the metal/oxide interface to the oxide/gas interface. Finally, the role of hydrogen in the oxide film was studied using XPS and nuclear reaction analysis (NRA). The results indicated that hydrogen was present in the film only in small relative amounts, likely as hydroxyl groups trapped at the metal/oxide interface. Detailed XPS spectra showed two distinct types of oxygen: one of these is assigned to be oxygen in a normal MgO lattice position; the other is ascribed to oxygen in a “defective” chemical environment.

Radiation damage in scientific charge-coupled devices

Two important classes of radiation damage to the scientific CCD are discussed, namely bulk and ionization effects. Bulk damage or displacement damage is a process in which silicon atoms are displaced from their normal lattice positions by high energy photons or particles. Single atomic displacements or cluster defect damage in silicon is produced depending on the energy and type of radiation experienced by the detector. Bulk damage creates trapping sites within the CCD's signal channel which in turn degrades CTE performance. Ionization-induced damage induces a buildup of charge in the CCD's gate insulator causing the sensor's drive operating windows to shift (i.e. flat-band shift). In addition, ionization damage creates unwanted electronic sites at the gate's interface causing the CCD's dark current to increase.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The hamiltonian for excitonphonon coupling in molecular crystals

Abstract Excitons in a molecular crystal are usually surrounded by a zone of unexcited molecules displaced from their normal lattice positions. The displacement can be presented as a change in the origins of lattice modes. It is shown that the second-quantized crystal hamiltonian can be expressed in a way to treat the displaced modes separately from the others. Two canonical transformations, both developments from earlier work, are given to simplify calculations based on the new hamiltonian.

ESR investigation of an ionized Mn2+-F centre pair in KMgF3

When KMgF3:Mn is irradiated at room temperature, Mn2+-F-centre pairs are created. A subsequent UV bleach at 77K ionizes the F electron leaving an Mn2+-fluorine vacancy centre. The ESR spectrum associated with this centre exhibits a large crystal-field splitting with principal axes along the (100) directions. A small deviation from purely axial symmetry is explained by considering the distortion of the remaining five fluorine ions from their normal lattice positions. A pulse anneal study indicates that the Mn2+-fluorine vacancy centre dissociates at 185K.

Direct identification of atomic binding sites on a crystal

The atomic resolution of the field ion microscope, in conjunction with its ability to remove and identify individual atomic layers, allowed us to map unambiguously the unit cell of the (111) plane of tungsten and to determine directly the location of single tungsten atoms adsorbed on this plane. Adatoms have been observed to occupy two binding sites only. The predominant site corresponds to a normal lattice position. The second site is of similar symmetry, in that the adatom sits between three first layer atoms; however, at this position the adatom is located above an atom in the second rather than the third lattice layer. The former site is favored energetically, but only by ≈ 0.5 eV. All observations have been made at high fields, however, it is shown from studies of migration and other effects that the binding sites identified in the field ion microscope are typical of a normal, field free environment.

Dielectric Behavior of NaOH-Doped Ice

The dielectric behavior of ice doped with different NaOH concentrations was investigated in the 0° to − 25°C temperature range. In the lower concentration range of 1–4 × 10−6M NaOH, the conductivity behavior was similar to pure ice. At higher concentrations in the range of 7 × 10−6M NaOH, significantly higher conductivities were observed with the low-frequency conductivity being about 5 × 10−8 (Ω·cm)−1 and exhibiting only a slight temperature dependence. At lower temperatures the static dielectric constant was found to decrease with increasing NaOH concentration. The dielectric relaxation times and corresponding activation energies were about the same as for pure ice. The over-all behavior is distinctly different from that of HF, HCl, and NH3-doped ice. The model postulated to explain the observed results involves the interstitial incorporation of Na bonded to an (OH) site in its normal lattice position. It is postulated that this mode of incorporation causes lattice distortion that increases either the concentration or mobility of the OH-ionic defects which in turn accounts for the increased conductivity.