The structure and optical properties of AlN nanocrystals prepared by Schlenk techniques at atmospheric pressure and low temperature

AlN nanocrystals were prepared in organic solvent at atmospheric pressure and low temperature by the Schlenk technique. Both hexagonal and cubic AlN nanocrystals were obtained. The hexagonal nano-AlN powder possessed a wurtzite structure with a=3.124 Å, c=5.024 Å, the average grain size was about 2 nm. The lattice constant of the cubic nano-AlN was a=9.171 Å, the average grain size was about 4 nm. The structural and optical properties of the obtained AlN were analyzed. The emission related to deep-level defects was investigated by using temperature-dependent photoluminescence.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

Young's modulus surface and Poisson's ratio curve for cubic metals

The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l, θ) along arbitrary loading direction [h k l] are given for cubic crystals. The representation surface for which the length of the radius vector in the [h k l] direction equals to E(h k l) and representation curve for which the length of the radius vector with angle θ deviated from the reference directions , and equals to υ(100, θ), υ(110, θ) and υ(111, θ) for example, are constructed for six FCC metals Ag, Al, Au, Cu, Ni, Pb and seven BCC metals Cr, Fe, Mo, Nb, Ta, V, W.     

Trends in stability, elastic and electronic properties of cubic Rh, Ir, Pd and Pt carbides depending on carbon content: MC versus M4C from first-principles calculations

First principles FLAPW-GGA calculations have been performed to understand the peculiarities of stability, elastic, electronic properties and chemical bonding for cubic carbides of four noble metals M=Rh, Pd, Ir and Pt depending on carbon stoichiometry: MC versus M₄C. Our main findings are as follows: (i) in contrast to mono-carbides MC with positive formation energies E_form>0, carbon-deficient sub-carbides M₄C are stable (E_form<0), thus carbon stoichiometry is one of the major factors determining successful synthesis of these materials, and (ii) as distinct from the majority of other 3d-5d metals (including Pd and Pt examined here), an unusual effect of Rh and Ir “metallization” and the increasing of ductility for these metals owing to the introduction of carbon has been established.     

Investigation of the vibrational properties of cubic yttria-stabilized zirconia: A combined experimental and theoretical study

A combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO₂ about the X-point. Compared to pure cubic ZrO₂, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO₂ at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO₂, thus providing support for the structural model considered.     

Crystal structure and compressibility of a high-pressure Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81, isomorphous with cubic zirconia

A Ti-rich oxide, (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)_∑=1.0O_1.81, was synthesized at 8.8 GPa and 1600 °C using a multi-anvil apparatus. Its crystal structure at ambient conditions and compressibility up to 10.58 GPa were determined with single-crystal X-ray diffraction. This high-pressure phase is isomorphous with cubic zirconia (fluorite-type) with space group Fm3¯m and unit-cell parameters a=4.8830(5) Å and V=116.43(4) Å³. Like stabilized cubic zirconia, the structure of (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 is also relaxed, with all O atoms displaced from the (, , ) position along 〈1 0 0〉 by 0.319 Å and all cations from the (0, 0, 0) position along 〈1 1 1〉 by 0.203 Å. No phase transformation was detected within the experimental pressure range. Fitting the high-pressure data (V vs. P) to a third-order Birch-Murnaghan EOS yields K₀=164(4) GPa, K′=4.3(7), and V₀=116.38(3) Å³. The bulk modulus of (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 is significantly lower than that (202 GPa) determined experimentally for cubic TiO₂ or that (~210 GPa) estimated for cubic ZrO₂. This study demonstrates that cubic TiO₂ may also be obtained by introducing various dopants, similar to the way cubic zirconia is stabilized below 2370 °C. Furthermore, (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 has the greatest ratio of Ti⁴⁺ content vs. vacant O²⁻ sites of all doped cubic zirconia samples reported thus far, making it a more promising candidate for the development of electrolytes in solid oxide fuel cells.     

Estimation of critical grain size at different temperatures and thermodynamic stability of γ→α in NC Fe by QDA and EOS methods

Thermodynamic properties of Nanocrystalline (NC) materials are essentially different from the conventional coarse-grained materials (with the same chemical composition). The role of grain boundary is very important in the characterization of thermodynamic functions and thermal properties of NC materials when the grain size is less than 100 nm. Therefore, the traditional thermodynamics being applied for coarse-grained materials is not applicable for NC materials. In this study, Quasiharmonic Debye Approximation (QDA) and Equation of State (EOS) methods are used to calculate the Gibbs free energy in NC Fe. Since the Gibbs free energy for Fe, predicted by EOS and QDA methods, is inaccurate (especially at temperatures higher than the ambient temperature), a term called as ΔG^Excess is proposed to modify the results. Thus, the Modified QDA (MQDA) and Modified EOS (MEOS) methods are introduced for this purpose. Thereafter, the change in the Gibbs free energy for γ-Fe to α-Fe phase transformation (ΔG^γ→α) via the grain size is calculated by MQDA and MEOS methods. The results obtained by the two methods are also compared and discussed. Finally, the critical grain size, at which ΔG^γ→α=0, can be estimated at different temperatures, is found to increase with increasing temperature.     

Preparation and characterization of Cd2Nb2O7 thin films on Si substrates

Thin Cd₂Nb₂O₇ films were grown on single-crystal p-type SiO₂/Si substrates by the metallo-organic decomposition (MOD) technique. The films were investigated by X-ray diffraction, X-ray energy-dispersive spectroscopy, and field emission scanning electron microscopy, and showed a single phase (cubic pyrochlore), a crack-free spherical grain structure, and nanoparticles with a mean size of about 68 nm. A Cauchy model was also used in order to obtain the thickness and index of refraction of the stack layers (transparent layer/SiO₂/Si) by spectroscopic ellipsometry (SE). The dielectric constant (K) of the films was calculated to be about 25 from the capacitance-voltage (C-V) measurements.     

Investigation of energy band gap and optical properties of cubic CdS epilayers

High quality cubic CdS epilayers were grown on GaAs (1 0 0) substrates by the hot-wall epitaxy method. The crystal structure of the grown epilayers was confirmed to be the cubic structure by X-ray diffraction patterns. The optical properties of the epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) and were studied in the transmittance spectra at a wavelength range of 400-700 nm at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, 〈?(E)〉 = 〈?₁(E)〉 + i〈?₂(E)〉, such as E₀, E₁, E₂, E^′₀, and E^′₁ structures. In addition, the optical properties related to the pseudodielectric function of CdS, such as the absorption coefficient α(E), were investigated. All the critical point structures were observed, for the first time, at 300 K by ellipsometric measurements for the cubic CdS epilayers. Also, the energy band gap was determined by the transmittance spectra of the free-standing film, and the results were compared with the E₀ structure obtained by SE measurement.     

Thermo-physical properties of body-centered cubic iron-magnesium alloys under extreme conditions

Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron-magnesium alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi-Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus (C^′). Magnesium alloying significantly increases C^′ of Fe, and bcc Fe-Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe-Mg alloy with 5% Mg agree well with those of the Earth’s inner core as provided by seismic observations.     

Synthesis of hexagonal C14/C36 and cubic C15 ZrCr2 Laves phases and thermodynamic stability of their hydrides

ZrCr₂ samples with well defined hexagonal (C14/C36) or cubic (C15) Laves phase structures have been synthesised by melting in an arc furnace followed by different treatments depending on the sample type. Hydrogen absorption capacity and thermodynamic stability of their respective hydrides have been investigated by solid–gas reaction. Pressure–composition–temperature (PCT) curves obtained for both structures at temperatures of 348, 373, 398 and 423 K, and H-pressures from 10¹ to 10⁶ Pa, showed that the cubic phase is less stable than the hexagonal ones. This result was related to Miedema's rule of reversed stability and also correlated to the volume of the interstitial hole for hydrogen.     

Structural investigations on iron containing natural Zincblende using EBSD

A sample of natural zinc sulfide containing iron (from Portugal, Albergaria, Velha) was systematically investigated with respect to its microstructure using XRD (X-ray diffraction) and EBSD (electron back scatter diffraction). The habitus of the black sample suggests a hexagonal crystal structure, i.e. the occurrence of the Wurtzite phase. Nevertheless, using XRD and EBSD allowed only detecting and localizing the cubic Zincblende structure within the sample with the fibrous habitus while the expected hexagonal Wurtzite structure and possibly a hexagonal FeS structure were missed. The macroscopic fibrous structures consist of non-uniform and elongated grain structures which possess a preferred orientation with the <224>-direction parallel to the fiber direction. Inside the grains, twinning occurs (Σ3-Twinning) as well as grain fragmentation. Iron is not distributed homogeneously; instead areas with unique iron concentrations occurred. They were arranged like twins with iron concentrations from 4.1 up to 5.1 at% as detected and localized using energy dispersive x-ray spectroscopy (EDS). Fe²⁺ is incorporated in lattice sites of Zn²⁺. Although the phase diagram FeS-Zn-S is not yet completely determined in all composition ranges of interest, coexisting phases (zincblende and FeS) should be expected at room temperatures. The results may contribute to further insights into the growth mechanisms of natural zinc sulfide, respectively to the discussion about. Furthermore, it was shown, that the crystal habitus not always allows concluding on the crystals symmetry with certainty.     

Phase stability in the LnCa2Mn2O7 (Ln=Pr, Nd, Sm and Gd) Ruddlesden-Popper series

The synthesis of the Ruddlesden-Popper series, LnCa₂Mn₂O₇, (Ln=Pr, Nd, Sm and Gd) is described and their structure and electronic properties investigated. The reduction in size of the A-site cation causes an increase in the distortion of their orthorhombic structures (space group Cmcm). All of these compounds form with a perovskite impurity, the amount of which increases on reduction of the cation size. The synthesis temperature also alters the amount of perovskite impurity in the phase, but only to a lower limit, implying the perovskite phase is intrinsic to the material and that a phase equilibrium exists between the layered Ruddlesden-Popper and perovskite phases, which is controlled by the cation size. The magnetic susceptibility show transitions characteristic of the perovskite phase, therefore little direct information can be obtained about the Ruddlesden-Popper phases, except that ferromagnetism is not observed in any of these materials.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

In situ Raman spectroscopy of cubic boron nitride to 90 GPa and 800 K

The temperature and pressure dependences of the Raman spectrum of the transverse-optical mode of cubic boron nitride were calibrated for applications to a Raman spectroscopy pressure sensor in optical cells to about 800 K and 90 GPa. A significant deviation from linearity of the pressure dependence is confirmed at pressures above 20 GPa. At ambient temperature, dv/dP slopes are 3.41(7) and 2.04(7) cm⁻¹/GPa at 0 and 90 GPa, respectively. A polynomial expression is used to fit the pressure–temperature dependence of the Raman line. The pressure dependence does not significantly change with temperature, as determined from experiments conducted up to 800 K. At 0 GPa, the dv/dP slope is 3.46(7) cm⁻¹/GPa at 800 K. At pressures above 90 GPa, the Raman spectrum of the transverse-optical mode cannot be observed because of an overlap of the signals of cubic boron nitride and diamond used as the anvils in the high-pressure cell.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

The evolution of octahedral rotations of orthorhombic LaVO3 in superlattices with cubic SrVO3

We have studied the octahedral rotations in LaVO₃/SrVO₃ superlattices, keeping the thickness of the orthorhombic LaVO₃ layers constant and increasing the thickness of cubic SrVO₃ layers. We have found that for a small thickness of SrVO₃, the octahedral rotations in LaVO₃ are maintained, while for an increasing thickness, these rotations are suppressed. This observation cannot be explained by purely elastic effects due to the lattice mismatch between the two materials, but the absence of rotations in SrVO₃ is a crucial ingredient, illustrating the concept of interface engineering of octahedral rotations.     

Influence of the Peierls relief on size effects in plastic deformation of micro- and nanocrystals of body-centered cubic metals

The influence of the Peierls relief and Peierls stresses on the size effects in micro- and nanocrystals of metals with a body-centered cubic (bcc) lattice has been theoretically discussed in the framework of the dislocation-kinetic approach. It has been found that, as compared to micro- and nanocrystals with a facecentered cubic (fcc) lattice, for the bcc crystals, the exponent n in the power law σ ～ D ^?n (where σ is the flow stress and D is the transverse size of the crystal) is the smaller, the higher is the critical temperature T _c above which the Peierls relief ceases to control the motion of dislocations in the bcc metals. The specific features of the influence of the Peierls relief on the coefficient of strain-rate sensitivity of the flow stresses in microcrystals of the bcc lattice have also been discussed. The theoretical results have been illustrated by the experimental data available in the literature.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Influence of pH on microstructural and optical properties of electrosynthesized CdSe thin films

Cadmium selenide (CdSe) thin films were electrosynthesized onto well cleaned stainless steel and fluorine-doped tin oxide (FTO) coated glass (10-15 Ω/cm²) substrates at different pH of the electrolytic solution. X-ray diffraction study reveals a cubic structure with preferential orientation along the (1 1 1) direction. The structural parameters such as grain size (D), lattice constant (a), strain (ε), dislocation density (δ), average internal stress (S) and degree of preferred orientation (I₁₁₀/I₂₂₀) in the film are calculated and they are found to be dependent on the pH of the depositing bath. EDAX analysis confirms nearly stoichiometric composition of the film deposited at pH 2.70. AFM analysis shows uniform deposition of the film over the entire substrate surface. In optical studies, the transition of the deposited film is found to be a direct allowed transition. The optical energy gaps are found to be in the range from 1.87 to 2.04 eV depending on the pH of the depositing bath. Photoluminescence (PL) spectrum shows blue shift in PL peak position and reduction in luminescence intensity for the film deposited at pH other than 2.70.