The effect of RO (R = Mg,Ca, Ba and Zn) on physical properties of Na2O/Al2O3/B2O3/SiO2 glasses for the preparation of gradient index micro lenses

Glasses in the system Na₂O–Al₂O₃–B₂O₃–SiO₂ doped with MgO, CaO, BaO or ZnO (concentrations: 2.5%–5 mol%) were melted from the raw materials and studied with respect to their densities, hydrolytic durabilities, crystal growth velocities, specific ionic conductivities, refractive indices, Abbe numbers and optical transmissions. The samples were ion exchanged in a NaNO₃/AgNO₃ salt melt in order to determine the Na⁺/Ag⁺ interdiffusion coefficients and the refractive indices, Abbe numbers and optical transmissions of the ion exchanged silver containing glasses. Glasses doped with ZnO are advantageous for the production of gradient index lenses due to their improved chemical durability and smaller crystal growth velocities at viscosities in the range of 10⁵–10⁶ dPa s. The optical properties of these glasses are similar to those of glasses without RO.     

On the effect of sublattice displacement in visualization of crystal structures

Electron-microscopy images of 〈001〉-GeNb₉O₂₅ obtained at the atomic resolution have been studied at the electron energy E = 80 keV. It is established that one of the sublattices on the image is displaced with respect to the other sublattices. The nature of this displacement is interpreted in terms of physics. The effect was studied in the formalism of Bloch waves. The important role of the valence Bloch state is also considered.     

Three-dimensional momentum density reconstructed from positron annihilation measurements of Si single crystals in comparison with compton effect results

The three-dimensional electron-positron pair momentum density ϱ_ep( p ) is reconstructed with the help of the MIJNARENDS '-analysis using angular correlation curves of 2γ-positron annihilation-radiation measured with p_z parallel to the [100], [110], and [111] direction of a Si single crystal. ϱ_ep( p ) is compared with the electron momentum density ϱ_e( p ) and ϱ_e( p ) which are calculated on the basis of the independent particle model. These momentum densities are taken from the literature. It is shown that the most important differences between the experimental momentum densities ϱ_ep( p ) and ϱ_e( p ) can be reproduced by the calculation on the basis of the independent particle model. Some experiences concerning the handling of experimental data and accuracy of reconstructed momentum densities are reported.     

Glide planes symmetry in the organization of some sulfide structures

The role of glide planes in the organization of structures is shown based on a crystallographic analysis of the monoclinic structures of TlAs₃S₅ and Tl₂(As,Sb)₃S₁₃ sulfides. In the first structure, cations and anions form systems (with identical geometries) of two face-centered sublattices, linked by the c plane, with the effect of unified “two-dimensional” (2D) ordering. The second structure, exhibiting signs of order–disorder (OD) type, is interpreted as a superposition of two noncentrosymmetric components with independent cation and anion sublattices, which, however, also form a regular 2D order due to the n plane. The stabilizing role of Tl cations in the geometry of cation matrices is indicated.     

Properties of porous glasses with embedded ferroelectric materials

Methods of introducing two ferroelectric materials, viz. NaNO₂ and (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB), into porous glass matrices and the results of dielectric, pyroelectric, thermal expansion and SEM investigations are presented. The best filling degree was achieved for NaNO₂ introduced from the melt, whereas a significantly worse filling effect was achieved for a water solution as the introducing agent. A splitting was observed in the temperature dependence of dielectric permittivity related to the incommensurate phase of NaNO₂ embedded in porous glass from a water solution, whereas no incommensurate phase was observed in the case of glass filled with NaNO₂ from the melt. The confined materials turned out to possess ferroelectric properties and exhibited phase transitions. The negative size effect was observed for porous glasses filled with NaNO₂ and PMACB. The above conclusions are based on the anomaly of dielectric, pyroelectric and dilatometric properties observed in the phase transition regions.     

Analysis of atomic structures as the development of Belov’s “lattice” crystallography

The theorems of lattice crystallography, which was developed by N.V. Belov, and the wave-mechanical concept of the crystalline state lie in the basis of the crystallographic analysis of structures, which determines the results of atomic ordering by sets of crystallographic planes with the formation of pseudotranslational sublattices (force skeletons of structures). The role of cationic and anionic sublattices is shown by the example of structures of natural sulfides: heyrovskyite Pb₆Bi₂S₉ and cannizzarite (Pb,Cd)₅(Bi,In)₆(S,Se)₁₄.     

Growth and characterization of GaP and GaAs1−xPx

The minority carrier lifetime at low and high excitation densities was determined in VPE layers of GaP and GaAs_?0.2 P_?0.8. The lifetime at high excitation densities, having a value up to ∽350 ns, is one to two orders of magnitude larger than the lifetime at low excitation densities. It is shown that impurities are involved in some saturable killer centres dominating at low excitation densities. In the case of the largest values of the minority carrier lifetime and at a dislocation density of > 10⁵ cm^?2, the non-radiative recombination at high excitation densities is shown to occur at dislocations; at lower values of the minority carrier lifetime the killer action may be due to microprecipitates. These findings also hold for LPE layers of GaP. It is shown that by measuring the minority carrier lifetime as a function of temperature a discrimination is possible between killer action due to diffusion of minorities towards sinks like dislocations or microprecipitates and due to capture by a normal point-defect type recombination centre.     

Nano-composite soda lime silicate glass prepared using silver ion exchange

Commercial soda lime silicate glasses have been subjected to ion exchange at different temperatures ranging from 320 to 500 °C in a molten mixture of AgNO₃ and NaNO₃ with molar ratio of 10:90, 02:98 and 50:50 for different time periods ranging from 40 to 180 min. Optical and structural properties of the ion exchanged glass are measured using UV–Vis–NIR absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and transmission electron microscopy (TEM). Signature of silver nanoparticle formation is obtained from the UV–Vis–NIR spectra, which shows a peak at 425 nm due to surface plasmon resonance (SPR). Replacement of Na⁺ ions by Ag⁺ ions is inferred from FTIR spectra. Fluorescence spectra reveal the formation of Ag⁰ atoms from Ag⁺ ions at higher temperatures. TEM image shows the silver nanoparticles of average size 3.75 nm. At exchange temperature of 500 °C Ag nanoparticles are formed without post-exchange annealing treatment.     

Numerical calculation of marangoni convection in a rectangular open boat

Numerical calculations of the flow caused by thermocapillary forces at the free surface in NaNO₃ and Si melts in an open boat geometry have been carried out. The results agree well with measured flow velocities in model experiments for crystal growth configurations with NaNO₃ melts. For Si melts a transition to a more complicated multiple eddy flow occurs. The projection-iteration method used for the calculations is much more effective than the finite difference methods applied until now and is especially advantageous for more complicated flow patterns.     

Potential energy landscape of Li+ ions in lithium silicate glasses: Implications on ionic transport

The potential energy landscapes of Li⁺ ions in Li₂O–SiO₂ glasses containing 3.3–15 mol% Li₂O have been studied using molecular dynamics simulation. It is shown for the first time that the densities of states for Li⁺ ions follow a nearly universal logarithmic distribution irrespective of the Li concentration. Such a functional form of the ionic density of states is shown to provide an explanation for the experimentally observed logarithmic dependence of the activation energy of dc conductivity on the modifier ion concentration in a wide variety of glasses.     

Electrical and Photoelectrical Properties of n-CuInS2 Single Crystals

The temperature dependences of free electron density and mobility in n-CuInS₂ single crystals have been determined from the Hall effect measurements. The ionization energy of donors and their densities have been estimated. The results of measurements show that the crystals are strongly compensated. The spectral distribution of photoconductivity, the dependence of photoconductivity on excitation intensity, and the photoconductivity decay with time have been measured in the n-CuInS₂ crystals at different temperatures. The results of photo-conductivity measurements suggest linear recombination of photo-electrons at weak excitation and quadratic recombination at high excitation intensity. The compensated deep acceptor probably acts as the recombination centre.     

Er photoluminescence enhancement in Ag-doped sol-gel planar waveguides

This paper reports on the study of the effects of silver (Ag^o) nanoparticles on the optical and spectroscopic properties of Er³⁺-doped silica-based gels and glasses, including active bulk materials and planar waveguides for integrated optics. Two different procedures for silver and erbium ion incorporation into the glassy matrices have been investigated: the direct incorporation of a metal salt (AgNO₃ and/or Er(NO₃)₃) into the sol-gel solution, as well as a modified sol-gel process, based on pore-doping of a precursor gel with AgNO₃ and/or Er(NO₃)₃ solutions. The study of the parameters determining the average size and size distribution of the nanoparticles, together with their influence on the sol-gel material densification and Er³⁺ photoluminescence at 1.5 μm, has been performed by means of transmission electron microscopy, plus ultra-violet/visible and photoluminescence spectroscopies. The Ag^o colloidal nanoparticles, obtained by thermal precipitation, were approximately spherical, homogeneously distributed and they exhibited an average size between ∼2 and 15 nm, depending on the silver content and heat treatment performed. They are shown to be responsible for a remarkable enhancement of the Er³⁺ photoluminescence intensity, which is mainly due to the increase of the local electric field around the Er³⁺ ions, due to the surface plasmon resonance of the Ag^o nanoparticles.     

Direct density determination of low- and high-density glassy polyamorphs following a liquid–liquid phase transition in Y2O3–Al2O3 supercooled liquids

Evidence has been presented for a density-driven phase transition occurring between supercooled liquids in the system Y₂O₃–Al₂O₃. The high- and low-density liquids were quenched to metastably coexisting glasses. Chemical analysis showed the compositions of the two glasses to be identical, and it was inferred that they differed in their densities and entropies. The entropy difference has been verified by calorimetry. Here, we confirm that the chemical compositions of the glassy materials derived from the high- and low-temperature liquids are identical. We present a direct density determination of the two glasses using sink-float techniques. The measured densities are 3.72(3) g/cm³ for the glass derived from the high-temperature liquid (i.e., the high-density amorphous or HDA polyamorph), and 3.58(1) g/cm³ for the low-temperature (low-density, LDA) polyamorph.     

Mechanism of thermal expansion of structural modifications of zinc pyrovanadate

Crystal chemical characteristics of the α and β modifications of Zn₂V₂O₇ are calculated based on in situ high-temperature X-ray measurements. The expansion of the structure is found to be strongly anisotropic up to the negative volumetric thermal expansion of the α-Zn₂V₂O₇ unit cell in the temperature range of 300–600°С, α _V =–17.94 × 10^–6 1/K. The transformations of the “hard” and “soft” sublattices with an increase temperature and at the phase transition are considered in detail. It is shown that the negative volumetric thermal expansion of α-Zn₂V₂O₇ is due to the degeneracy of the zigzag-like shape of zinc–oxygen columns at constant distances between their vertices.     

Bridgman growth and characterization of birefringent crystal NaNO3

With a bigger birefringence than calcite, sodium nitrate (NaNO₃) single crystal might have potential applications in fiber‐optic polarizer, opto‐isolators and polarizing prisms. However, it is difficult to grow large‐size NaNO₃ crystals for their thermal conductivity anisotropy and phase transition at 275°C. In this paper, crack‐free NaNO₃ crystals with size of Φ20×100 mm³ were prepared by Bridgman method with lower growth rate and lower interface temperature gradient. The dependence of the transmittance on the sample exposure time was measured and studied. The principal refractive indices of NaNO₃ crystal at the wavelengths 0.4730, 0.5320, 0.6328, 1.064 and 1.338 μm were measured by auto‐collimation method. From which, we calculated and obtained the Sellmeier's equation of NaNO₃ crystal. Moreover, the photoluminescence spectra were detected under the excitation at 240 nm, and NaNO₃ crystal presented its fluorescence around 416 nm.     

An experimental charge density study of mesulergine hydrochloride,a dopamine agonist

Carefully measured high-resolution X-ray diffraction measurements at 120(2) K have been used to map the electron deformation density of mesulergine hydrochloride, a dopamine agonist. A total of 52,508 intensities were measured with MoK radiation to a sin/)_max = 0.84~Å^–1. Averaging symmetry equivalent reflections yielded 9,418 unique reflections. A least squares refinement procedure was used in which multipole parameters were added to describe the distortions of the atomic electron distributions from spherical symmetry. Maps of the model electron deformation density distribution show the distribution of electron density in mesulergine hydrochloride. Molecular electrostatic potentials have been calculated from the electron densities determined in the least-squares multipole refinement. A region of negative electrostatic potential near N23 in the NHSO₂N(CH₃)₂ substituent suggests that the nitrogen atom would be reactive toward electrophilic attack. Net atomic charges calculated from refined monopole population parameters yielded charges of +0.17 for the NHSO₂N(CH₃)₂ substituent and +0.37 for the ergoline fused ring system. There does not seem to be an area of the molecule that, based on the results of this study, is more electron rich or electron deficient than others. Crystallographic data: C₁₈H₂₆N₄O₂SHCl, P2₁, a = 12.1281(3) Å, b = 6.1357(4) Å, c = 13.5935(4) Å, = 105.14(5), V = 974.14(5) ų, Z = 2.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Characterization of deep levels in a-plane GaN epi-layers grown using various growth techniques

To study the correlation between defects and deep levels in a-plane GaN films grown on r-plane sapphire substrates, transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) have been performed on three types of a-plane GaN samples grown using modified two-step growth (sample I), SiN_x interlayer (sample II), and patterned insulator on sapphire substrate (sample III). From the microstructure evolution in cross-sectional TEM images, it was shown that combination of growth techniques is highly efficient in the reduction of dislocation densities. Average dislocation densities of samples I, II, and III were about 2.2×10⁹ cm^?2, 1.1×10⁹ cm^?2, and 3.4×10⁸ cm^?2, respectively. The trap a_t E_c–E_t～0.13 eV (E1) was observed in only sample I, and three electron traps at 0.28–0.33 eV (E2), 0.52–0.58 eV (E3), and 0.89–0.95 eV (E4) from the conduction band edge were measured common to all the samples. The analysis of trap properties indicated that E2 and E3 trap levels are strongly associated with the partial dislocations in a-plane GaN films.     

Structural conditionality for the quadratic nonlinear susceptibility of Sr1−x Ba x Nb2O6 crystals

The intensity of the second harmonic generated in powdered samples of the compositions Sr_0.75Ba_0.25Nb₂O₆, Sr_0.61Ba_0.39Nb₂O₆, and Sr_0.50Ba_0.50Nb₂O₆ under irradiation by a pulsed-periodic YAG: Nd laser is measured. The effective values of the quadratic nonlinear susceptibility of these materials with respect to the reference sample of potassium dihydrophosphate crystals are estimated. The results obtained are compared with the precise parameters of the atomic structure of the single crystals with the corresponding compositions. It is established that the dominant contribution to the nonlinear optical susceptibility of the single crystals under investigation comes from the chains aligned parallel to the c axis of the crystal and composed of Nb octahedra with regularly alternating increased and decreased Nb-O interatomic distances. The occurrence of such a sequence in the crystal structure suggests an alternation of stronger and weaker chemical bonds in the chains ?O=Nb?O=Nb?O=. These bonds are characterized by different charge electron densities, a phenomenon confirmed by the X-ray diffraction analysis, which indicates not only a difference in the Nb-O bond lengths but also an anharmonicity in the thermal vibrations of the niobium atoms. The revealed anharmonicity is associated with the difference between the chemical bonds of the niobium atoms with the oxygen atoms located in trans positions in the octahedra.